JP7623650B2 - Anionic water-soluble polymer dispersion and method for producing same - Google Patents

Description

The present invention relates to an anionic water-soluble polymer dispersion prepared by dispersion polymerization in a salt solution in the presence of a polymeric dispersant, and to a method for producing the same.

Regarding ionic water-soluble polymer dispersions produced by dispersion polymerization in the presence of an ionic polymer dispersant in an aqueous salt solution, various techniques have been reported up to now, such as in Patent Document 1.
Among these, anionic water-soluble polymer dispersions are widely used as flocculants for wastewater treatment and sludge dewatering, as retention aids and drainage aids for papermaking, as soil solidification agents for civil engineering, etc. For these applications, there is a particular demand for high molecular weight anionic water-soluble polymers.
However, compared with cationic water-soluble polymer dispersions, it is difficult to produce stable dispersions by precipitating polymers in salt water and producing them by dispersion polymerization in general high molecular weight anionic water-soluble polymer dispersions. In the case of cationic monomers, it is easy to synthesize polymers that are insoluble in salt water by polymerizing monomers that have hydrophobic groups such as benzyl groups or long-chain alkyl groups, and the molecular weight is also sufficient for use as a flocculant. On the other hand, in the case of anionic monomers, it is difficult to introduce hydrophobic groups into the molecules, so a method of copolymerizing hydrophobic monomers is considered, but copolymerizing these hydrophobic monomers often does not result in high molecular weights.
Accordingly, various reports have been made on techniques for producing anionic water-soluble polymer dispersions.
For example, Patent Documents 2 and 3 disclose anionic water-soluble polymer dispersions which are polymerized in a salt solution in the presence of an anionized water-soluble polymer stabilizer.
Patent Document 4 discloses the provision of a stable dispersion of a high molecular weight sulfonic acid group-containing anionic water-soluble polymer.
Patent Document 5 discloses a method for producing an anionic water-soluble polymer dispersion using a low molecular weight sulfonic acid group-containing polymer as a polymer dispersant.
However, when these anionic water-soluble polymer dispersions are produced, ammonium sulfate is generally used as an inorganic salt, and the total nitrogen content derived from the ammonium sulfate salt constituting the dispersion is about 75,000 ppm (relative to the water-soluble polymer dispersion product). The total nitrogen content has a great influence on agricultural crops. Plants absorb nitrogen in the form of ammonium nitrogen or nitrate nitrogen and use it for their growth. However, it is known that an excess of nitrogen has a negative effect, and a reduction in the content is required. In addition, since ammonium compounds are designated as harmful substances following the amendment of the Water Pollution Prevention Act in 2014, anionic water-soluble polymer dispersions that can reduce the amount of ammonium sulfate, particularly those with high molecular weight and high versatility, are desired.
Furthermore, when an anionic water-soluble polymer dispersion product is stored for a long period of time, solidification or separation of the product can be a problem, and there is a demand for an improvement in the separation stability of the product.

Japanese Unexamined Patent Publication No. 62-20511 Special Publication No. 2001-508473 Special Publication No. 2003-508598 JP 2002-302521 A JP 2004-231822 A

The present invention aims to provide an anionic water-soluble polymer dispersion that is obtained by dispersion polymerization in a salt solution in the presence of a polymeric dispersant, and has excellent product stability without using ammonium sulfate, and a method for producing the same.

As a result of intensive research to solve the above problems, it has been found that an anionic water-soluble polymer dispersion can be obtained by dispersing and polymerizing a monomer or a monomer mixture that contains a specific monomer as an essential component in an aqueous salt solution containing magnesium sulfate in the presence of a polymer dispersant whose constituent units are 2-acrylamido-2-methylpropanesulfonic acid or a salt thereof.

The dispersion of the present invention is an anionic water-soluble polymer dispersion and its manufacturing method that does not use ammonium sulfate, and can suppress the release of salt-derived nitrogen into the environment, and can obtain an anionic water-soluble polymer dispersion with high molecular weight and excellent long-term stability.

一般式（１）
Ｒ_１は水素、メチル基又はカルボキシメチル基、ＱはＳＯ_３、Ｃ_６Ｈ_４ＳＯ_３、ＣＯＮＨＣ（ＣＨ_３）_２ＣＨ_２ＳＯ_３、Ｃ_６Ｈ_４ＣＯＯあるいはＣＯＯ、Ｒ_２は水素又はＣＯＯＹ_２、Ｙ_１あるいはＹ_２は水素又は陽イオンをそれぞれ表わす。 The water-soluble polymer dispersion in the present invention is an anionic water-soluble polymer dispersion obtained by dispersing and polymerizing, in an aqueous salt solution, an aqueous monomer or monomer mixture solution essentially containing a monomer represented by the following general formula (1) in the presence of a polymer dispersant soluble in the aqueous salt solution under stirring:

General formula (1)
_R1 represents hydrogen, a methyl group or _a carboxymethyl group, Q represents _SO3 , _C6H4SO3 , _CONHC ( _CH3 ) _2CH2SO3 , _C6H4COO or _COO , _R2 represents hydrogen or _COOY2 , and _Y1 and _{Y2 represent} hydrogen or _a cation.

The anionic monomer represented by the general formula (1) used in producing the anionic water-soluble polymer dispersion of the present invention is in the range of 1 to 100 mol %. In order to obtain the effect of the anionic group and obtain a high molecular weight, 5 to 80 mol % is preferable, and 5 to 60 mol % is more preferable. Examples of anionic monomers include vinyl sulfonic acid, vinylbenzene sulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, methacrylic acid, acrylic acid, itaconic acid, maleic acid, phthalic acid, p-carboxystyrene acid, and the like, and their salts. Two or more of these may be used in combination. During production, the anionic monomer can be neutralized to any degree of neutralization with an alkaline substance such as an alkali metal hydroxide, such as sodium hydroxide, or an amine, and polymerized.

An anionic monomer represented by general formula (1) and a nonionic monomer may be used. Examples of nonionic monomers used when copolymerizing an anionic monomer represented by general formula (1) and a nonionic monomer include (meth)acrylamide, N,N-dimethylacrylamide, acrylonitrile, 2-hydroxyethyl (meth)acrylate, diacetone acrylamide, N-vinylpyrrolidone, N-vinylformamide, N-vinylacetamide, and acryloylmorpholine. Two or more of these may be used in combination.

When producing the water-soluble polymer dispersion of the present invention, a cationic monomer can be used within a range that does not inhibit the effect of the anionic group.

The cationic monomer is a quaternary product of dimethylaminoethyl (meth)acrylate or dimethylaminopropyl acrylamide with a halide of a lower alkyl group such as methyl chloride or ethyl chloride. For example, (meth)acryloyloxyethyl trimethylammonium chloride, (meth)acryloyloxyethyl dimethylbenzylammonium chloride, (meth)acryloylaminopropyl trimethylammonium chloride, (meth)acryloylaminopropyl dimethylbenzylammonium chloride, (meth)acryloyloxy-2-hydroxypropyl trimethylammonium chloride, etc. Two or more of these may be combined. The cationic monomer is preferably less than 10 mol%, more preferably less than 5 mol%. In addition, the anionic monomer represented by the general formula (1), the cationic monomer, and the nonionic monomer may be copolymerized.

The dispersion polymerization in salt water in the present invention can be carried out by a conventional method disclosed in JP-A-62-20511, JP-A-10-212320, JP-A-2004-231822, etc. In the present invention, a monomer or monomer mixture aqueous solution essentially containing a monomer represented by general formula (1) is dispersion polymerized in a salt aqueous solution in the presence of a polymer dispersant having 2-acrylamido-2-methylpropanesulfonic acid or a salt thereof as a constituent unit, and using magnesium sulfate as an inorganic salt.
When dispersion polymerization is performed, the addition of a polymerization retarding substance in an amount of 0.5 to 5% by mass based on the total monomers has the effect of suppressing thickening, and the polymerizable composition can be produced by adding the polymerization retarding substance appropriately. Examples of the polymerization retarding substance include itaconic acid, maleic acid, and phthalic acid.

In the present invention, the polymer dispersant used in dispersion polymerization in salt water contains 2-acrylamido-2-methylpropanesulfonic acid or its salt as a constituent unit. Poly 2-acrylamido-2-methylpropanesulfonic acid or its salt may be used, but copolymers of 2-acrylamido-2-methylpropanesulfonic acid or its salt with nonionic monomers can also be used. Examples of nonionic monomers include acrylamide, N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone, N,N-dimethylacrylamide, acrylonitrile, diacetoneacrylamide, 2-hydroxyethyl (meth)acrylate, etc., but copolymers with acrylamide are preferred. In addition, other cationic monomers, such as (meth)acryloyloxyethyltrimethylammonium chloride, may also be contained in the polymer dispersant composition.

The molecular weight of the polymer dispersant is undesirably high, as it increases the viscosity of the dispersion. Therefore, it is between 5,000 and 2 million, and preferably between 50,000 and 1 million. The addition rate of the polymer dispersant is between 1 and 20% by mass, and preferably between 3 and 20% by mass, relative to the monomer. It is preferable that the amount is less than 5% by mass relative to the dispersion. This is because a content of 5% by mass or more is economically disadvantageous and may inhibit the function of the polymer.

Ammonium sulfate is generally used as an inorganic salt for dispersion polymerization in salt water. In the present invention, magnesium sulfate is used as an inorganic salt for polymerization. By using magnesium sulfate as an essential ingredient, it is possible to suppress significant thickening and produce a water-soluble polymer dispersion with high molecular weight and excellent separation stability. An amount of magnesium sulfate is added that is 5% by mass or more relative to the total amount of the water-soluble polymer dispersion and reaches a saturated concentration in the salt water solution. As for magnesium sulfate, a hydrate such as the heptahydrate can be preferably used.
As other salts, sodium chloride, potassium chloride, calcium chloride, lithium chloride, sodium sulfate, sodium carbonate, calcium carbonate, etc. may be contained, but magnesium sulfate must be 50 mass% or more of the total amount of inorganic salts during polymerization, preferably 70 mass% or more. The monomers are dissolved in these inorganic salt aqueous solutions, and a polymer dispersant is further added to adjust the pH to 2 to 5, followed by nitrogen replacement and polymerization is initiated by a polymerization initiator.

The present invention has discovered that by using a polymer dispersant that contains magnesium sulfate as an inorganic salt during polymerization and 2-acrylamido-2-methylpropanesulfonic acid or its salt as a structural unit, thickening can be suppressed and a high molecular weight anionic water-soluble polymer dispersion can be stably produced.

The use of a polymer dispersant containing magnesium sulfate as an inorganic salt during polymerization and containing 2-acrylamido-2-methylpropanesulfonic acid or a salt thereof as a constituent unit has the effect of suppressing thickening. There are many theoretically unknown aspects that have not yet been explained, but the phenomenon can be estimated as follows.
That is, as the polymerization proceeds in the salt solution, the concentration of the polymer produced exceeds the solubility and the polymer particles begin to precipitate, but before that, the viscosity of the polymer itself (polymerization system) also increases due to the dissolved polymer, resulting in a state in which the dissolved polymer and precipitated particles coexist. After this, the proportion of precipitated polymer increases, the viscosity of the polymer gradually decreases, and the phase changes to a dispersed state. In this coexisting state, the main role of the dispersant during the phase transition period is thought to be to improve the slip between the precipitated particles and the gel-like dissolved polymer, and to smoothly transition the phase change from the thickened state before the phase change to the dispersed state. In the early stage of polymerization, the dispersant's constituent unit 2-acrylamido-2-methylpropanesulfonic acid or its salt participates in the phase separation of the relatively low molecular weight of the polymer produced, smoothly progressing the phase change, and in the latter half of the polymerization, the action of magnesium sulfate, which has a strong salting-out power, promotes the salting-out effect with the relatively high molecular weight of the polymer produced, resulting in a stable production of a polymer dispersion. These are believed to be due to a synergistic effect of the action of 2-acrylamido-2-methylpropanesulfonic acid or a salt thereof to smooth the phase change and the action of magnesium sulfate to promote the salting-out effect.

Ammonium sulfate is generally used as an inorganic salt for water-soluble polymer dispersions such as those of the present invention, but general-purpose products contain a total nitrogen content derived from ammonium sulfate salt of about 75,000 ppm (for water-soluble polymer dispersion products). By eliminating the nitrogen derived from ammonium sulfate salt, the total nitrogen content can be reduced to about 20,000 ppm (for water-soluble polymer dispersion products). The total nitrogen content has a significant impact on agricultural crops. Plants absorb nitrogen in the form of ammonium nitrogen or nitrate nitrogen and use it for their growth, but it is known that excess nitrogen has a negative effect, and there is a demand to reduce the content. In addition, ammonium compounds have been designated as harmful substances following the amendment of the Water Pollution Prevention Act in 2014, raising concerns about their impact on the environment, and the water-soluble polymer dispersion of the present invention is extremely useful because it does not use ammonium ammonium sulfate.

The polymerization concentration is 2% to 25% by mass as the monomer concentration. This is because a low monomer concentration is less practical, and a high monomer concentration is more economically advantageous in terms of transportation costs, but if the monomer concentration exceeds 25% by mass, the viscosity increases significantly during production, making it difficult to obtain a dispersion. It is preferably 5% to 20% by mass. The monomer may be supplied all at once at the start of polymerization, or in appropriate portions.

Polymerization conditions are usually determined appropriately depending on the monomers and copolymerization mol% used, and the temperature is in the range of 0 to 100°C. A radical polymerization initiator is used to start the polymerization. These initiators may be either oil-soluble or water-soluble, and polymerization can be carried out with any of the azo, peroxide, and redox initiators. Examples of oil-soluble azo initiators include 2,2'-azobisisobutyronitrile, 1,1'-azobis(cyclohexanecarbonitrile), 2,2'-azobis(2-methylbutyronitrile), 2,2'-azobis(2-methylpropionate), and 2,2'-azobis(4-methoxy-2,4-dimethyl)valeronitrile, which are dissolved in a water-miscible solvent and added.

Examples of water-soluble azo initiators include 2,2'-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis(2-amidinopropane)dihydrochloride, 4,4'-azobis(4-cyanovaleric acid), etc. Examples of redox initiators include combinations of ammonium persulfate with sodium sulfite, sodium hydrogen sulfite, trimethylamine, tetramethylethylenediamine, etc. Examples of peroxides include ammonium or potassium persulfate, hydrogen peroxide, benzoyl peroxide, lauroyl peroxide, octanoyl peroxide, succinic peroxide, t-butylperoxy-2-ethylhexanoate, etc.

The addition rate of the azo-based initiator or peroxide-based initiator is 50 to 500 ppm, preferably 70 to 200 ppm, per monomer at the start of polymerization. However, since a single addition results in a low polymerization rate, it is preferable to add it in several portions.
When copolymerizing with a redox initiator, if polymerization is started under conditions of 40°C or higher, it is difficult to control the polymerization, and a rapid temperature rise or clumping of the polymerization liquid occurs, making it difficult to obtain a stable dispersion with a high degree of polymerization; therefore, 15 to 35°C is preferred. The addition rate of this initiator is 5 to 100 ppm, preferably 10 to 100 ppm, per monomer at the start of polymerization. However, since a single addition results in a low polymerization rate, it is preferred to add it several times. The number of additions is 2 to 5 times, preferably 2 to 3 times.

In addition, it is effective to use sodium formate, isopropyl alcohol, etc. in an amount of 0.1 to 5 mass% relative to the monomer to adjust the degree of polymerization.

After the polymerization reaction is complete, salt can be added to adjust the stabilization of the product. The salt to be added may be magnesium sulfate, but sodium chloride, potassium chloride, calcium chloride, lithium chloride, sodium sulfate, sodium thiosulfate, sodium carbonate, calcium carbonate, etc. can also be used. They may also be used in combination. In particular, one or more salts selected from magnesium sulfate, sodium sulfate, and sodium thiosulfate are preferred. These salts are added in an amount of 1 to 10% by mass based on the total amount of the water-soluble polymer dispersion.

The anionic water-soluble polymer dispersion of the present invention can be used as an agent to be added when flocculating and treating raw sludge, excess sludge, flocculated sludge, digested sludge or a mixture of these sludges from urban sewage, sewage, general industrial wastewater, or when flocculating and dehydrating them using a decanter, belt press, filter press, screw press dehydrator or the like; an oil separating agent used in an oil refining process or an oil separation process and treatment of industrial wastewater containing oil; a papermaking agent such as a drainage improver, a retention improver, or a valuable material recovery agent in white water used in a papermaking process; and in order to function as such in these applications, the 0.4 mass% salt solution viscosity, which is an indicator of the molecular weight of the water-soluble polymer dispersion of the present invention, i.e., the aqueous solution viscosity in a 4 mass% sodium chloride aqueous solution (adjusted to pH 8.5) at 0.4 mass% of the water-soluble polymer constituting the dispersion measured at 25°C, is in the range of 5 to 200 mPa·s. In particular, in the use of flocculants in wastewater treatment and sludge dewatering, papermaking chemicals as retention and drainage improvers, civil engineering chemicals as soil solidification agents, etc., high molecular weight anionic water-soluble polymers are often desired, in which case 10 to 60 mol% of the anionic monomer is generally used, and the aqueous solution viscosity of 0.4 mass% in a 4 mass% sodium chloride aqueous solution is preferably 20 to 200 mPa·s, more preferably 50 to 200 mPa·s, and even more preferably 80 to 200 mPa·s. The viscosity of the 0.4 mass% salt aqueous solution is a value measured with a B-type viscometer (Toki Sangyo TVB-10M, etc.) using a No. 2 rotor at 60 rpm.
The intrinsic viscosity of the water-soluble polymer dispersion at 0.5% by weight measured at 25° C. in a 4% by weight aqueous sodium chloride solution (adjusted to pH 8.5) is preferably in the range of 10 to 30 dl/g.

When storing an anionic water-soluble polymer dispersion product for a long period of time, solidification or separation of the product may be a problem. As an indicator of the separation stability of the product, it can be measured by centrifugal separation under certain conditions using a centrifuge. In this measurement test, the anionic water-soluble polymer dispersion of the present invention has superior separation stability and can be stored for a long period of time compared to a general-purpose product that uses ammonium sulfate as the polymerization salt.

The anionic water-soluble polymer dispersion of the present invention and its manufacturing method are specifically described below, but the present invention is not limited to the following examples.

(Example 1) 91.4 g of demineralized water and 71.8 g of magnesium sulfate heptahydrate were added to a 0.5 L separable flask equipped with an anchor blade stirrer, a cooling tube, and a nitrogen inlet tube, and dissolved uniformly under stirring. Next, 15.9 g of 80% by mass acrylic acid, 2.4 g of 48% by mass sodium hydroxide, 58.6 g of 50% by mass acrylamide, 0.4 g of sodium formate, 0.4 g of itaconic acid, and 15.4 g of 15% by mass sodium poly 2-acrylamide-2-methylpropanesulfonate were added to form a uniform solution, and the temperature was stabilized by immersing in a 33°C water bath. Next, under a nitrogen atmosphere, 4% by mass of an aqueous solution of ammonium persulfate and 4% by mass of an aqueous solution of sodium hydrogensulfite were added as polymerization initiators at 55 ppm per monomer, and polymerization was carried out at 30°C for 10 hours under stirring. After that, 300 ppm of the above polymerization initiator was added, and polymerization was carried out for 5 hours. After the reaction was completed, 5.0 g of sodium sulfate, 38.8 g of magnesium sulfate heptahydrate, and 0.9 g of sodium thiosulfate were added to the obtained dispersion, and the mixture was stirred until no residue was left behind to obtain a water-soluble polymer dispersion. The molar composition ratio of this water-soluble polymer was acrylamide/acrylic acid = 70/30 mol%. The viscosity of this dispersion was 390 mPa·s, and the viscosity when the water-soluble polymer dispersion was dissolved in a 4 mass% sodium chloride aqueous solution to a concentration of 0.4 mass% was 120 mPa·s. The theoretical total nitrogen content of this water-soluble polymer dispersion was 19650 ppm (relative to the dispersion). In addition, a centrifugation operation was performed to conduct a separation stability test. This is shown in Table 1 as Example 1.

(Example 2) 65.0 g of demineralized water and 67.7 g of magnesium sulfate heptahydrate were added to a 0.5 L separable flask equipped with an anchor blade stirrer, a cooling tube, and a nitrogen inlet tube, and the mixture was dissolved uniformly under stirring. Next, 21.6 g of 80% by mass acrylic acid, 3.2 g of 48% by mass sodium hydroxide, 79.5 g of 50% by mass acrylamide, 0.4 g of sodium formate, 0.6 g of itaconic acid, and 20.9 g of 15% by mass sodium poly 2-acrylamide-2-methylpropanesulfonate were added to form a uniform solution, and the temperature was stabilized by immersing the solution in a 33°C water bath. Next, under a nitrogen atmosphere, 4% by mass of an aqueous solution of ammonium persulfate and 4% by mass of an aqueous solution of sodium hydrogensulfite were added as polymerization initiators at 55 ppm per monomer, and the mixture was polymerized at 30°C for 10 hours under stirring. After that, 300 ppm of the above polymerization initiator was added, and the mixture was polymerized for 5 hours. After the reaction was completed, 4.7 g of sodium sulfate, 36.5 g of magnesium sulfate heptahydrate, and 0.9 g of sodium thiosulfate were added to the obtained dispersion, and the mixture was stirred until no residue was left behind to obtain a water-soluble polymer dispersion. The molar composition ratio of this water-soluble polymer was acrylamide/acrylic acid = 70/30 mol%. The viscosity of this dispersion was 1050 mPa·s, and the viscosity when the water-soluble polymer dispersion was dissolved in a 4 mass% sodium chloride aqueous solution to a concentration of 0.4 mass% was 131 mPa·s. The theoretical total nitrogen content of this water-soluble polymer dispersion was 26670 ppm (relative to the dispersion). In addition, a centrifugation operation was performed to conduct a separation stability test. This is shown in Table 1 as Example 2.

(Example 3) 110.0 g of demineralized water and 75.2 g of magnesium sulfate heptahydrate were added to a 0.5 L separable flask equipped with an anchor blade stirrer, a cooling tube, and a nitrogen inlet tube, and the mixture was dissolved uniformly under stirring. Next, 7.6 g of 80% by mass acrylic acid, 0.8 g of 48% by mass sodium hydroxide, 47.9 g of 50% by mass acrylamide, 0.4 g of sodium formate, 0.3 g of itaconic acid, and 12.0 g of 15% by mass sodium poly 2-acrylamide-2-methylpropanesulfonate were added to form a uniform solution, and the temperature was stabilized by immersing the solution in a 33°C water bath. Next, under a nitrogen atmosphere, 4% by mass of an aqueous solution of ammonium persulfate and 4% by mass of an aqueous solution of sodium hydrogensulfite were added as polymerization initiators at 55 ppm per monomer, and the mixture was polymerized at 30°C for 10 hours under stirring. After that, 300 ppm of the above polymerization initiator was added, and the mixture was polymerized for 5 hours. After the reaction was completed, 5.3 g of sodium sulfate, 40.6 g of magnesium sulfate heptahydrate, and 0.9 g of sodium thiosulfate were added to the obtained dispersion, and the mixture was stirred until no residue was left behind to obtain a water-soluble polymer dispersion. The molar composition ratio of this water-soluble polymer was acrylamide/acrylic acid = 80/20 mol%. The viscosity of this dispersion was 360 mPa·s, and the viscosity when the water-soluble polymer dispersion was dissolved in a 4 mass% sodium chloride aqueous solution to a concentration of 0.4 mass% was 31.6 mPa·s. This is shown in Table 1 as Example 4. The theoretical total nitrogen content of this water-soluble polymer dispersion was 16050 ppm (relative to the dispersion). In addition, a centrifugation operation was performed to conduct a separation stability test. This is shown in Table 1 as Example 3.

(Example 4) 90.2 g of demineralized water and 71.8 g of magnesium sulfate heptahydrate were added to a 0.5 L separable flask equipped with an anchor blade stirrer, a cooling tube, and a nitrogen inlet tube, and the mixture was dissolved uniformly under stirring. Next, 13.3 g of 80% by mass acrylic acid, 2.0 g of 48% by mass sodium hydroxide, 62.8 g of 50% by mass acrylamide, 0.4 g of sodium formate, 0.4 g of itaconic acid, and 15.4 g of 15% by mass sodium poly 2-acrylamide-2-methylpropanesulfonate were added to form a uniform solution, and the temperature was stabilized by immersing the solution in a 33°C water bath. Next, under a nitrogen atmosphere, 4% by mass of an aqueous solution of ammonium persulfate and 4% by mass of an aqueous solution of sodium hydrogensulfite were added as polymerization initiators at 55 ppm per monomer, and the mixture was polymerized at 30°C for 10 hours under stirring. After that, 300 ppm of the above polymerization initiator was added, and the mixture was polymerized for 5 hours. After the reaction was completed, 5.0 g of sodium sulfate, 38.8 g of magnesium sulfate heptahydrate, and 0.9 g of sodium thiosulfate were added to the obtained dispersion, and the mixture was stirred until no residue was left behind to obtain a water-soluble polymer dispersion. The molar composition ratio of this water-soluble polymer was acrylamide/acrylic acid = 75/25 mol%. The viscosity of this dispersion was 460 mPa·s, and the viscosity when the water-soluble polymer dispersion was dissolved in a 4 mass% sodium chloride aqueous solution to a concentration of 0.4 mass% was 86.3 mPa·s. The theoretical total nitrogen content of this water-soluble polymer dispersion was 21040 ppm (relative to the dispersion). In addition, a centrifugation operation was performed to conduct a separation stability test. This is shown in Table 1 as Example 4.

(Example 5) 114.3 g of demineralized water and 75.2 g of magnesium sulfate heptahydrate were added to a 0.5 L separable flask equipped with an anchor blade stirrer, a cooling tube, and a nitrogen inlet tube, and the mixture was dissolved uniformly under stirring. Next, 15.1 g of 80% by mass acrylic acid, 2.2 g of 48% by mass sodium hydroxide, 35.8 g of 50% by mass acrylamide, 0.4 g of sodium formate, 0.3 g of itaconic acid, and 10.8 g of 15% by mass sodium poly 2-acrylamide-2-methylpropanesulfonate were added to form a uniform solution, and the temperature was stabilized by immersing the solution in a 33°C water bath. Next, under a nitrogen atmosphere, 4% by mass of an aqueous solution of ammonium persulfate and 4% by mass of an aqueous solution of sodium hydrogensulfite were added as polymerization initiators at 55 ppm per monomer, and the mixture was polymerized at 30°C for 10 hours under stirring. After that, 300 ppm of the above polymerization initiator was added, and the mixture was polymerized for 5 hours. After the reaction was completed, 5.3 g of sodium sulfate, 40.6 g of magnesium sulfate heptahydrate, and 0.9 g of sodium thiosulfate were added to the obtained dispersion, and the mixture was stirred until no residue was left behind to obtain a water-soluble polymer dispersion. The molar composition ratio of this water-soluble polymer was acrylamide/acrylic acid = 60/40 mol%. The viscosity of this dispersion was 183 mPa·s, and the viscosity when the water-soluble polymer dispersion was dissolved in a 4 mass% sodium chloride aqueous solution to a concentration of 0.4 mass% was 46.6 mPa·s. The theoretical total nitrogen content of this water-soluble polymer dispersion was 12060 ppm (relative to the dispersion). In addition, a centrifugation operation was performed to conduct a separation stability test. This is shown in Table 1 as Example 5.

(Example 6) 116.5 g of demineralized water and 75.2 g of magnesium sulfate heptahydrate were added to a 0.5 L separable flask equipped with an anchor blade stirrer, a cooling tube, and a nitrogen inlet tube, and the mixture was dissolved uniformly under stirring. Next, 26.4 g of 80% by mass acrylic acid, 7.8 g of 48% by mass sodium hydroxide, 17.8 g of 50% by mass acrylamide, 0.4 g of sodium formate, 0.3 g of itaconic acid, and 12.0 g of 15% by mass sodium poly 2-acrylamide-2-methylpropanesulfonate were added to form a uniform solution, and the temperature was stabilized by immersing the solution in a 33°C water bath. Next, under a nitrogen atmosphere, 55 ppm of a 4% by mass aqueous solution of ammonium persulfate and a 4% by mass aqueous solution of sodium hydrogensulfite were added as polymerization initiators per monomer, and the mixture was polymerized at 30°C for 10 hours under stirring. After that, 300 ppm of the above polymerization initiator was added, and the mixture was polymerized for 5 hours. After the reaction was completed, 5.3 g of sodium sulfate and 0.8 g of sodium thiosulfate were added to the obtained dispersion, and the mixture was stirred until no residue was left behind to obtain a water-soluble polymer dispersion. The molar composition ratio of this water-soluble polymer was acrylamide/acrylic acid = 30/70 mol%. The viscosity of this dispersion was 250 mPa·s, and the viscosity when the water-soluble polymer dispersion was dissolved in a 4 mass% sodium chloride aqueous solution to a concentration of 0.4 mass% was 25.3 mPa·s. The theoretical total nitrogen content of this water-soluble polymer dispersion was 7120 ppm (relative to the dispersion). In addition, a centrifugation operation was performed to conduct a separation stability test. This is shown in Table 1 as Example 6.

(Example 7) 115.5 g of demineralized water and 75.2 g of magnesium sulfate heptahydrate were added to a 0.5 L separable flask equipped with an anchor blade stirrer, a cooling tube, and a nitrogen inlet tube, and the mixture was dissolved uniformly under stirring. Next, 30.1 g of 80% by mass acrylic acid, 8.9 g of 48% by mass sodium hydroxide, 11.9 g of 50% by mass acrylamide, 0.4 g of sodium formate, 0.3 g of itaconic acid, and 12.0 g of 15% by mass sodium poly 2-acrylamide-2-methylpropanesulfonate were added to form a uniform solution, and the temperature was stabilized by immersing the solution in a 33°C water bath. Next, under a nitrogen atmosphere, 4% by mass of an aqueous solution of ammonium persulfate and 4% by mass of an aqueous solution of sodium hydrogensulfite were added as polymerization initiators at 55 ppm per monomer, and the mixture was polymerized at 30°C for 10 hours under stirring. After that, 300 ppm of the above polymerization initiator was added, and the mixture was polymerized for 5 hours. After the reaction was completed, 5.3 g of sodium sulfate and 0.8 g of sodium thiosulfate were added to the obtained dispersion, and the mixture was stirred until no residue was left behind, to obtain a water-soluble polymer dispersion. The molar composition ratio of this water-soluble polymer was acrylamide/acrylic acid = 20/80 mol%. The viscosity of this dispersion was 300 mPa·s, and the viscosity was 9.5 mPa·s when the water-soluble polymer dispersion was dissolved in a 4 mass% sodium chloride aqueous solution to a concentration of 0.4 mass%. The theoretical total nitrogen content of this water-soluble polymer dispersion was 4920 ppm (relative to the dispersion). In addition, a centrifugation operation was performed, and a separation stability test was performed. This is shown in Table 1 as Example 7.

(Example 8) 91.4 g of demineralized water and 71.8 g of magnesium sulfate heptahydrate were added to a 0.5 L separable flask equipped with an anchor blade stirrer, a cooling tube, and a nitrogen inlet tube, and the mixture was dissolved uniformly under stirring. Next, 15.9 g of 80% by mass acrylic acid, 2.4 g of 48% by mass sodium hydroxide, 58.6 g of 50% by mass acrylamide, 0.4 g of sodium formate, 0.4 g of itaconic acid, and 15.4 g of 15% by mass acrylamide/sodium 2-acrylamido-2-methylpropanesulfonate (5/95 mol%) copolymer were added to form a uniform solution, and the temperature was stabilized by immersing the solution in a 33°C water bath. Next, under a nitrogen atmosphere, 4% by mass of an aqueous solution of ammonium persulfate and 4% by mass of an aqueous solution of sodium hydrogensulfite were added as polymerization initiators at 55 ppm per monomer, and the mixture was polymerized at 30°C for 10 hours under stirring. After that, 300 ppm of the above polymerization initiator was added, and the mixture was polymerized for 5 hours. After the reaction was completed, 5.0 g of sodium sulfate, 38.8 g of magnesium sulfate heptahydrate, and 0.9 g of sodium thiosulfate were added to the obtained dispersion, and the mixture was stirred until no residue was left behind to obtain a water-soluble polymer dispersion. The molar composition ratio of this water-soluble polymer was acrylamide/acrylic acid = 70/30 mol%. The viscosity of this dispersion was 180 mPa·s, and the viscosity when the water-soluble polymer dispersion was dissolved in a 4 mass% sodium chloride aqueous solution to a concentration of 0.4 mass% was 120 mPa·s. The theoretical total nitrogen content of this water-soluble polymer dispersion was 19650 ppm (relative to the dispersion). In addition, a centrifugation operation was performed to conduct a separation stability test. This is shown in Table 1 as Example 8.

(Comparative Example 1) 94.1 g of demineralized water and 71.8 g of magnesium sulfate heptahydrate were added to a 0.5 L separable flask equipped with an anchor blade stirrer, a cooling tube, and a nitrogen inlet tube, and dissolved uniformly under stirring. Next, 15.9 g of 80% by mass acrylic acid, 10.9 g of 48% by mass sodium hydroxide, 58.6 g of 50% by mass acrylamide, 0.4 g of sodium formate, 0.4 g of itaconic acid, and 12.6 g of 15% by mass polyacrylic acid were added to make a uniform solution, and the temperature was stabilized by immersing in a water bath at 33°C. Next, under a nitrogen atmosphere, 55 ppm of a 4% by mass aqueous solution of ammonium persulfate and a 4% by mass aqueous solution of sodium hydrogensulfite were added as polymerization initiators per monomer, and polymerization was started at 30°C under stirring. During the reaction, the viscosity of the reaction solution increased, making it difficult to stir and solidifying. This is shown in Table 1 as Comparative Example 1.

(Comparative Example 2) 92.4 g of demineralized water, 24.5 g of 80% acrylic acid, and 1.9 g of 48% sodium hydroxide were added to a 0.5 L separable flask equipped with an anchor blade stirrer, a cooling tube, and a nitrogen inlet tube, and dissolved uniformly under stirring. Next, 90.3 g of 50% acrylamide, 0.4 g of sodium formate, 0.7 g of itaconic acid, 23.8 g of 15% poly 2-acrylamide-2-methylpropanesulfonic acid sodium, and 50.4 g of ammonium sulfate were added to make a uniform solution, and the temperature was stabilized by immersing in a 33°C water bath. Next, under a nitrogen atmosphere, 4% ammonium persulfate aqueous solution and 4% sodium hydrogensulfite aqueous solution were added as polymerization initiators at 55 ppm per monomer, and polymerization was performed at 30°C for 10 hours under stirring. After that, 300 ppm of the above polymerization initiator was added, and polymerization was performed for 5 hours. After the reaction was completed, 13.2 g of ammonium sulfate and 0.9 g of sodium thiosulfate were added to the obtained dispersion, and the mixture was stirred until no residue was left behind to obtain a water-soluble polymer dispersion. The molar composition ratio of this water-soluble polymer was acrylamide/acrylic acid = 70/30 mol%. The viscosity of this dispersion was 486 mPa·s, and the viscosity when the water-soluble polymer dispersion was dissolved in a 4 mass% sodium chloride aqueous solution to a concentration of 0.4 mass% was 134 mPa·s. The theoretical total nitrogen content of this water-soluble polymer dispersion was 74870 ppm (relative to the dispersion). In addition, a centrifugation operation was performed to conduct a separation stability test. This is shown in Table 1 as Comparative Example 2.

(Comparative Example 3) 137.5 g of demineralized water, 16.0 g of 80% acrylic acid, and 2.4 g of 48% sodium hydroxide were added to a 0.5 L separable flask equipped with an anchor blade stirrer, a cooling tube, and a nitrogen inlet tube, and dissolved uniformly under stirring. Next, 59.0 g of 50% acrylamide, 0.4 g of sodium formate, 0.4 g of itaconic acid, 15.5 g of 15% poly 2-acrylamide-2-methylpropanesulfonic acid sodium, and 55.2 g of ammonium sulfate were added to make a uniform solution, and the temperature was stabilized by immersing in a 33°C water bath. Next, under a nitrogen atmosphere, 4% ammonium persulfate aqueous solution and 4% sodium hydrogensulfite aqueous solution were added as polymerization initiators at 55 ppm per monomer, and polymerization was carried out at 30°C for 10 hours under stirring. After that, 300 ppm of the above polymerization initiator was added, and polymerization was carried out for 5 hours. After the reaction was completed, 14.5 g of ammonium sulfate and 0.9 g of sodium thiosulfate were added to the resulting dispersion, and the mixture was stirred until no residue remained, to obtain a water-soluble polymer dispersion. The molar composition of this water-soluble polymer was acrylamide/acrylic acid = 70/30 mol%. The viscosity of this dispersion was 67 mPa·s. The theoretical total nitrogen content of this water-soluble polymer dispersion was 68,700 ppm (relative to the dispersion). In addition, a centrifugation operation was performed, and a separation stability test was conducted. This is shown in Table 1 as Comparative Example 3.

(Comparative Example 4) 108.6 g of demineralized water, 21.7 g of 80% acrylic acid, and 3.2 g of 48% sodium hydroxide were added to a 0.5 L separable flask equipped with an anchor blade stirrer, a cooling tube, and a nitrogen inlet tube, and uniformly dissolved under stirring. Next, 79.9 g of 50% acrylamide, 0.4 g of sodium formate, 0.6 g of itaconic acid, 21.01 g of 15% poly 2-acrylamide-2-methylpropanesulfonic acid sodium, and 52.0 g of ammonium sulfate were added to make a uniform solution, and the temperature was stabilized by immersing in a 33°C water bath. Next, under a nitrogen atmosphere, 4% ammonium persulfate aqueous solution and 4% sodium hydrogensulfite aqueous solution were added as polymerization initiators at 55 ppm per monomer, and polymerization was performed at 30°C for 10 hours under stirring. After that, 300 ppm of the above polymerization initiator was added, and polymerization was performed for 5 hours. After the reaction was completed, 13.6 g of ammonium sulfate and 0.9 g of sodium thiosulfate were added to the resulting dispersion, and the mixture was stirred until no residue was left behind, yielding a water-soluble polymer dispersion. The molar composition of this water-soluble polymer was acrylamide/acrylic acid = 70/30 mol%. The viscosity of this dispersion was 265 mPa·s. The theoretical total nitrogen content of this water-soluble polymer dispersion was 72790 ppm (relative to the dispersion). A centrifugation operation was also performed, and a separation stability test was carried out. This is shown in Table 1 as Comparative Example 4.

単量体組成；ＡＡＭ：アクリルアミド、ＡＡＣ：アクリル酸
高分子分散剤；
ｐ－ＡＭＰＳ：ポリ２－アクリルアミド－２－メチルプロパンスルホン酸ナトリウム
ＡＡＭ／ＡＭＰＳ：アクリルアミド／２－アクリルアミド－２－メチルプロパンスルホン酸ナトリウム（５／９５モル％）共重合体
ｐ－ＡＡＣ：ポリアクリル酸、高分子分散剤添加率（質量％）：対単量体
重合時塩：ａ；硫酸マグネシウム７水和物、ｂ；硫酸アンモニウム
単量体濃度：水溶性重合体分散液に対する単量体の質量割合
無機塩濃度：水溶性重合体分散液に対する無機塩の質量割合
分散液粘度：水溶性重合体分散液の２５℃において測定した粘度
０．４質量％塩水溶液粘度：４質量％塩化ナトリウム水中に高分子濃度が０．４質量％になるように溶解したときの２５℃において測定した粘度（ｐＨ８．５）。
固有粘度：水溶性重合体分散液の２５℃において測定した０．５質量％における、４質量％食塩水溶液中の固有粘度（ｐＨ８．５）
遠沈上澄み：分散液４０ｇを遠沈管に採取、４０００ｒｐｍで１０分間、遠心分離した後に、上澄み液の高さを測定し、分散液の高さに対する割合を算出した。
遠沈残渣：分散液４０ｇを遠沈管に採取、４０００ｒｐｍで１０分間、遠心分離した後に、遠沈管を１２０秒間転倒させて得た沈殿残渣物質量を測定し、分散液に対する質量％を算出した。 (Table 1)

Monomer composition: AAM: acrylamide, AAC: acrylic acid polymer dispersant;
p-AMPS: poly sodium 2-acrylamido-2-methylpropanesulfonate AAM/AMPS: acrylamide/sodium 2-acrylamido-2-methylpropanesulfonate (5/95 mol%) copolymer p-AAC: polyacrylic acid, polymer dispersant addition rate (mass%): relative to monomer during polymerization Salts: a; magnesium sulfate heptahydrate, b; ammonium sulfate Monomer concentration: mass ratio of monomer to water-soluble polymer dispersion Inorganic salt concentration: mass ratio of inorganic salt to water-soluble polymer dispersion Dispersion viscosity: viscosity of water-soluble polymer dispersion measured at 25°C Viscosity of 0.4 mass% salt aqueous solution: viscosity measured at 25°C when dissolved in 4 mass% sodium chloride water to give a polymer concentration of 0.4 mass% (pH 8.5).
Intrinsic viscosity: Intrinsic viscosity of the water-soluble polymer dispersion in a 4% by weight saline solution (pH 8.5) at 0.5% by weight measured at 25° C.
Centrifugation supernatant: 40 g of the dispersion was collected in a centrifuge tube and centrifuged at 4,000 rpm for 10 minutes. The height of the supernatant was then measured and its ratio to the height of the dispersion was calculated.
Centrifugation residue: 40 g of the dispersion was placed in a centrifuge tube and centrifuged at 4,000 rpm for 10 minutes. The amount of precipitated residue obtained by inverting the centrifuge tube for 120 seconds was measured and the mass % relative to the dispersion was calculated.

In Examples 1 to 8, which are anionic water-soluble polymer dispersions obtained by using magnesium sulfate as an inorganic salt during polymerization and a polymer having 2-acrylamido-2-methylpropanesulfonic acid or a salt thereof as a constituent unit as a dispersant, water-soluble polymer dispersions were obtained without using ammonium sulfate during polymerization. Moreover, compared with the comparative example of a general-purpose product using ammonium sulfate as an inorganic salt during polymerization, the amounts of the centrifugal supernatant and centrifugal residue in the separation stability test were small, and it was found that the product stability was good.
On the other hand, in Comparative Example 1, in which magnesium sulfate was used as the inorganic salt during polymerization but a polymer dispersant not containing 2-acrylamido-2-methylpropanesulfonic acid or a salt thereof as a constituent unit was used, the dispersion thickened and solidified, and no dispersion was obtained. It was found that by using magnesium sulfate as the inorganic salt during polymerization and a polymer containing 2-acrylamido-2-methylpropanesulfonic acid or a salt thereof as a constituent unit as a dispersant, thickening can be suppressed without using ammonium sulfate, and an anionic water-soluble polymer dispersion having good product stability can be obtained.

Claims (3)

An anionic water-soluble polymer dispersion is obtained by dispersing and polymerizing an aqueous solution of a monomer or a monomer mixture essentially containing a monomer represented by the following general formula (1) in an aqueous salt solution containing 70% by mass or more of magnesium sulfate based on the total amount of inorganic salts during polymerization under stirring while allowing a polymer dispersant having 2-acrylamido-2-methylpropanesulfonic acid or a salt thereof as a constituent unit to coexist in the aqueous salt solution.

General formula (1)
_R1 represents hydrogen, a methyl group or _a carboxymethyl group, Q represents _SO3 , _C6H4SO3 , _CONHC ( _CH3 ) _2CH2SO3 , _C6H4COO or _COO , _R2 represents hydrogen or _COOY2 , and _Y1 and _{Y2 represent} hydrogen or _a cation. The anionic water-soluble polymer dispersion according to claim 1, characterized in that the viscosity of a 4% by weight aqueous sodium chloride solution at 0.4% by weight of the anionic water-soluble polymer measured at 25°C is in the range of 20 to 200 mPa·s. A method for producing an anionic water-soluble polymer dispersion, characterized in that an aqueous solution of a monomer or monomer mixture essentially containing a monomer represented by the following general formula (1) is made to coexist with a polymer dispersant having 2- acrylamido-2-methylpropanesulfonic acid or a salt thereof as a constituent unit in an aqueous salt solution containing 70% by mass or more of magnesium sulfate based on the total amount of inorganic salts during polymerization, and dispersion-polymerized under stirring to obtain the anionic water-soluble polymer dispersion.

General formula (1)
_R1 represents hydrogen, a methyl group or _a carboxymethyl group, Q represents _SO3 , _C6H4SO3 , _CONHC ( _CH3 ) _2CH2SO3 , _C6H4COO or _COO , _R2 represents hydrogen or _COOY2 , and _Y1 and _{Y2 represent} hydrogen or _a cation.