JPH11130968A - Water-absorbent resin and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a granular water-absorbent resin which has a large granular size and a high agglomeration strength and a process for producing the same. SOLUTION: This granular water-absorbent resin has an average granular size of 200-10,000 μm and is formed by agglomerating bead-like water-absorbent resin primary particles having two or more frequency distributions and having a ratio of the max. median particle size to a median particle size lower than the max. median particle size of (3,000/1)-(1.5/1). The bead-like water-absorbent resin pimary paticles, having two or more frequency distributions and used for preparing the granular water- absorbent resin, are obtd. by subjecting a water-soluble ethylenically unsatd. monomer to reversed phase suspension polymn. of a water-in-oil type by using a water-soluble free-radical polymn. initiator in the presence of a dispersant in a hydrophobic org. solvent, optionally in the presence of a crosslinker, to form water-contg. bead-like water-absorbent resin primary particles, then adding a water-soluble ethylenically unsatd. monomer to a slurry of the polymn. system contg. the water-contg. bead-like water-absorbent resin to case the primary particles to absorb the water soln., conducting the additional suspension polymn., and repeating the above-mentioned process at least twice.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a water-absorbent resin and a method for producing the same. Specifically, a granular water-absorbent resin having a large particle size and a high binding strength, which is obtained by binding bead-like water-absorbent resin single particles having two or more frequency distributions and a specific median particle diameter ratio, and production thereof About the method.

According to the production method of the present invention, the average particle size is 20
A product granule of 0 to 10000μ can be obtained arbitrarily, and the obtained granule has a small amount of fine powder, a large binding force between single particles, and a high water absorption rate, so that hygiene such as disposable diapers and sanitary napkins can be obtained. It can be used not only as a material, but also as a soil water retention agent in the field of agriculture, and as a water-stopping material, a lubricating material, an anti-condensation material, and as a material for civil engineering and construction.

[0003]

2. Description of the Related Art In recent years, water-absorbing resins have been used not only for sanitary materials such as disposable diapers and sanitary goods, but also for industrial applications such as water-stopping materials, anti-condensation materials, freshness-retaining materials, and solvent dehydrating materials, greening, agricultural and horticultural applications, and the like. And various types have been proposed so far. As this kind of water absorbent resin,
Hydrolysates of starch-acrylonitrile graft copolymer, cross-linked carboxymethyl cellulose, cross-linked polyacrylic acid (salt), acrylic acid (salt) -vinyl alcohol copolymer, cross-linked polyethylene oxide, and the like are known.

However, none of these water-absorbent resins has been satisfactory in terms of particle size. In particular, there are various problems with the polymer obtained by reverse phase suspension polymerization. For example, in a water-in-oil type (hereinafter referred to as W / O type) reverse phase suspension polymerization of an alkali metal acrylate, HLB described in JP-B-54-30710 is used as a dispersant.
3-6 sorbitan fatty acid esters, JP-A-57-16
Nonionic surfactants of HLB 6 to 9 described in JP-A-7302 or HLB described in JP-B-60-25045
When surfactants of 8 to 12 are used, all of them can only obtain a fine water-absorbent resin having a particle size of about 10 to 100 μm, so that the particle size is too small to handle and the fine powder comes out of the diaper. there were. In order to solve this problem, a method of increasing the particle size by using a specific dispersant and a method of granulating after polymerization have been proposed.

The former method is disclosed in Japanese Patent Publication No. 63-363.
No. 21, JP-B-63-36322 discloses a method in which a lipophilic carboxyl group-containing polymer is used as a dispersant.
JP-B-1-17482, JP-A-57-15821
No. 0 discloses a method using an oil-soluble cellulose ester or cellulose ether as a dispersant. JP-A-62-2
No. 172006 proposes a method of using a polyglycerol fatty acid ester of HLB 2 to 16 as a dispersant,
Although a polymer having a large particle size was obtained, bulk polymerization was easily caused in, and the dispersant remaining during drying was melted, and the polymer was easily aggregated or adhered to the vessel wall. When considered, it was hardly an advantage.

On the other hand, Japanese Patent Application Laid-Open No. 61-9 / 1986 discloses the latter method.
No. 7,333 and JP-A-61-101536 propose a method of granulating single particles of a water-absorbing resin using a binder such as water or a water-soluble polymer. However, the method using water as a binder in this method has a problem that the binding force between the single particles is extremely weak, and the single particles bound in general transportation and handling are easily broken. Further, the method using polyvinyl alcohol requires a large amount of polyvinyl alcohol, although the binding property is improved, and requires a special apparatus, which increases the cost, and is not necessarily an advantageous method. .

JP-A-62-230813 discloses that a dispersion of water-absorbent resin seed particles formed from a first monomer in a non-aqueous liquid is formed, and the particles are added to a water-soluble ethylenically unsaturated second particle. A seed polymerization method is disclosed which comprises absorbing a monomer and then polymerizing a second monomer. According to this method, a water-absorbent resin having a larger particle diameter than the seed particles or a large particle diameter obtained by agglomeration of the seed particles can be obtained.

On the other hand, JP-A-3-227301 discloses a similar seed polymerization method, in which the slurry liquid after completion of the first-stage reverse phase suspension polymerization is cooled, and a surfactant and / or a polymer protective colloid is cooled. A method for producing a water-absorbent resin has been proposed in which a second-stage monomer aqueous solution is added in such a manner as to precipitate in a solvent, and after the liquid absorption, the polymerization of the second and subsequent stages is performed. In this method, a surfactant and / or a polymer protective colloid is precipitated by cooling a first-stage polymerization solution, that is, a surfactant solution, that is, a second-stage monomer aqueous solution is suspended, ie, W / O-type emulsion is not formed, liquid absorption is sufficiently performed, and when this is polymerized, the amount of fine powder is small.
Moreover, it is stated that a water-absorbent resin having a sharp particle size distribution can be obtained.

Japanese Patent Application Laid-Open No. 6-184212 discloses that the first-stage polymerization is carried out using a specific radical-polymerization-reactive surfactant, and the radical-polymerization-reactive surfactant used in the first stage is used in the seed polymer. A method is disclosed in which a surfactant is incorporated, and in the liquid absorption and polymerization after the second stage, the surfactant is eliminated from the medium. Also, JP-A-8-28331
No. 6 discloses that a coagulant monomer obtained by adding a specific hydrophilic additive to a slurry of an aqueous polymer dispersed in a hydrocarbon liquid is introduced into a medium at a temperature higher than the polymerization temperature of the monomer (decomposition temperature of the initiator). Methods for obtaining agglomerated particles have been proposed.

[0010] Japanese Patent Application Laid-Open No. 9-12613 discloses that the slurry liquid after the completion of the first-stage reverse phase suspension polymerization has an HLB of 7 or more and an HLB higher than the HLB of the surfactant used in the first stage. A nonionic surfactant or an anionic surfactant or a mixture thereof is added, a second-stage monomer aqueous solution is added to cause demulsification, and after the monomer aqueous solution is absorbed, the second and subsequent stages are added. A method for producing a water-absorbent resin for performing polymerization has been proposed.

[0011]

However, there is a problem that it is difficult to obtain granules having excellent granule particle size and binding strength between single particles by using any of the seed polymerization methods. ing. An object of the present invention is to provide a granular water-absorbent resin having a large particle size and a high binding strength, and a method for producing the same by a reversed-phase suspension polymerization method.

[0012]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, they have two or more frequency distributions produced by reversed-phase suspension polymerization and a specific median particle diameter ratio. The present inventors have found that by binding bead-shaped water-absorbent resin single particles, a granular water-absorbent resin having a large particle size, a high binding strength, and a significantly improved water absorption rate can be obtained. Was completed.

That is, the gist of the present invention is as follows. It has two or more frequency distributions, and the ratio of the smaller median particle size to its maximum median particle size is 1
/ 3000 to 1 / 1.5 bound to a single particle of the water-absorbent resin beads, and has an average particle diameter of 200 to 100.
1. a granular water-absorbent resin having a particle size of 00 μm; Water-soluble ethylenically unsaturated monomer is used in a hydrophobic organic solvent in the presence of a dispersant, and in the presence of a crosslinking agent, if necessary, using a water-soluble radical polymerization initiator to form a water-in-oil reverse phase suspension polymerization reaction. To form a single particle of the water-containing bead-like water-absorbent resin, and then further adding a water-soluble ethylenically unsaturated monomer to the polymerization slurry containing the water-containing bead-like water-absorbent resin to form the aqueous solution. After absorbing the liquid into the water-absorbent resin single particles, the operation of performing an additional suspension polymerization reaction is repeated once or more, and in order to obtain a granular water-absorbent resin, a bead-shaped water absorbent having two or more frequency distributions A method for producing a water-absorbent resin, which comprises using single resin particles. Hereinafter, the present invention will be described in detail.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION

(1) Granular water-absorbent resin The granular water-absorbent resin of the present invention has two or more frequency distributions as described above, and the ratio of the median particle diameter smaller than that to the maximum median particle diameter is 1 / 3000
To 1.5 / 1.5, preferably 1/250 to 1 / 1.6, more preferably 1/25 to 1 / 1.8. Diameter 200
-10,000 μm, preferably 200-5000 μm,
More preferably, it refers to a granular material having a size of 200 to 2000 μm.

The bead-shaped water-absorbent resin single particles include:
Its maximum median particle size is 30-300 μm, preferably 30-250 μm, more preferably 50-250 μm
And the smaller median particle size is 0.1 to 10
0 μm, preferably 1 to 100 μm, more preferably 1 μm
0 to 100 μm. Note that the granular state means that the state in which the seed particles are bound to each other is a grape state.

FIG. 1 shows an example of the particle size distribution of the beaded water-absorbent resin single particles having two or more frequency distributions used herein. Here, FIG. 1 shows an example of a water absorbent resin having three frequency distributions. The median particle diameter is a particle diameter when the cumulative weight% is 50% by weight in one frequency distribution of the seed particles, and has a median diameter for each frequency distribution.

(2) Method for producing granular water-absorbing resin The granular water-absorbing resin of the present invention can be prepared, for example, by adding a water-soluble ethylenically unsaturated monomer to a hydrophobic organic solvent in the presence of a dispersant.
If necessary, a water-in-oil type reverse phase suspension polymerization reaction is carried out using a water-soluble radical polymerization initiator in the presence of a crosslinking agent to form seed particles, and then a water-soluble ethylenically unsaturated monomer is further added. After adding to the polymerization slurry containing the seed particles and allowing the aqueous solution to be absorbed by the generated seed particles,
An operation of performing an additional suspension polymerization reaction is repeated one or more times to obtain a granular water-absorbent resin, which is produced by the method of the present invention, which comprises using seed particles having two or more frequency distributions. can do.

This method can be summarized as follows: a step of producing seed particles having two or more frequency distributions obtained by repeating a reverse phase suspension polymerization reaction operation of an aqueous solution of a water-soluble ethylenically unsaturated monomer two or more times (hereinafter referred to as a "step"). This step is simply referred to as a “first step”) and a step of absorbing, aggregating, and polymerizing an aqueous solution of a water-soluble ethylenically unsaturated monomer with the seed particles (hereinafter, this step is simply referred to as a “second step”). ), And by appropriately selecting the ratio of the total weight of the largest seed particles of the seed particles having two or more frequency distributions to the total weight of the small seed particles, the average granulated particle diameter, binding strength, etc. Can be changed arbitrarily. Further, the product granules obtained by the present invention have various characteristics such as an extremely high water absorption rate.

(Water-soluble ethylenically unsaturated monomer) As the water-soluble ethylenically unsaturated monomer used in the present invention, basically any water-soluble or water-miscible monomer can be used. For example, (meth) acrylic acid and / or its alkali metal salt, ammonium salt,
Ionic monomers such as (meth) -acrylamide-2-methylsulfonic acid and / or alkali metal salts thereof; (meth) acrylamide, N, N-dimethylacrylamide, 2-hydroxyethyl (meth) acrylate, N-methylol (meth) Nonionic monomers such as acrylamide; unsaturated monomers having an amino group such as diethylaminoethyl (meth) acrylate and dimethylaminopropyl (meth) acrylate, and quaternized products thereof; and a kind selected from these groups Alternatively, two or more kinds can be used. Here, "(meth) acryl"
The term shall mean both "acryl" and "methacryl".

Of these, (meth) acrylic acid and / or its alkali metal salts and ammonium salts are preferred.
(Meth) acrylamide. Examples of the alkali metal salt include a sodium salt, a potassium salt, a lithium salt, a rubidium salt, and the like.
From the viewpoint of industrial availability and safety, sodium salts or potassium salts are preferred.

The monomer concentration in the aqueous solution of these water-soluble ethylenically unsaturated monomers is generally 20% by weight.
As described above, the content is preferably 25 to 75% by weight. In addition, (meth) acrylic acid, 2-methyl-acrylamide-2-sulfonic acid and the like are used in a form in which a part or the whole amount is neutralized by an alkali metal compound or an ammonium compound. The ratio (hereinafter referred to as the degree of neutralization) is 20
-100 mol%, preferably 30-100 mol%.

In the present invention, as the water-soluble ethylenically unsaturated monomer as described above, the monomer components used in the first and second stages are the same or different from the monomer components used in the first stage. May be used. Furthermore, when the monomer component used in the second stage is the same as the monomer component used in the first stage, the monomer concentration in the aqueous solution, the degree of neutralization, etc. may be changed, and various conditions as well as arbitrary types are employed. it can.

(Dispersant used in the first stage) The dispersant used in the first stage of the present invention is used in the reversed-phase suspension polymerization system of the first stage. Any of these can be used as long as they have solubility or affinity in a hydrophobic solvent and basically form a W / O type emulsification system. Such dispersants are specifically non-ionic and / or anionic, generally having an HLB of 1 to 9, preferably less than 2 to 7.

Specific examples of the dispersant include sorbitan fatty acid esters, polyoxysorbitan fatty acid esters,
Sucrose fatty acid ester, polyglycerin fatty acid ester, polyoxyethylene alkyl phenyl ether, ethyl cellulose, ethyl hydroxyethyl cellulose,
Polyethylene oxide, anhydrous maleated polyethylene, anhydrous maleated polybutadiene, anhydrous maleated ethylene
Examples include propylene diene terpolymer, copolymer of α-olefin and maleic anhydride or a derivative thereof, and polyoxyethylene alkyl ether phosphoric acid. The amount of the dispersant used is 0.05 to 1 with respect to the hydrophobic solvent.
0% by weight, preferably 0.1 to 1% by weight.

(Hydrophobic solvent) The hydrophobic solvent used in the present invention is basically hardly soluble in water, and any solvent can be used as long as it is inert to polymerization. Examples thereof include aliphatic hydrocarbons such as n-pentane, n-hexane, n-heptane and n-octane; alicyclic hydrocarbons such as cyclohexane and methylcyclohexane; and aromatic hydrocarbons such as benzene, toluene and xylene. And the like. Among these, n-hexane, n-heptane and cyclohexane can be mentioned as preferred solvents in view of the stability and quality of industrial availability. The amount of the hydrophobic solvent to be used is 0.5 to 10 times by weight, preferably 0.6 to 5 times by weight, based on the aqueous solution of the water-soluble ethylenically unsaturated monomer used in the first stage.

(Crosslinking Agent) In the present invention, a crosslinking agent can be used as required in the first and second stages.
If necessary, in the present invention, so-called self-crosslinking by the monomer itself occurs even without a crosslinking agent depending on, for example, monomer conditions (type of monomer, concentration of monomer in aqueous solution, degree of neutralization, etc.) Thereby, a water-absorbing resin can be formed. However, a cross-linking agent may be required depending on the required performance, for example, the water absorption capacity and the water absorption rate. Examples of the crosslinking agent used in the present invention include a crosslinking agent having two or more polymerizable unsaturated groups and / or reactive functional groups.

Examples of the crosslinking agent having two or more polymerizable unsaturated groups include ethylene glycol, propylene glycol,
Di- or tri (meth) acrylates of polyols such as trimethylolpropane, glycerin polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; and reaction of the polyols with unsaturated acids such as maleic acid and fumaric acid Unsaturated polyesters, bisacrylamides such as N, N'-methylenebisacrylamide, di- or tri (meth) acrylates obtained by reacting polyepoxide with (meth) acrylic acid, tolylene diester Di (meth) acrylate carbamyl esters obtained by reacting polyisocyanates such as isocyanate and hexamethylene diisocyanate with hydroxyethyl (meth) acrylate, allylated starch, allylated cellulose, diallyl phthalate, and others Tiger allyloxy ethane, pentaerythritol triallyl ether, trimethylolpropane triallyl ether, diethylene glycol diallyl ether, polyvalent allyl system, such as triallyl trimellitate and the like. Among these, in the present invention, ethylene glycol di (meth) acrylate, polyethylene glycol di (meth) acrylate, propylene glycol di (meth) acrylate, polypropylene glycol di (meth) acrylate, N, N'-methylenebis (meth) acrylamide, etc. Is usually used.

Examples of the crosslinking agent having two or more reactive functional groups include diglycidyl ether compounds, haloepoxy compounds and isocyanate compounds. Among these, a diglycidyl ether compound is particularly preferred.
Specific examples of the diglycidyl ether compound include ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, glycerin diglycidyl ether, and polyglycerin diglycidyl ether. Among them, ethylene glycol diglycidyl ether is preferable. Other examples of haloepoxy compounds include epichlorohydrin, epibromohydrin, β-methylepichlorohydrin and the like, and isocyanate compounds such as 2,4
-Tolylene diisocyanate, hexamethylene diisocyanate and the like, which can be used in the present invention. The amount of the crosslinking agent to be used is generally 0 to 10% by weight, preferably 0.001 to 5% by weight, based on the ethylenically unsaturated monomer.

(Water-soluble radical polymerization initiator) The polymerization initiator used in the present invention is a water-soluble radical polymerization initiator. Examples include hydrogen peroxide, potassium persulfate, sodium persulfate, persulfates such as ammonium persulfate,
2,2'-azobis- (2-amidinopropane) dihydrochloride, 2,2'-azobis- (N, N'-dimethyleneisobutylamidine) dihydrochloride, 2,2'-azobis {2methyl-N -[1,1-bis (hydroxymethyl) -2-
[Hydroxyethyl] propionamide}. These water-soluble radical initiators may be used as a mixture. In addition, hydrogen peroxide and persulfate can be used as a redox-type initiator by combining a reducing substance such as sulfite, L-ascorbic acid, and amines. These polymerization initiators are used in an amount of 0.001 to 5% by weight, preferably 0.03% by weight, based on the amount of the ethylenically unsaturated monomer.
It is appropriate to use it in the range of 1 to 1% by weight.

(Polymerization reaction operation and conditions) First stage: Method and conditions for preparing seed particles having two or more frequency distributions: In the present invention, first, an aqueous solution of a water-soluble ethylenically unsaturated monomer is mixed with the W / O type aqueous solution. In the presence of a dispersant, in a hydrophobic solvent,
If necessary, the first-stage reverse phase suspension polymerization is carried out using a water-soluble radical initiator in the presence of a crosslinking agent. The polymerization method at this time may be any of a batch polymerization method in which the monomer aqueous solution is charged into the reactor from the beginning and a dropping method in which the monomer aqueous solution is dropped into the hydrophobic solvent under reflux. The median particle size of the obtained seed particles can be generally controlled by the amount of the dispersant, the number of rotations for stirring, and the like. For example, a method of reducing the median particle diameter by increasing the amount of the dispersant during the polymerization, or reducing the median particle diameter by increasing the stirring rotation speed during the polymerization can be performed. In the case of the batch polymerization method, set the number of rotations for stirring low, first create the maximum seed particles, and then in the same reactor under conditions that do not absorb the monomer to be added (for example, conditions under which the dispersant does not precipitate). By adding an unsaturated monomer and polymerizing at a rotation speed higher than the rotation speed, small seed particles may be obtained. Also, after setting the amount of the dispersant low and preparing the maximum seed particles first, the amount of the dispersant is added so that the concentration of the dispersant is higher than the amount of the dispersant, and the conditions for not absorbing the added monomer ( For example, the conditions under which the dispersant does not precipitate)
By adding a water-soluble ethylenically unsaturated monomer in the same reactor for polymerization, small seed particles may be obtained. Further, the dispersant and the number of rotations may be changed at the same time. In addition, an operation of obtaining the largest seed particles after creating small seed particles which is the reverse of these operations may be used.

In addition, in the drop-type polymerization, as in the case of batch polymerization, the stirring speed is set low and the maximum seed particles are first prepared, and then the added monomer is not absorbed (for example, the dispersant is not precipitated). Small seed particles may be obtained by adding a water-soluble ethylenically unsaturated monomer in the same reactor and polymerizing at a rotation speed higher than the rotation speed. The reverse operation may be used. Also, by changing the rotation speed during the drop polymerization, the maximum seed particles and the small seed particles may be simultaneously produced.

The median particle size of the largest seed particles having two or more frequency distributions is 30 to 300 μm,
Preferably from 30 to 250 μm, more preferably from 50 to 2
At 50 μm, the small seed particles have a median particle size of 0.
It is 1-100 μm, preferably 1-100 μm, more preferably 10-100 μm. The weight ratio of W ₁ / W ₂ when the total weight of the largest seed particles of the seed particles is W ₁ and the total weight of the small seed particle group is W ₂ ,
1 <W ₁ / W ₂ <100, preferably 1 <W ₁ / W ₂ <
75, more preferably 1 <W ₁ / W ₂ <50.

Second stage: The water-soluble ethylenically unsaturated monomer aqueous solution for absorbing the water-soluble ethylenically unsaturated monomer aqueous solution to the seed particles is the water-soluble ethylenically unsaturated monomer aqueous solution used for preparing the seed particles. And may be the same or different. For example, when the monomer types are completely different, for example, an aqueous solution of sodium acrylate is used as a seed particle forming monomer, and an aqueous solution of acrylamide is used as a monomer to be absorbed by the seed particles. Furthermore, even if the monomer species is the same, there may be mentioned, for example, a case where the aqueous sodium acrylate solution is used for preparing the seed particles and the monomer aqueous solution is used for absorbing the seed particles with different degrees of neutralization or the monomer concentration in the aqueous solution. The amount of the aqueous monomer solution used for absorbing the seed particles is 5 to 300% by weight, preferably 10 to 150% by weight, based on the aqueous monomer solution used for preparing the seed particles.

A crosslinking agent and a water-soluble radical polymerization initiator are not necessarily required in the aqueous monomer solution at the time of absorbing the seed particles, and may be appropriately determined according to required product quality. In particular, even if the water-soluble radical polymerization initiator is not newly added to the seed particles in the aqueous monomer solution at the time of liquid absorption, the water-soluble radical polymerization initiator is absorbed in the slurry for preparing the seed particles and is easily polymerized when the temperature is set to a predetermined temperature. What is important in the additional addition of the water-soluble ethylenically unsaturated monomer is that the mixture is subjected to a temperature lower than the decomposition temperature of the radical polymerization initiator after the liquid absorption treatment.

In the operation for uniformly absorbing the monomer, for example, a method in which a surfactant or the like is present in the system to cause demulsification to absorb the monomer as described in JP-A-9-12613, Also, JP-A-3-227
As described in JP-A-301, there is a method in which the temperature in the system is set to a temperature lower than the temperature at which the dispersant precipitates and the monomer is absorbed.

When a surfactant is present in the system, the HLB of the surfactant used is 7 or more and is higher than the HLB of the dispersant used in the first stage.
These are mixtures between the nonionic or anionic surfactants of the LB. HLB of this surfactant
Is preferably one or more higher than the HLB of the dispersant used for the polymerization.

Specific examples of the nonionic surfactant include polyoxyethylene alkyl phenyl ether, polyoxyethylene alkyl ether, polyoxyethylene fatty acid ester, polyoxyethylene alkyl phenyl ether, and polyoxyethylene polyoxypropylene block. Examples include polymers, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene alkylamine ethers, fatty acid diethanolamine, sucrose fatty acid esters, polyglycerin fatty acid esters, and the like.

Examples of the anionic surfactants include fatty acid salts such as sodium oleate, castor oil potash, semi-hardened tallow soda, sodium lauryl sulfate, higher alcohol sulfate ester soda salt, lauryl alcohol sulfate ester / triethanolamine salt, Higher alcohol sulfates such as ammonium lauryl alcohol sulfate, alkyl benzene sulfonate and alkyl naphthalene sulfonate such as sodium dodecylbenzene sulfonate and sodium alkyl naphthalene sulfonate, condensate of naphthalene sulfonate formalin, sodium dialkyl sulfosuccinate Dialkyl sulfosuccinates such as
(Di) alkyl phosphate salts, polyoxyethylene alkyl sulfate soda, polyoxyethylene alkyl phenyl sulfate soda salt, etc., polyoxyethylene sulfate salts, polyoxyethylene polyoxypropylene glycol ether sulfate ammonium salts, polyoxyethylene distyrenated phenyl Ether sulfate ammonium salts and other high-molecular special anions are exemplified.

Among the above surfactants, one or a mixture of two or more selected from polyoxyethylene alkylphenyl ether, polyoxyethylene polyoxypropylene block polymer, and alkylbenzene sulfonate is preferred. Further, the HLB is preferably 9 or more. The amount of these surfactants used varies depending on the type of monomer used, other operating conditions, and the like, but is generally 0.05 to 10% by weight, preferably 0.1 to 10% by weight, based on the dried water-absorbent resin after repolymerization. 5% by weight.

These surfactants are added to the slurry before the second-stage monomer aqueous solution is added, or simultaneously with the second-stage monomer aqueous solution or after the second-stage monomer aqueous solution is added. Can also be performed. The addition can be carried out by using the surfactant as it is, as an aqueous solution, or by dissolving or mixing in the second-stage monomer, or by dissolving or mixing in a hydrophobic solvent. In the present invention, it is preferable to add a surfactant by dissolving or mixing it in a second-stage monomer aqueous solution. The temperature at this time can be set to any temperature, but it is desirable that the temperature after liquid absorption be equal to or lower than the polymerization start temperature as described above. On the other hand, the temperature at which the monomer is absorbed by setting the temperature in the system to be equal to or lower than the temperature at which the dispersant is precipitated is preferably equal to or lower than the temperature at which the dispersant is precipitated. is there. The temperature of the second-stage monomer aqueous solution varies depending on the presence or absence, type, etc. of the radical polymerization initiator in the aqueous solution, but is generally preferably around room temperature, that is, 15 to 40 ° C.

By absorbing the second-stage monomer aqueous solution, the whole system becomes a viscous slurry state by a liquid-absorbing gel. At this time, it is important to absorb the second-stage monomer as uniformly as possible. is there. The gel that has absorbed the second-stage monomer generally has the property of being slightly tacky and easy to aggregate. Therefore, if the liquid absorption is non-uniform, or if there is uneven liquid absorption, the liquid absorption gel partially agglomerates, locally adheres or stays as a large lump, This has a great effect on not only the particle size distribution but also the stability of continuous production.

An important operating factor in the liquid absorbing operation is to increase the number of rotations at the time of liquid absorption, and to achieve uniform liquid absorption efficiently, a high number of rotations is preferable. By the above-mentioned operation, a gel is obtained in which the second-stage monomer aqueous solution is uniformly absorbed, and the gel has a property of easily aggregating as described above. Therefore, when the polymerization of the gel slurry is started by elevating the temperature or the like under stirring, a grape gel in which substantially spherical gel particles containing water adhere to each other is obtained.

When the monomer aqueous solution after the second-stage monomer aqueous solution is further absorbed, basically the same method and operation as in the method of absorbing the second-stage monomer aqueous solution are performed. The slurry after polymerization is subjected to direct dehydration or azeotropic dehydration with a hydrophobic solvent according to a known method, and then, if necessary, surface cross-linking or surface modification as surface treatment. For example, as a crosslinking agent used for surface crosslinking, a carboxyl group and / or
Alternatively, any crosslinking agent having two or more functional groups capable of reacting with a carboxylate group can be used. For example, a polyglycidyl ether compound, a haloepoxy compound, an aldehyde compound, an isocyanate compound and the like can be used, and a polyglycidyl ether compound is generally used. The use amount of these crosslinking agents is not particularly limited, but is usually used in the range of 0.005% to 5% by weight based on the polymer. The compounds used for surface modification include:
Examples include silane compounds represented by the following general formula.

[0044]

Embedded image X (R) _m Si (Y) _3-m

(Where X represents a functional group capable of reacting with a carboxyl group and / or a carboxylate group in the superabsorbent polymer, R represents a hydrocarbon group, and Y represents a hydrolyzable group. m is 0, 1 or 2.) Specific examples of the silane compound represented by the above formula include γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, β- (3 , 4-
Epoxycyclohexyl) ethyltrimethoxysilane,
γ- (2-aminoethyl) aminopropyltrimethoxysilane, γ- (2-aminoethyl) aminopropyltriethoxysilane, N-phenyl-γ-aminopropyltrimethoxysilane, γ-mercaptopropyltrimethoxysilane, γ- Mercaptopropylmethyldimethoxysilane, γ-chloropropyltrimethoxysilane, γ-chloropropylmethyldimethoxysilane, octadecyldimethyl [3- (trimethoxysilyl) propyl] ammonium chloride and the like. Further, a polyvalent metal compound can be used as a surface modifier. Examples of the polyvalent metal compound include compounds of divalent metals such as Mg, Ca, Ba, and Zn, and compounds of trivalent metals such as Al and Fe. Specific examples include magnesium sulfate, aluminum sulfate, ferric chloride, and chloride. Examples thereof include calcium, magnesium chloride, aluminum chloride, polyaluminum chloride, iron nitrate, calcium nitrate, aluminum nitrate, and aluminum hydroxide. The use amount of the silane compound and the polyvalent metal compound is not particularly limited, but is usually 0.001 to 10% by weight based on the polymer. Next, the product is dried and passed through a sieve.

Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples and the like as long as the gist of the present invention is not exceeded. The particle size distribution / average particle size, binding strength, water absorption capacity, and water absorption rate of the water-absorbent resin described above are measured by the following methods. (1) Average particle size / particle size distribution The average particle size and the particle size distribution of the seed particles were measured using a laser diffraction particle size analyzer (SYMPTEC HEL, manufactured by Nippon Laser Co., Ltd.).
OS model). The granulated granules were passed through an ASTM standard sieve from above with 8 mesh (2830 μm), 12 mesh (1680 μm), 20 mesh (840 μm), 4
0 mesh (420 μm), 60 mesh (250 μm)
m), 80 mesh (177 μm), 100 mesh (149 μm), 150 mesh (105 μm), 20
0 mesh (74 μm), 325 mesh, and a tray were combined in this order. About 50 g of the water-absorbing resin was put into the uppermost sieve, and the mixture was shaken with a low-tap type automatic sieve shaker for 1 minute. The weight of the water-absorbent resin remaining on each sieve is weighed, and the ratio of the total amount to 100% is calculated on a mass basis.

(2) Bonding strength 8 cm at the center of a SUS plate having a size of 10 cm × 10 cm
0.5 g of a water-absorbent resin of a 20-mesh pass to 80-mesh-on fraction was uniformly sprayed within × 8 cm, and a SUS plate as above was further placed thereon, and the water-absorbent resin was sandwiched therebetween.
A pressure of kgf / cm ² is applied for 10 minutes. The water-absorbent resin after decompression was recovered, and the water-absorbent resin was collected with the low tap type automatic sieve shaker.
Shake for minutes. The amount that has passed through an 80 mesh sieve is measured, and this ratio is calculated as a percentage by weight (a smaller value indicates less crushing of the granulated particles, ie, a higher binding strength).

(3) Water-absorbing ability About 0.5 g of the water-absorbing resin was precisely weighed and placed in a 250-mesh nylon bag (20 cm × 10 cm size).
Soak in 1 cc of artificial urine for 1 hour. Thereafter, the nylon bag was pulled up, drained for 15 minutes, weighed, blank-corrected, and the water absorption capacity was calculated according to the following equation.

[0049]

(Equation 2)

The composition of the artificial urine is as follows. Artificial urine composition Urea 1.94% Sodium chloride 0.80% Calcium chloride 0.06% Magnesium sulfate 0.11% Pure water 97.09%

(4) Water absorption rate 0.9% by weight of physiological saline 50 was placed in a 100 ml beaker.
ml and add a rotor (length 3) with a magnetic stirrer.
3 mm), and after adding 2 g of the water absorbent resin,
The time until the rotor stopped was measured and defined as the water absorption rate. Reference Example: Preparation of Seed Particles (First Stage) 325.4 g of cyclohexane was put into a 2 liter four-necked round bottom flask equipped with a stirrer, a reflux condenser, a thermometer, and a nitrogen gas inlet tube, and HLB was added thereto. =
1.627 g of 4.7 sorbitan monostearate was added and dissolved, and the internal temperature was adjusted to 20 ° C. in a nitrogen gas atmosphere. Separately, add acrylic acid to a 1 liter conical beaker.
Take 4 g, add 77.4 g of water while cooling from the outside,
Further, 413.2 g of 25% sodium hydroxide was added to neutralize 80% of the carboxyl groups. In this case, the monomer concentration with respect to water corresponds to 40% by weight as the monomer concentration after neutralization. Next, 0.1162 g of N, N'-methylenebisacrylamide and 0.193 g of potassium persulfate were added thereto.
4 g, and 0.0464 g of sodium hypophosphite hydrate as a water-soluble chain transfer agent were added and dissolved, and the temperature was adjusted to 20 ° C.

To the contents of the 2-liter four-necked round-bottomed flask, 361.5 g of the contents of the 1-liter conical beaker was added and suspended by stirring. The stirring was performed at 200 rpm using a full zone blade. Then
When the temperature was raised to about 55 ° C. at the same rotation speed, polymerization started and peaked at about 77 ° C. Thereafter, the temperature was kept at 70 ° C. for 30 minutes.

Next, the agitation was increased to 300 rpm,
The remaining monomer aqueous solution (36.2 g) in the 1-liter conical beaker was taken, added dropwise to the contents of the 2-liter 4-necked round-bottom flask, and kept at 70 ° C. for 30 minutes. After completion of the first stage, the rotation speed was set to 500 rpm and the mixture was further heated, and the produced polymer was dehydrated to 7% by azeotropic distillation with cyclohexane. As a result, true spherical single particles having two frequency distributions were obtained. Maximum median diameter 143μm, smaller median diameter 31
μm, weight ratio of beads-like water-absorbent resin single particles W ₁ / W ₂ =
It was 10.

Reference Example 2 The amount of the monomer in the first polymerization was 361.5 g, and the number of rotations was 1
At 90 rpm, the amount of monomer used for the second polymerization is 12.
The operation and method were all the same as in Reference Example 1 except that the stirring speed was changed to 0 g and the rotation speed at 390 rpm. As a result, true spherical single particles having two frequency distributions were obtained. The maximum median diameter was 145 μm, the smaller median diameter was 15 μm, the minimum median diameter was 15 μm, and the weight ratio W ₁ / W _{2 of} single beads of the water-absorbent resin was 30.

Reference Example 3 The amount of the monomer in the first polymerization was 300.0 g and the number of rotations was 16
0 rpm, the amount of the monomer used for the second polymerization was 100.
Except for changing to 0 g and the number of rotations of 400 rpm, all operations and methods were the same as those in Reference Example 1. As a result, a distribution of true spherical single particles having two frequency distributions was obtained. Maximum median diameter 195 μm, smaller median diameter 15 μm,
The weight ratio of the bead-shaped water-absorbent resin single particles was W ₁ / W ₂ = 3.

REFERENCE EXAMPLE 4 Except that the amount of monomer in the first polymerization was changed to 330.0 g, the same operation and procedure as in Reference Example 1 were carried out, and the temperature was maintained at 70 ° C. after the second polymerization. Next, stir 450
rpm, and take 30.0 g of the remaining monomer aqueous solution of the 1 liter conical beaker, and drop it into the contents of the 2 liter four-necked round bottom flask,
C. for 30 minutes. The obtained polymer was further dehydrated to 7% by azeotropic distillation with cyclohexane at a rotation speed of 500 rpm. As a result, a distribution of single spherical particles having three frequency distributions was obtained. Maximum median diameter 143 μm, smaller median diameters 13 and 27 μm, weight ratio W ₁ / W of single beads of water-absorbent resin
₂ = 5.

Reference Example 5 The same procedure as in Reference Example 1 was carried out except that the stirring speed in the first polymerization was changed to 420 rpm, the monomer amount was changed to 16.0 g, the second stirring speed was changed to 160 rpm, and the monomer amount was changed to 361.5 g. Operation and method were performed. As a result, a distribution of true spherical single particles having two frequency distributions was obtained. Maximum median diameter 2
10 μm, a smaller median diameter of 12 μm, and a weight ratio W ₁ / W ₂ = 23 of single beads of the water-absorbent resin.

REFERENCE EXAMPLE 6 325.4 g of cyclohexane was placed in a two-liter four-necked round-bottomed flask equipped with a stirrer, a reflux condenser, a thermometer, and a nitrogen gas inlet tube, and sorbitan mono of HLB = 4.7 was added thereto. 1.627 g of stearate was added and dissolved, and the internal temperature was adjusted to 20 ° C. in a nitrogen gas atmosphere. Separately, 232.4 g of acrylic acid was placed in a 1-liter conical beaker, 77.4 g of water was added while cooling from the outside, and 413.2 g of 25% caustic soda was further added to neutralize 80% of the carboxyl groups. In this case, the monomer concentration with respect to water corresponds to 40% by weight as the monomer concentration after neutralization. Then, N, N'-methylenebisacrylamide was added to the mixture.
1162 g, potassium persulfate 0.1934 g, and sodium hypophosphite hydrate 0.04 as a water-soluble chain transfer agent
64 g was added and dissolved, and the temperature was adjusted to 20 ° C.

After heating the content of the 2-liter four-necked round-bottom flask to 73 ° C., 90.4 g of the content of the 1-liter conical beaker was first added dropwise over about 30 minutes, followed by stirring. Suspension polymerization was performed. The stirring was performed at 400 rpm using a full zone blade. The contents of the 2 liter four-necked round bottom flask were heated to 73 ° C.
361.5 of the contents of a 1-liter conical beaker
g was dropped over about 1 hour and stirred at 200 rpm.
Reverse phase suspension polymerization was performed. Thereafter, the temperature was kept at 70 ° C. for 30 minutes. The mixture was further heated at a rotation speed of 500 rpm, and azeotroped with cyclohexane to 7% of the produced polymer.
Dehydration was performed until. As a result, true spherical single particles having two frequency distributions were obtained. The maximum median diameter was 143 μm, the smaller median diameter was 15 μm, and the weight ratio W ₁ / W _{2 of} the beads-like water-absorbent resin single particles was 4.

Reference Example 7 Amount of Monomer in First Polymerization by the Same Operation and Method as Reference Example 1
Once at 361.5 g and stirring speed of 200 rpm
After polymerization of only the eye, the same operation and procedure as in Reference Example 1
To make the amount of monomer in the first polymerization 36.2 g,
With the number of turns being 370 rpm, only the first polymerization is polymerized.
Was. After that, half of each reactor was stirred by stirrer and cooled by reflux
2 liter capacity equipped with a vessel, thermometer and nitrogen gas inlet tube
In a four-necked round-bottomed flask, and set the rotation speed to 500 rpm.
And further heated and formed by azeotropy with cyclohexane.
The polymer thus obtained was dehydrated to 7%. The result
As a result, a distribution of true spherical single particles with two frequency distributions was obtained.
Was. Maximum median diameter 143μm, smaller media
Ann diameter 25 μm, weight ratio W of bead-shaped water-absorbent resin single particles
₁/ W_Two= 10.

Reference Example 8 The amount of monomer in the first polymerization was obtained by the same operation and procedure as in Reference Example 1.
Once with 361.5 g and stirring speed of 220 rpm
After polymerization of only the eye, the same operation and procedure as in Reference Example 1
To make the amount of monomer in the first polymerization 36.2 g,
With the number of turns being 265 rpm, only the first polymerization is polymerized.
Was. After that, half of each reactor was stirred by stirrer and cooled by reflux
2 liter capacity equipped with a vessel, thermometer and nitrogen gas inlet tube
In a four-necked round-bottomed flask, and set the rotation speed to 500 rpm.
And further heated and formed by azeotropy with cyclohexane.
The polymer thus obtained was dehydrated to 7%. The result
As a result, a distribution of true spherical single particles with two frequency distributions was obtained.
Was. Maximum median diameter 140μm, smaller media
Weight ratio W of bead-shaped water-absorbent resin single particles with an diameter of 75 μm
₁/ W_Two= 10.

REFERENCE EXAMPLE 9 Only the first polymerization was carried out by the same operation and procedure as in Reference Example 1. As a result, true spherical single particles were obtained. The frequency distribution of the bead-shaped water-absorbing resin single particles is one, and the median diameter is 143.
μm. Example 1 (Second Stage) After the first stage polymerization under the same operation and conditions as in Reference Example 1, the temperature of the polymerization content was directly set to 50 ° C. Next, 160.0 g of the remaining monomer aqueous solution of the 1 liter conical beaker was taken, and a polyoxyethylene polypropylene block polymer 0.76 g was added thereto as a surfactant.
g (HLB = 10.1, manufactured by Toho Chemical Co., Ltd., Pepole B)
184) and dodecylbenzenesulfonic acid triethanolamine salt (anionic surfactant, Daiichi Kogyo Seiyaku Co., Ltd.)
Addition of 3.47 g of Onegen T)
The polymerization content was added at a stirring rotation speed of 300 rpm. After the addition, the temperature of the content became about 40 ° C., and the system became a slightly viscous slurry. After the addition, the liquid was almost completely absorbed into the first-stage hydrous polymer gel particles. Then, when the temperature was increased to 500 rpm with stirring,
Polymerization started at about 550 ° C., and peaked at about 69 ° C. Thereafter, the mixture was kept at 70 ° C. for about 15 minutes, further heated at the same rotation speed, and azeotroped with cyclohexane to dehydrate the produced polymer to 7%. After dehydration,
When the stirring was stopped, the polymer particles settled at the bottom of the flask and could be easily separated by decantation. The separated polymer was heated to 90 ° C. to remove attached cyclohexane and some water. The obtained dried polymer was a powdery granule in which smooth single particles were bound in a grape-like manner.

Example 2 After the first stage polymerization under the same operation and conditions as in Reference Example 2, the temperature of the polymerization content was directly set at 50 ° C. Next, a powdery granulated product in which single particles that were smooth under the same operation and conditions as in Example 1 were bound in a grape-like manner was obtained. Example 3 After the first stage polymerization under the same operation and conditions as in Reference Example 3, the temperature of the polymerization content was directly set to 50 ° C. Next, a powdery granulated product in which single particles that were smooth under the same operation and conditions as in Example 1 were bound in a grape-like manner was obtained.

Example 4 After the first stage polymerization under the same operation and conditions as in Reference Example 4, the temperature of the polymerization content was directly set at 50 ° C. Next, a powdery granulated product in which single particles that were smooth under the same operation and conditions as in Example 1 were bound in a grape-like manner was obtained. Example 5 After the first stage polymerization under the same operation and conditions as in Reference Example 5, the temperature of the polymerization content was directly set to 50 ° C. Next, a powdery granulated product in which single particles that were smooth under the same operation and conditions as in Example 1 were bound in a grape-like manner was obtained.

Example 6 After the first stage polymerization under the same operation and conditions as in Reference Example 6, the temperature of the polymerization content was directly set at 50 ° C. Next, a powdery granulated product in which single particles that were smooth under the same operation and conditions as in Example 1 were bound in a grape-like manner was obtained. Example 7 After the first stage polymerization under the same operation and conditions as in Reference Example 7, the temperature of the polymerization content was directly set to 50 ° C. Next, a powdery granulated product in which single particles that were smooth under the same operation and conditions as in Example 1 were bound in a grape-like manner was obtained.

Example 8 After the first stage polymerization under the same operation and conditions as in Reference Example 1, the temperature of the polymerization content was cooled to 20 ° C. to precipitate sorbitan monostearate as a dispersant. Next, the remaining monomer aqueous solution 17 of the 1 liter conical beaker was used.
6.3 g was weighed out, added at 20 ° C. to the polymerization content at a rotation speed of 50 rpm, and kept at the same temperature for 30 minutes to absorb liquid. Then, when the temperature was increased to 150 rpm with stirring, polymerization was started at about 55 ° C., and further heating was performed at the same rotation speed, and azeotropic distillation with cyclohexane was performed to dehydrate the produced polymer to 7%. The subsequent operation was the same as in Example 1, and a powdery granulated product was obtained by the same method.

Comparative Example 1 After the first stage of polymerization under the same operation and conditions as in Reference Example 7, the temperature of the polymerization content was directly set at 50 ° C. A powdery granulated product was obtained by the same operation and in the same manner as in Example 1. Comparative Example 2 After the first stage polymerization under the same operation and conditions as in Reference Example 8, the temperature of the polymerization content was directly set to 50 ° C. The subsequent operation was the same as in Example 1, and a powdery granulated product was obtained by the same method.

Example 9 The powdery granules obtained in Example 1 were adjusted to a water content of 20% by weight, and γ-glycidoxypropyltrimethoxysilane was used as a surface treatment agent in an amount of 1000 to the dried water-absorbent resin.
ppm was added to perform surface crosslinking, and after drying, a powdery granulated product was obtained. Comparative Examples 3 and 4 Competitor products A and B, which are granulated products obtained based on the reverse phase suspension polymerization method, were used as Comparative Examples 3 and 4, respectively. The polymers of Example 9 and Comparative Examples 3 and 4 were evaluated for water absorption capacity and water absorption rate. Table 2 shows the results.

As shown in Examples 1 to 8 and Comparative Examples 1 and 2 (Table 1), in the present invention, the monomer aqueous solution is absorbed by using seed particles having two or more frequency distributions. As a result, a granulated product can be obtained very efficiently, the binding strength is large, the particle size distribution is narrow, and the fine particle component (# 80 pass product) is small. In addition, as shown in Example 9 and Comparative Examples 3 and 4, it can be seen that the water-absorbent resin granules obtained by the present invention have a particularly high water absorption rate.

[0070]

[Table 1]

[0071]

[Table 2]

[0072]

According to the present invention, a granulated product having the above-mentioned characteristics is obtained by absorbing a monomer aqueous solution and polymerizing the same with water-containing water-absorbent resin single particles having two or more frequency distributions. Can be manufactured with extremely simple operation and at low cost. And such a thing is suitable for a disposable diaper, a sanitary napkin, and a soil water retention agent, for example.

[Brief description of the drawings]

FIG. 1 shows an example of a particle size distribution of a granular water absorbent resin having three frequency distributions.

Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI // C08F 20/06 C08F 20/06

Claims (8)

[Claims] 1. A bead-like water-absorbent resin having two or more frequency distributions and having a ratio of a median particle diameter smaller than the maximum median particle diameter in a range of 1/3000 to 1 / 1.5. The particles are bound and the average particle diameter is 20
A granular water-absorbent resin having a particle size of 0 to 10000 μm. 2. The granular water-absorbing material according to claim 1, wherein the single-particle water-absorbent resin particles have a maximum median particle size of 30 to 300 μm and a smaller median particle size of 0.1 to 100 μm. Resin. 3. The single bead-shaped water-absorbent resin particles,
The total weight of the frequency distribution beaded water-absorbent resin single particles having the maximum median particle diameter is W ₁ , and the total weight of the frequency distribution beaded water-absorbent resin single particles having a smaller median particle diameter is W ₂ when, granular water absorbent resin according to claim 1 or 2 W ₁ and W ₂ using beads water-absorbing resin single particle satisfying the following ratio. [Equation 1] 1 ≦ W ₁ / W ₂ ≦ 100 4. A water-in-oil type reverse phase using a water-soluble radical polymerization initiator in a hydrophobic organic solvent in the presence of a dispersant and optionally a cross-linking agent in the presence of a dispersant. The suspension is subjected to a polymerization reaction to form single particles of a water-containing bead-like water-absorbent resin, and then a water-soluble ethylenically unsaturated monomer is further added to a polymerization slurry containing the water-containing bead-like water-absorbent resin, and the aqueous solution is added. After absorbing the water-containing beaded water-absorbent resin single particles produced, the operation of performing an additional suspension polymerization reaction is repeated once or more, and in order to obtain a granular water-absorbent resin, two or more frequency distributions are obtained. A method for producing a granular water-absorbent resin, comprising using single beads of a water-absorbent resin having the following formula: 5. A method for polymerizing a water-soluble ethylenically unsaturated monomer aqueous solution after absorbing the aqueous solution of the water-soluble ethylenically unsaturated monomer into the single bead-shaped water-absorbent resin particles, and adding the water-soluble ethylenically unsaturated monomer to be absorbed to the bead-shaped water-absorbent resin monolayer. 5. The method according to claim 4, wherein the polymerization reaction system is added in a proportion of 5 to 300% by weight of the particles. 6. A nonionic surfactant having an HLB of 7 or more is added to the water-soluble ethylenically unsaturated monomer aqueous solution to be absorbed when the additional water-soluble ethylenically unsaturated monomer aqueous solution is absorbed into the single-particle bead-shaped water-absorbent resin particles. 5. The method according to claim 4, wherein an additional anionic surfactant or a mixture thereof is added to carry out additional suspension polymerization. 7. In the additional suspension polymerization, the temperature of the reaction mixture after the additional water-soluble ethylenically unsaturated monomer aqueous solution is subjected to the liquid-absorbing treatment of the bead-shaped water-absorbent resin single particles is controlled by the water-soluble radical polymerization initiator. 5. The method according to claim 4, wherein the addition is performed so as to be lower than the decomposition temperature. 8. The method according to claim 4, wherein the water-soluble ethylenically unsaturated monomer is at least one selected from acrylic acid or a salt thereof, methacrylic acid or a salt thereof, acrylamide and methacrylamide.