JPH06279511A - Production of highly water-absorbing polymer - Google Patents

Abstract

PURPOSE:To produce the polymer having a large particle diameter and a narrow particle diameter distribution. CONSTITUTION:A process for performing reversed-phase suspension polymerization of the following monomers in a water-in-oil dispersion comprising an aqueous solution containing an acrylic acid monomer based on acrylic acid and its alkali metal salt, a polymerization initiator and optionally a crosslinking agent and an organic solvent containing a dispersant, which process comprises forming the water-in-oil dispersion with the aid of a dispersant in an amount necessary for dispersing the monomers, heating the dispersion, and further adding a dispersant in an amount necessary for dispersing the polymer to be formed just before starting the polymerization.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing a super absorbent polymer. The superabsorbent polymer obtained by the production method of the present invention is spherical, has a large particle size, has an extremely narrow particle size distribution, and has a very good fluidity as a free-flowing material. Therefore, it can be suitably used for various water absorbing materials such as sanitary materials, industrial materials, and agriculture and horticulture.

[0002]

2. Description of the Related Art A reverse phase suspension polymerization method for producing a superabsorbent polymer by polymerizing an acrylic acid type monomer in an organic solvent which is substantially incompatible with an aqueous solution containing the monomer in the presence of a dispersant. Is widely known in general, and various polymerization methods characterized by the type of dispersant used, the type of organic solvent and other polymerization conditions have been reported. For example, as a dispersant, a nonionic surfactant having an HLB of 3 to 6 (Japanese Patent Publication No.
30710, JP 61-157513 A), HLB 6-9 nonionic surfactants (JP 57 57).
No. 167302), nonionic surfactants of HLB 8 to 12 (Japanese Patent Publication No. 60-25045), HLB2.
To 16 polyglycerin fatty acid ester (JP-A-62-1
172006), sucrose fatty acid ester (JP-A-61-87702, JP-A-61-43606), cellulose ester or cellulose ether (JP-B-1-17482, JP-A-57-1582).
No. 10, JP-A-58-32607), synthetic polymers such as methacrylic acid-methyl methacrylate-diethylhexyl acrylate copolymer (JP-A-57-94011).
Gazette), a copolymer of α-olefin and α, β-unsaturated polycarboxylic acid anhydride (JP-A-62-53310, JP-A-62-62807, JP-A-62-95).
No. 308), a polymer containing a lipophilic carboxyl group (Japanese Patent Publication No. 63-36321 and Japanese Patent Publication No. 63-322322).
No.), a lipophilic basic nitrogen-containing polymer (Japanese Patent Publication No. 63-
No. 36323), styrenes-dialkylaminoalkyl (meth) acrylates copolymers (JP-A-61-161).
No. 40309) is known.

The polymers obtained by these various methods can be obtained as relatively large spherical polymers having a primary particle size of several μ to several hundred μ, but generally they are so-called fine powder polymers of several tens μ or less. Generation is unavoidable, and the particle size distribution is quite wide. Therefore, when handling this powder, it is necessary to take measures against dust. Further, generally, when the amount of the finely divided polymer increases, a so-called "mamako" phenomenon is likely to occur when absorbing the liquid to be absorbed, and the absorption becomes very difficult. Further, when used as a paper diaper or a sanitary napkin, for example, when mixing and holding a water-absorbent polymer in crushed pulp, a non-woven fabric or the like, there is a problem that fine powder easily falls off.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the above problems and provides a method for producing a super absorbent polymer having a large particle size and a narrow particle size distribution.

[0005]

[Means for Solving the Problems]

[Summary of the Invention] The inventors of the present invention have conducted diligent research on the reversed-phase suspension polymerization method for the purpose of solving the above-mentioned problems, and as a result, the dispersion droplet diameter depends on the dispersant concentration and the suspension time, and The smaller the amount, the larger the droplet size was obtained, and the phenomenon that the dispersed droplets were fragmented and atomized with the passage of suspension time was captured. It was found that a superabsorbent polymer having a large particle size can be obtained by forming dispersed droplets, raising the temperature, and adding an amount of a dispersant necessary for dispersing the polymer immediately before the initiation of the polymerization, and arrived at the present invention. did. That is, the method for producing a superabsorbent polymer of the present invention contains an acrylic acid-based monomer containing acrylic acid and its alkali metal salt as a main component, a polymerization initiator, and an aqueous solution and a dispersant containing a crosslinking agent if desired. When reverse-phase suspension polymerization of the monomer in a water-in-oil dispersion comprising an organic solvent, a water-in-oil dispersion is prepared by using a dispersant in an amount necessary to disperse the aqueous monomer solution. It is characterized in that it is formed, heated, and an additional amount of a dispersant necessary for dispersing the produced polymer is added immediately before the initiation of polymerization.

[Detailed Description of the Invention] <Acrylic Acid-Based Monomer> The acrylic acid-based monomer used in the polymerization reaction of the present invention contains acrylic acid and its alkali metal salt as main components. Here, “to be a main component” means that this acrylic acid and its alkali metal salt account for 95 mol% or more. A preferred specific example of such an acrylic acid-based monomer is 20% or more, preferably 50% or more, of all carboxyl groups in the preparation.
Is mainly composed of an alkali metal salt neutralized. In this case, if the degree of neutralization is less than 20%, the water absorption capacity will be small and the strength of the resulting water-absorbent gel will be extremely small. The upper limit of the degree of neutralization is about 90%.

As an alkali agent for neutralizing acrylic acid into an alkali metal salt, an alkali metal hydroxide or bicarbonate can be used, but an alkali metal hydroxide is preferably used. . Specific examples of such alkali metal hydroxides include sodium hydroxide, potassium hydroxide and lithium hydroxide. Sodium hydroxide is the most preferable in terms of industrial availability, price and safety. The higher the concentration of the acrylic acid-based monomer used in the present invention in the aqueous solution, the better. Specifically, the monomer concentration after neutralization is 20% by weight or more,
Preferably 30% by weight or more, more preferably 35% by weight
That is all. The higher the monomer concentration, the greater the yield per unit batch, and the easier the dehydration operation after polymerization, which is economically advantageous.

In the present invention, in addition to the above main monomer,
For example, the following monomers having a copolymerizable double bond can be used together. Methacrylic acid, itaconic acid, maleic acid, fumaric acid, 2-acrylamido-2-methylpropanesulfonic acid, 2-acryloylethanesulfonic acid, 2-acryloylpropanesulfonic acid and its salts, itaconic acid, maleic acid, fumaric acid, etc. Alkyl or alkoxyalkyl esters of dicarboxylic acids, (meth) acrylamide, vinyl sulfonic acid and its salts, methyl acrylate, ethyl acrylate, etc., hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, polyethylene glycol mono (Meth) acrylate,
N-methylol (meth) acrylamide, glycidyl (meth) acrylate and the like. The “salt” of the above-mentioned acid compound refers to an alkali metal salt, an alkaline earth metal salt, an ammonium salt and the like. The amount of these monomers used is 5 mol% or less based on all acrylic acid-based monomers.

Further, in the present invention, in addition to the above acrylic acid type monomers, the following monomers can be used in combination as a crosslinking agent. Double bonds in the molecule such as polyethylene glycol di (meth) acrylate, polypropylene glycol di (meth) acrylate, glycerin tri (meth) acrylate, trimethylolpropane triacrylate, N, N'-methylenebis (meth) acrylamide, etc. Acrylic acid monomers having two or more and having copolymerizability with acrylic acid monomers, such as ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, di- or polyglycidyl ethers of aliphatic polyhydric alcohols, etc. A compound having two or more functional groups, such as a carboxyl group, which can react during the polymerization or during the drying after the polymerization. The amount of these cross-linking agents used is usually 5 mol% or less with respect to the acrylic acid-based monomer. In the examples of the monomer and the crosslinking agent compound, “(meth) acrylic” is used.
The terms “(meth) acrylate” and “(meth) acrylate” are generic terms for “acrylic” and “methacrylic” and “acrylate” and “methacrylate”.

<Dispersant> The dispersant used in the present invention includes nonionic surfactants such as sorbitan fatty acid ester, sucrose fatty acid ester, polyglycerin fatty acid ester, and the like, cellulose derivatives such as cellulose ether and cellulose ester. And the like, synthetic polymers such as copolymers of α-olefin and maleic anhydride or their derivatives, and the like. Preferably, nonionic surfactants of HLB 2 to 7, more preferably sorbitan fatty acid esters of HLB 2 to 7 are used, and those which are solid at room temperature, particularly sorbitan monostearate are preferable.

The amount of the dispersant to be added to the organic solvent in advance is preferably as low as possible as long as it is at least the minimum amount necessary to disperse the aqueous solution containing the acrylic acid type monomer. This minimum required amount is generally estimated by the critical micelle concentration (CMC) measured under static conditions. C
Since MC varies depending on conditions such as the type of aqueous solution, dispersant and organic solvent, the amount of dispersant used cannot be specified uniformly. As an example, when sorbitan monostearate is used as the dispersant and cyclohexane is used as the organic solvent, the CMC obtained by measuring the interfacial tension is 0.01% by weight based on the organic solvent.
Therefore, the amount of the dispersant used may be 0.01 to 0.2% by weight, preferably 0.02 to 0.1% by weight. In this case, the amount of dispersant used is 0.2% by weight.
If it exceeds the above range, a large particle size cannot be obtained because the dispersion proceeds excessively.

The sum of the amount of the dispersant added in advance and the amount of the dispersant added additionally, that is, the total amount of the dispersant used is not less than the minimum amount necessary to disperse the polymer obtained by the polymerization. Good. In general, the viscosity of acrylic acid-based monomers increases with polymerization, and when the polymerization rate is low to medium, the droplets tend to fuse together, so the use necessary to disperse the aqueous solution containing the monomers If only the amount is used, the adhesion of the polymer to the wall of the polymerization tank and the agglomeration of the polymer particles become remarkable, and sometimes the polymer may be agglomerated during the polymerization. The required minimum amount of the total amount used also differs depending on the conditions such as the type of aqueous solution, the dispersant and the organic solvent, and therefore cannot be unconditionally specified. For example, when sorbitan monostearate is used as the dispersant and cyclohexane is used as the organic solvent, the amount of the polymer that can be adhered to the wall of the polymerization tank becomes the permissible limit, and the minimum amount used is 0% of the organic solvent. 0.3 to 10% by weight, preferably 0.35 to 3
It is exemplified as wt%. In this case, if the total amount of the dispersant is 10% by weight or more, the dispersing effect is saturated and it is uneconomical.

<Organic solvent> The organic solvent used in the present invention may be hydrophobic and inert, and examples thereof include aliphatic hydrocarbons such as n-pentane, n-hexane, n-heptane and n-octane. Alicyclic hydrocarbons such as cyclohexane, cyclooctane, methylcyclohexane, etc., aromatic hydrocarbons such as benzene, toluene, xylene, etc., halogenated hydrocarbons such as carbon tetrachloride, trichloroethane, dichloroethane, benzene chloride, etc. Can be mentioned. Of these, cyclohexane is particularly preferable.

The amount of the organic solvent used is 0.5 in weight ratio with respect to the aqueous acrylic acid monomer solution in order to make the polymerization reaction system a water-in-oil type and to remove the heat of the polymerization reaction. It is desirable to set it to -5. In the present invention, a dispersant is additionally added. This additionally added dispersant is also desirably added as an organic solvent solution. The amount of the organic solvent used for suspending or dissolving the additionally added dispersant is usually 1 to 20% by weight based on the total amount of the organic solvent used.

<Polymerization Initiator> In the present invention, a radical generator is used as a polymerization initiator. As such a polymerization initiator, any conventionally known one can be used as long as it is soluble in an acrylic acid-based monomer aqueous solution, and examples thereof include persulfates such as potassium persulfate and ammonium persulfate, and 2,2′-azobis. (2-amidinopropane) dihydrochloride, 2,2'-azobis (N, N'-dimethyleneisobutylamidine) dihydrochloride, 4,4'-azobis (4-
Examples thereof include azo compounds such as (cyanovaleric acid), hydrogen peroxide, tertiary butyl hydroperoxide, hydroperoxides such as cumene hydroperoxide, and the like. The amount of these polymerization initiators used is generally 0.001 to 0.1 mol% with respect to the acrylic acid-based monomer.

In the present invention, a redox initiator composed of a combination of an oxidizing agent such as persulfate or hydroperoxide and a reducing agent may be used as the radical generator. Examples of the reducing agent include hypophosphorous acid and salts thereof, phosphorous acid and salts thereof, sulfites, pyrosulfites, L-ascorbates, amines and the like. Here, the “salt” is one or more alkali metals, alkaline earth metals, ammonium salts and the like. The amount of these reducing agents used is usually 0.001 to 20 in molar ratio with respect to the oxidizing agent. Even if the amount of reducing agent used exceeds the above range, the effect is saturated.

<Polymerization Method> An example of a specific embodiment relating to the polymerization method of the present invention is shown below. Acrylic acid is neutralized in advance to form an aqueous solution of an alkali metal salt, and a crosslinking agent or the like is added and dissolved therein to obtain an aqueous solution of an acrylic acid-based monomer. A polymerization initiator is added and dissolved therein, and an inert gas such as nitrogen is introduced to degas. Separately, a predetermined amount of a dispersant is added to an organic solvent, and if necessary, slightly warmed to suspend, and then an inert gas such as nitrogen is introduced to degas. An aqueous solution containing the above acrylic acid-based monomer is added to this at a low temperature at which polymerization does not occur, for example, 10 to 30 ° C., and heating is started. After reaching a predetermined temperature described later, a dispersant previously suspended or dissolved in an organic solvent is additionally added to polymerize the monomer. The polymer after polymerization is
It is composed of swollen bead-like particles, which can be easily separated from the dispersion medium directly or after azeotropic dehydration and then by a decantation or evaporation operation. Then, the swollen polymer is dried at a temperature of 120 ° C. or lower, for example, to obtain a powdery polymer.

<Time of Additional Addition of Dispersant> The present invention is characterized by the additional addition of the dispersant. The timing of the additional addition of the dispersant is before the initiation of the polymerization and at a time as late as possible so that the dispersant can be sufficiently diffused in the system by the initiation of the polymerization. Here, “polymerization initiation” means that the polymerization rate is 1
It means the time when it becomes%. Specifically, when the polymerization initiation temperature at which the polymerization rate is 1% is 55 ° C., the system temperature is 35 to 54.
When it is ℃, it can be exemplified. If the addition time is early,
The particle size obtained is small and does not meet the purpose of the present invention. On the other hand, when the addition time is late, polymerization occurs in the state where the dispersant is insufficient, so that fusion of the produced polymer particles and adhesion of the polymer to the wall of the polymerization tank or the stirring blade become remarkable.

[0019]

EXAMPLES The following examples serve to explain the present invention more specifically. The following measurements were performed on the superabsorbent polymers obtained in the examples. <Water absorption ratio> About 0.5 g of superabsorbent polymer was precisely weighed and 2
50 mesh nylon bag (10cm x 10cm size)
And soak in 500 cc of 0.9% saline for 1 hour. Then, the nylon bag was pulled up, drained for 15 minutes, then weighed, and the water absorption capacity was calculated according to the following formula.

<Particle size distribution> Average particle size 100 g of the superabsorbent polymer was sieved for 10 minutes by a low tap type shaker using a JIS standard sieve, and the weight average particle size was calculated. Dispersion The logarithmic standard deviation σ was calculated according to the following formula as the dispersion of the particle size. σ = 0.5 × (lnD _15.9 −lnD _84.1 ) D _15.9 : Particle size at residual rate 15.9% D _84.1 : Particle size at residual rate 84.1%

Example 1 A 5-liter round-bottomed flask with a capacity of 1 liter equipped with a stirrer, a reflux condenser, a thermometer, a nitrogen gas inlet tube, and a dropping funnel was charged with 324 g of cyclohexane, and sorbitan monostearate (as a dispersant was used. HLB 4.7) 0.17 g
After (0.05% by weight of organic solvent) was added and dissolved, nitrogen gas was blown in to expel dissolved oxygen. Separately, use acrylic acid 1 in a conical beaker with a capacity of 500 ml.
To a mixed solution of 24 g and 18 g of water, 193 g of a 25% aqueous solution of sodium hydroxide was added under ice cooling to neutralize 70% of carboxyl groups. Then, N, N'as a crosslinking agent
-Methylenebisacrylamide (0.006 g) and potassium persulfate (0.087 g) as a polymerization initiator were added and dissolved, and then nitrogen gas was blown thereinto to expel dissolved oxygen. Add the volume of 5 to the contents of the five-necked round bottom flask described above.
The contents of a conical beaker (00 ml) were added, the mixture was stirred and dispersed at room temperature, and the temperature inside the flask was raised by an oil bath while bubbling nitrogen gas. When the internal temperature reached 40 ° C, sorbitan monostearate 1.50 was previously prepared.
g (0.45% by weight to organic solvent) and cyclohexane 10
The solution prepared from g was supplied from a dropping funnel, and the temperature was raised. When the internal temperature reached 55 ° C, heat generation due to the polymerization was observed, and reached a value of 76 ° C after a few minutes. Then, the internal temperature was maintained at 65 to 70 ° C., and stirring was continued. The stirring was performed at 200 rpm. One hour after the initiation of the polymerization, the oil bath was continuously heated to 100 ° C. and azeotropic dehydration was performed for 4 hours. When the stirring was stopped, the wet polymer particles settled to the bottom of the round bottom flask, and could be easily separated from the cyclohexane phase by decantation. 1 for the obtained wet polymer
It was dried under reduced pressure at 00 ° C. to obtain a free-flowing superabsorbent polymer.

COMPARATIVE EXAMPLE 1 (Effect of Dispersant Addition Mode) In Example 1, 334 g of cyclohexane and 1.67 g of sorbitan monostearate were initially added ( compared to organic solvent of 0.
5% by weight) was used, and the same operation was performed except that the additional addition of the dispersant was not carried out to obtain a superabsorbent polymer.

Example 2 (Influence of Addition Time of Additional Dispersant) A superabsorbent polymer was obtained in the same manner as in Example 1 except that the temperature at the time of additional addition of the dispersant was 53 ° C.

Comparative Example 2 (Influence of Addition Time of Dispersant) In Example 1, the same operation was performed except that the temperature at the time of adding the dispersant was 56 ° C. A water absorbing polymer was obtained.

Example 3 (Effect of Initial Addition Amount of Dispersant) In Example 1, the amount of sorbitan monostearate initially added was 0.067 g (0.02 g of organic solvent).
%), And the additional amount of sorbitan monostearate used is 1.60 g (0.48% by weight of organic solvent).
The same operation was performed except that the above was followed to obtain a superabsorbent polymer.

Example 4 (Influence of Initial Addition Amount of Dispersant) In Example 1, the amount of sorbitan monostearate initially added was 0.33 g (0.1% by weight of organic solvent), and additional addition was carried out. The same operation was performed except that the amount of sorbitan monostearate used was 1.34 g (0.4% by weight with respect to the organic solvent) to obtain a superabsorbent polymer.

Comparative Example 3 (Effect of Initial Addition Amount of Dispersant) In Example 1, the amount of sorbitan monostearate initially added was 0.017 g (0.000 of organic solvent).
(5% by weight), but the same operation was performed, but a good suspension state was not obtained, and therefore polymerization could not be performed.

Comparative Example 4 (Effect of Initial Addition Amount of Dispersant) In Example 1, the amount of sorbitan monostearate initially added was 1.00 g (0.3% by weight of organic solvent), and additional addition was carried out. The same operation was performed except that the amount of sorbitan monostearate used was 0.67 g (0.2 wt% with respect to the organic solvent) to obtain a superabsorbent polymer.

Example 5 (Influence of the amount of total dispersant used) In Example 1, the amount of sorbitan monostearate additionally added was 6.67 g (2% by weight of organic solvent),
The same operation was performed except that the amount of cyclohexane used was 20 g to obtain a superabsorbent polymer.

Example 6 (Influence of Total Dispersant Amount Used) The same as Example 1 except that the amount of sorbitan monostearate additionally added was 1.00 g (0.3% by weight of organic solvent). The operation was performed to obtain a super absorbent polymer.

Comparative Example 5 (Influence of Total Dispersant Amount Used) The same as Example 1 except that the amount of sorbitan monostearate additionally added was 0.77 g (0.23% by weight of organic solvent). When the operation was performed, a super absorbent polymer in which some particles were fused was obtained.

Example 7 A 5-liter round-bottomed flask having a capacity of 1 liter, equipped with a stirrer, a reflux condenser, a thermometer, a nitrogen gas introduction tube, and a dropping funnel, was charged with 310 g of cyclohexane, and was used as a dispersant, DIACALNER 30 (Mitsubishi Corporation). 0.12 g (manufactured by Kasei Kogyo Co., Ltd., 0.038% by weight of organic solvent) and 0.12 g of sorbitan monostearate (0.038%, by weight of organic solvent) were added and dissolved, and then nitrogen gas was blown in, Dissolved dissolved oxygen. Separately, in a conical beaker with a capacity of 500 ml, to a mixed solution of 114 g of acrylic acid and 38 g of water, under ice cooling,
203 g of a 25% aqueous solution of sodium hydroxide was added to neutralize 80% of the carboxyl groups. Then, N, N'-methylenebisacrylamide 0.080 as a crosslinking agent
g, 0.109 g of sodium hypophosphite monohydrate and 0.119 g of potassium persulfate as a polymerization initiator were added and dissolved, and then nitrogen gas was blown thereinto to expel dissolved oxygen. Add the volume of 5 to the contents of the five-necked round bottom flask described above.
The contents of a conical beaker (00 ml) were added, the mixture was stirred and dispersed at room temperature, and the temperature inside the flask was raised by an oil bath while bubbling nitrogen gas. When the internal temperature reached 45 ° C, 1.08 g (0.34% by weight of organic solvent) of Diacarna 30 and 1.08 g (0.34% by weight of organic solvent) of sorbitan monostearate and 10 g of cyclohexane were used. The prepared solution was supplied from a dropping funnel and the temperature was raised. When the internal temperature reached 53 ° C, heat generation due to the polymerization was recognized, and reached to 75 ° C after a few minutes.
Then, the internal temperature was maintained at 65 to 70 ° C., and stirring was continued. The stirring was performed at 220 rpm. After 1 hour from the start of polymerization, the oil bath was heated to 100 ° C. for 4 hours
It was azeotropically dehydrated over a period of minutes. When the stirring was stopped, the wet polymer particles settled to the bottom of the round bottom flask, and could be easily separated from the cyclohexane phase by decantation. The obtained wet polymer was dried under reduced pressure at 100 ° C. to obtain a free-flowing super absorbent polymer.

Example 8 In Example 7, 1.20 g (0.38% by weight of organic solvent) of Diacalner 30 alone was used as a dispersant to be initially added, and sorbitan monostearate was added as a dispersant to be additionally added. 1.20 g (compared with organic solvent 0.3
(8% by weight) The same operation was performed except that the superabsorbent polymer was obtained.

Comparative Example 6 (Effect of Dispersant Addition Mode) In Example 7, 320 g of cyclohexane, 1.20 g of Diakarna 30 (0.38% by weight of organic solvent), and 1.20 g of sorbitan monostearate ( compared to the initial amount) A high water-absorbent polymer was obtained by performing the same operation except that the organic solvent was used (0.38% by weight) and the dispersant was not additionally added.

Table 1 shows the analysis results of the super absorbent polymers obtained in the above Examples and Comparative Examples.

[Table 1]

[0036]

According to the method of the present invention, the particle size is large,
Moreover, a super absorbent polymer having a narrow particle size distribution can be produced. Such superabsorbent polymer contains almost no fine powder, and therefore can be suitably used especially for sanitary products such as disposable diapers and sanitary napkins.

Front page continuation (72) Inventor Osamu Nakajima 1 Toho-cho, Yokkaichi-shi, Mie Mitsubishi Oil Chemicals Co., Ltd. Yokkaichi Research Institute (72) Inventor Katsuhiko Itakura 1-place, Toho-cho, Yokkaichi-shi, Mie Mitsubishi Petrochemical Incorporated company Yokkaichi Research Institute

Claims (1)

[Claims] 1. Water-in-oil droplets comprising an acrylic acid-based monomer containing acrylic acid and its alkali metal salt as main components, a polymerization initiator, and optionally an aqueous solution containing a crosslinking agent and an organic solvent containing a dispersant. Upon reverse-phase suspension polymerization of the monomer in a mold dispersion, a water-in-oil type dispersion is first formed by using a dispersant in an amount necessary to disperse the aqueous monomer solution, and the temperature is raised to perform polymerization. A method for producing a superabsorbent polymer, which comprises adding an amount of a dispersant necessary for dispersing a produced polymer just before starting.