TWI869351B - Dispersing aid for suspension polymerization and method for producing vinyl resin - Google Patents

Abstract

The present invention provides a dispersing aid for suspension polymerization, which has excellent operability and, when used in suspension polymerization of vinyl compounds, can obtain vinyl resin particles with less coarse particles and high plasticizer absorption. The dispersing aid for suspension polymerization of vinyl compounds of the present invention contains a vinyl ester polymer having an acid value (KOH mg/g) of 5 or more and less than 100.

Description

Dispersing aid for suspension polymerization and method for producing vinyl resin

The present invention relates to a dispersing aid for suspension polymerization, and more particularly to a dispersing aid suitable for suspension polymerization of vinyl compounds, especially vinyl chloride.

Suspension polymerization is widely used in the industry to produce vinyl polymers such as vinyl chloride resins. The suspension polymerization method involves dispersing vinyl compounds in a water-soluble medium in the presence of a dispersant and polymerizing them using an oil-soluble catalyst. The main factors that usually determine the quality of the resin include polymerization rate, water-monomer ratio, polymerization temperature, the amount or feeding period of the polymerization initiator, the form of the polymerization tank, stirring speed or the type and amount of the dispersant, etc. Among these main factors, the dispersant has a great influence.

The properties required of a dispersant for suspension polymerization of vinyl compounds include: (1) significantly superior suspension polymerization stability when used in small amounts, and the ability to make the particle size distribution of the resulting vinyl polymer particles as narrow as possible; (2) increasing the rate of absorption of the plasticizer into the polymer particles to facilitate polymer processability, facilitate removal of monomers such as vinyl chloride remaining in the polymer particles, and adjust the porosity to prevent the formation of fish eyes in the molded product; (3) forming polymer particles with a high specific gravity; (4) good operability, etc.

The requirements for these properties (1) to (4) are becoming increasingly higher. In particular, with regard to the removability of the monomer component described in (2) above, the amount of residual monomers is extremely limited from an environmental point of view. In addition, in the case where it is difficult to remove the vinyl chloride monomers remaining in the particles during the drying step after the completion of polyvinyl chloride polymerization, a long time or high temperature drying is required to remove the residual monomers, but there is also a concern that the polyvinyl chloride resin may deteriorate. In addition, with regard to the operability described in (4) above, due to environmental issues, in recent years, organic solvents such as methanol have not been used, and it is hoped that the dispersant for suspension polymerization can be used in the form of an aqueous liquid.

As the dispersant for suspension polymerization, a main dispersant of polyvinyl alcohol resin (hereinafter sometimes referred to as PVA) can be used. Among them, PVA with a saponification degree of 65 mol% to 99 mol% can be used as the main dispersant, and PVA with a saponification degree of less than 65 mol% is often used as a dispersant. The main dispersant is used mainly to expect the performance of (1) or (3), and the dispersant is used to expect the performance of (2). In addition, the performance of (4) is required for all dispersants used.

As the dispersing aid, polyvinyl alcohol polymers having an ionic group at one end and a saponification degree of 10 mol% to 85 mol% (Patent Document 1), polyvinyl alcohol polymers obtained by saponifying a polymer composed of vinyl acetate and an unsaturated carboxylic acid (Patent Document 2), or vinyl alcohol polymers having a saponification degree of 35 mol% to 65 mol% and having an aliphatic alkyl group with a carbon number of 6 to 12 at the end (Patent Document 3) have been proposed. [Prior Art Documents] [Patent Documents]

Patent document 1: Japanese Patent Publication No. 10-168128. Patent document 2: Japanese Patent Publication No. 2013-203994. Patent document 3: International Publication No. 2015/019614.

(The problem that the invention wants to solve)

However, it is difficult to find a dispersing aid of vinyl compounds that fully meets the required performances (1) to (4) that have been rising in recent years, including the dispersing aid using PVA described in Patent Documents 1 to 3. In addition, the known dispersing aid is produced by polymerizing vinyl ester monomers to obtain vinyl ester polymers, and then saponifying them with alkali, etc., which has the following disadvantages: there is a concern that the solvent used in the saponification reaction will remain in the product, or the process is long and costly.

Therefore, one of the problems of the present invention is to provide a dispersing aid for suspension polymerization, which has excellent operability and when the dispersing aid for suspension polymerization is used for suspension polymerization of ethylene compounds, ethylene resin particles with less formation of coarse particles and high plasticizer absorption can be obtained. Furthermore, another problem of the present invention is to provide a method for producing ethylene resins, which uses the dispersing aid for suspension polymerization. (Means for solving the problem)

The inventors of the present invention have conducted intensive research to solve the above problems and have found that it is effective to use a vinyl ester polymer having an acid value (KOH mg/g) of 5 or more and less than 100 as a dispersing aid for suspension polymerization of vinyl compounds.

Therefore, the present invention is, in one aspect, a dispersing aid for suspension polymerization, comprising a vinyl ester polymer having an acid value (KOH mg/g) of 5 or more and less than 100.

In one embodiment of the dispersing aid for suspension polymerization of the present invention, the vinyl ester polymer is a vinyl acetate polymer.

In another embodiment of the dispersing aid for suspension polymerization of the present invention, the viscosity average polymerization degree of the vinyl ester polymer is 100 to 1000.

In still another embodiment of the dispersing aid for suspension polymerization of the present invention, the saponification degree of the vinyl ester polymer is 5 mol % or less.

In still another embodiment of the dispersing aid for suspension polymerization of the present invention, 60 mass % or more of the above-mentioned vinyl ester polymer is contained.

In still another embodiment of the dispersing aid for suspension polymerization of the present invention, the acid value (KOH mg/g) of the vinyl ester polymer is 10 or more and less than 40.

On the other hand, the present invention is a method for producing a vinyl resin, comprising: using the dispersing aid for suspension polymerization of the present invention to disperse a vinyl compound monomer, or a mixture of a vinyl compound monomer and a monomer copolymerizable with the vinyl compound monomer, in water for suspension polymerization. (Effect of the invention)

The present invention can obtain a dispersing aid for suspension polymerization, which has excellent operability and can produce vinyl resin particles with less coarse particle formation and high plasticizer absorption when the dispersing aid for suspension polymerization is used in suspension polymerization of vinyl compounds.

The following is a detailed description of the implementation of the present invention. In addition, the present invention is not limited to the implementation described below.

From the viewpoint of water solubility and performance, it is important that the vinyl ester polymer contained in the dispersing aid for suspension polymerization of one embodiment of the present invention has an acid value (KOH mg/g) of 5 or more and less than 100. When the acid value is less than 5 mg/g, the water solubility becomes insufficient, which becomes a practical problem in terms of handling. Therefore, the acid value must be 5 mg/g or more, preferably 7 mg/g or more, and more preferably 10 mg/g or more. In addition, if the acid value becomes 100 mg/g or more, the dispersion system becomes unstable, and the resulting vinyl resin particles are coarse particles, etc., which becomes a practical problem. Therefore, the acid value must be less than 100 mg/g, preferably less than 90 mg/g, more preferably less than 80 mg/g, further preferably less than 70 mg/g, further preferably less than 60 mg/g, further preferably less than 50 mg/g, particularly preferably less than 40 mg/g, and even more preferably less than 30 mg/g.

The acid value is measured in accordance with JIS (Japanese Industrial Standard) K0070: 1992. That is, it is determined from the amount of potassium hydroxide required for neutralization of the vinyl ester polymer.

The saponification degree of the vinyl ester polymer contained in the dispersing aid for suspension polymerization of one embodiment of the present invention is preferably 5 mol% or less, more preferably 2 mol% or less, and even more preferably 1 mol% or less. The saponification degree can also be set to 0 mol%. Reducing the saponification degree can omit or shorten the saponification step, which is economically advantageous and also beneficial for increasing the acid value.

The saponification degree of the vinyl ester polymer is measured by a method in accordance with JIS K6726: 1994. That is, it is determined by back titration using N/10 sulfuric acid specified in JIS K8951: 2006 and N/10 sodium hydroxide solution specified in JIS K8576: 2019.

The method for producing the vinyl ester polymer contained in the dispersing aid for suspension polymerization of one embodiment of the present invention is not particularly limited, but a simple method is to perform free radical polymerization of the vinyl ester monomer in the coexistence of an unsaturated carboxylic acid.

Examples of the unsaturated carboxylic acid used in the present invention include, but are not limited to, crotonic acid, isomethacrylic acid, itaconic acid, maleic acid, fumaric acid, (meth)acrylic acid, and the like. The unsaturated carboxylic acid may be used alone or in combination of two or more. In terms of the reactivity ratio with vinyl ester or the ease of handling, crotonic acid is preferably used.

The vinyl ester monomers are not limited and include vinyl acetate, vinyl formate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl trimethylacetate, and vinyl versatate. The vinyl ester monomers may be used alone or in combination of two or more. In terms of ease of acquisition, handling, and economy, vinyl acetate is most preferably used.

The viscosity average degree of polymerization of the vinyl ester polymer contained in the dispersing aid for suspension polymerization is preferably 100 or more from the viewpoint of dispersion stability or production efficiency when the vinyl compound is subjected to suspension polymerization. When the viscosity average degree of polymerization is less than 100, the dispersion system tends to become unstable when the vinyl compound is subjected to suspension polymerization, and the particle size of the obtained vinyl resin tends to become coarse. In addition, the softening temperature of the dispersing aid for suspension polymerization becomes low, so it tends to agglomerate in the drying step and manufacturing becomes difficult. Therefore, the viscosity average degree of polymerization is preferably 100 or more, more preferably 150 or more, and even more preferably 200 or more. In addition, the viscosity average degree of polymerization of the vinyl ester polymer contained in the dispersing aid for suspension polymerization is preferably 1000 or less, more preferably 800 or less, and even more preferably 600 or less in order to prevent the viscosity of the aqueous solution from becoming high and becoming difficult to handle or to increase the porosity of the particles obtained when the vinyl compound is subjected to suspension polymerization, thereby improving the plasticizer absorption.

The viscosity average degree of polymerization is measured in accordance with JIS K6725: 1977. That is, it is determined from the limiting viscosity [η] of the vinyl ester polymer contained in the dispersion aid for suspension polymerization measured in acetone at 30°C.

The polymerization method of the vinyl ester polymer contained in the dispersing aid for suspension polymerization of one embodiment of the present invention is not particularly limited. The polymerization method may be any of batch polymerization, semi-batch polymerization, continuous polymerization, and semi-continuous polymerization. The monomer may be fed all at once, or fed in batches, or may be added continuously or intermittently. In addition, any known polymerization method such as solution, emulsification, suspension, and bulk polymerization may be used. In terms of no residual solvent in the product and easy drying, a method in which no residual solvent is left after polymerization and the polymer is obtained in the form of particles is more suitable, and the suspension polymerization method is preferred.

At this time, other monomers may be further copolymerized within the scope that does not impair the effects of the present invention. Specific examples of other monomers include: olefins such as ethylene and propylene; dialkyl maleates; dialkyl fumarates; alkyl vinyl ethers, etc.

The polymerization initiator used in the free radical polymerization of vinyl ester monomers is not particularly limited, and the following compounds may be used alone or in combination: azo compounds such as azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, azobis(4-methoxy-2,4-dimethylvaleronitrile), azobisdimethylvaleronitrile, and azobismethoxyvaleronitrile; peroxides such as acetyl peroxide, benzoyl peroxide, lauryl peroxide, acetylcyclohexylsulfonyl peroxide, and 2,4,4-trimethylpentyl 2-peroxyphenoxyacetate; percarbonate compounds such as diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and diethoxyethyl peroxydicarbonate; peroxyester compounds such as α-isopropylphenyl peroxyneodecanoate and t-butyl peroxyneodecanoate, etc.

In addition, when copolymerization is performed at a high temperature, aldehydes are generated due to the oxidative decomposition of vinyl ester monomers to form condensates, resulting in coloration, etc. In this case, an antioxidant such as citric acid may be added to the polymerization system in an amount of 1 ppm to 100 ppm (relative to the total mass of vinyl ester monomers) to prevent coloration.

From the viewpoint of exerting the required performance, the dispersing aid for suspension polymerization of one embodiment of the present invention preferably contains 60 mass % or more of a vinyl ester polymer having an acid value (KOH mg/g) of 5 or more and less than 100, more preferably 80 mass % or more, and even more preferably 90 mass % or more. In addition, the dispersing aid for suspension polymerization of the present invention may also contain various additives within the scope that does not damage the gist of the present invention. Examples of such additives include: polymerization regulators such as aldehydes, halides, and thiols; polymerization inhibitors such as phenol compounds, sulfur compounds, and N-peroxide compounds; pH adjusters; crosslinking agents; preservatives; anti-mold agents, anti-adhesive agents; defoaming agents, etc.

The dispersing aid for suspension polymerization of one embodiment of the present invention can be particularly suitably used in the suspension polymerization of vinyl compounds. Examples of vinyl compounds include: halogenated vinyls such as vinyl chloride; vinyl esters such as vinyl acetate and vinyl propionate; acrylic acid, methacrylic acid, esters and salts thereof; maleic acid, fumaric acid, esters and anhydrides thereof; styrene, acrylonitrile, vinylidene chloride, vinyl ether, etc. Among them, the dispersing aid for suspension polymerization of the present invention is particularly suitably used when vinyl chloride is subjected to suspension polymerization alone, or when vinyl chloride is subjected to suspension polymerization together with a monomer copolymerizable with vinyl chloride. Monomers that can be copolymerized with vinyl chloride include: vinyl esters such as vinyl acetate and vinyl propionate; (meth)acrylates such as methyl (meth)acrylate and ethyl (meth)acrylate; α-olefins such as ethylene and propylene; unsaturated dicarboxylic acids such as maleic anhydride and itaconic acid; acrylonitrile, styrene, vinylidene chloride, vinyl ether, etc.

The dispersing aid for suspension polymerization of one embodiment of the present invention is suitable for the production of vinyl chloride resin particles with excellent plasticizer absorption, and is suitable for the production of soft vinyl chloride resin. However, in terms of excellent monomer removal and particle size distribution, it can also be applied to the production of hard vinyl chloride resin.

The dispersing aid for suspension polymerization of one embodiment of the present invention can be used alone, but can also be used together with other stabilizers such as cellulose derivatives, surfactants, etc.

The dispersing aid for suspension polymerization of one embodiment of the present invention has a large plasticizer absorption amount, so by using the dispersing aid for suspension polymerization, a vinyl chloride resin having excellent physical properties such as porous resin particles and few fish eyes can be obtained. The following is a specific description of the polymerization method of vinyl compounds, but it is not limited to these.

When producing vinyl compound resin particles such as vinyl chloride resin particles, in one embodiment, 0.01 to 0.3 parts by mass, preferably 0.02 to 0.10 parts by mass of the above-mentioned suspension polymerization dispersing aid is added to 100 parts by mass of the vinyl compound monomer. In addition, the ratio of the vinyl compound to water can be set to vinyl compound: water = 1:0.9 to 1:3, preferably vinyl compound: water = 1:1 to 1:1.5.

The polymerization initiator may also be a polymerization initiator previously used for the polymerization of vinyl compounds. As for the polymerization initiator, the following compounds may be used alone or in combination: percarbonate compounds such as diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and diethoxyethyl peroxydicarbonate; peroxyester compounds such as tert-butyl peroxyneodecanoate and α-isopropylphenyl peroxyneodecanoate; peroxides such as acetylcyclohexylsulfonyl peroxide and 2-peroxyphenoxyacetic acid-2,4,4-trimethylpentyl ester; azo compounds such as azobis-2,4-dimethylvaleronitrile and azobis(4-methoxy-2,4-dimethylvaleronitrile); and potassium persulfate, ammonium persulfate, hydrogen peroxide, and the like.

Furthermore, a polymerization regulator, a chain transfer agent, a gelling agent, an antistatic agent, a pH adjuster, etc. suitable for the polymerization of vinyl compounds may be added arbitrarily.

When carrying out the polymerization of ethylene compounds, the feed ratio of each component, the polymerization temperature, etc. can be set according to the conditions previously used for suspension polymerization of ethylene compounds, and there is no reason to specifically limit them. [Example]

The present invention is described in more detail below with reference to examples. In addition, unless otherwise specified, "parts" and "%" refer to "parts by mass" and "% by mass", respectively.

(Example 1) 〈Preparation of a dispersing aid for suspension polymerization〉 97 parts of vinyl acetate, 180 parts of water, 0.3 parts of polyvinyl alcohol (W-20N manufactured by Denka Co., Ltd.) as a dispersing agent, 1.6 parts of n-butylaldehyde (nBA) as a chain transfer agent, 3 parts of crotonic acid, and 0.9 parts of azobisisobutyronitrile were added to a polymerization tank, heated and polymerized at 60°C for 7 hours. The polymerization rate at the end of the polymerization was measured by the method shown below. Unpolymerized vinyl acetate was then removed by conventional methods, and the resulting vinyl ester polymer was flow dried at 40°C to obtain a dispersing aid A for suspension polymerization. No additives were added in this example, and the vinyl ester polymer itself constituted the dispersing aid A for suspension polymerization. The viscosity average degree of polymerization, saponification degree, acid value and water solubility of the obtained dispersing aid A for suspension polymerization were evaluated by the following methods. The evaluation results are shown in Table 1.

[Polymerization rate] The polymerization rate is obtained by sampling a small amount of the vinyl ester monomer and vinyl ester polymer particles uniformly dispersed in water, drying the sampled solution at 150°C for 30 minutes, and then calculating it by weight method.

[Viscosity average degree of polymerization] According to JIS K6725:1977, it is determined from the limiting viscosity [η] of the obtained suspension polymerization dispersing aid A measured in acetone at 30°C.

[Saponification degree] According to JIS K6726:1994, the saponification degree of the obtained dispersing aid A for suspension polymerization was determined.

[Acid value] According to JIS K0070:1992, it is determined from the amount of potassium hydroxide required to neutralize the obtained dispersing aid A for suspension polymerization.

[Water solubility] After adding 10 parts of dispersing aid A for suspension polymerization to 90 parts of water, sodium carbonate was added to make the pH value 7.5 to 8.5 and stirred at room temperature for 4 hours. Stirring was stopped and the presence or absence of precipitation was confirmed by visual inspection. Evaluation was performed according to the following criteria. ○: A uniform solution or dispersion was obtained and no precipitation occurred. ×: Precipitation occurred.

<Suspension polymerization of vinyl chloride> In a 30L stainless steel autoclave equipped with a stirrer, 14kg of 30°C water, 12g of polyvinyl alcohol (W-20N manufactured by Denka Co., Ltd.) with a saponification degree of 80 mol% as a main dispersant, 2g of the above-obtained dispersant A for suspension polymerization, 4.6g of tert-butyl peroxyneodecanoate as a polymerization initiator, and 1g of α-isopropylphenyl peroxyneodecanoate were added under stirring. After the autoclave was degassed in vacuum, 10kg of vinyl chloride monomer was added and polymerization was carried out at 57°C for 4 hours.

<Evaluation of vinyl chloride resin> The average particle size, particle size distribution, plasticizer absorption and volume specific gravity of the obtained vinyl chloride resin were evaluated by the following method. The evaluation results are shown in Table 1.

[Average particle size, particle size distribution] According to JIS Z8815:1994, the obtained vinyl chloride resin was classified using 60 mesh (pore size 250μm), 80 mesh (pore size 180μm), 100 mesh (pore size 150μm), 150 mesh (pore size 106μm), and 200 mesh (pore size 75μm) sieves, and the particle size with a cumulative frequency of 50% (D50) was set as the average particle size, and the difference between the particle size with a cumulative frequency of 80% (D80) and the particle size with a cumulative frequency of 20% (D20) was set as the particle size distribution.

[Plasticizer absorption] Measured in accordance with ISO (International Organization for Standardization) 4608:1998(E).

[Volume specific gravity] Measured in accordance with JIS K6720-2:1999.

[Table 1]

(Example 2 to Example 3) Except that the amount of n-butyraldehyde added and the amount of crotonic acid added were changed to the conditions described in Table 1, dispersing aids B and C for suspension polymerization were obtained in the same manner as in Example 1. Subsequently, suspension polymerization of vinyl chloride was carried out under the same conditions as in Example 1 except that the obtained dispersing aids B and C for suspension polymerization were used. The evaluation results of dispersing aids B and C for suspension polymerization and the vinyl chloride resin obtained using the dispersing aids B and C for suspension polymerization are shown in Table 1.

(Example 4) 97 parts of vinyl acetate, 180 parts of water, 0.3 parts of polyvinyl alcohol (W-20N manufactured by Denka Co., Ltd.) as a dispersant, 1.4 parts of n-butylaldehyde (nBA) as a chain transfer agent, and 0.9 parts of azobisisobutyronitrile were added to a polymerization tank, heated to 60°C, and then itaconic acid was added continuously for 6 hours in a manner such that the total addition amount became 1.0 part while polymerization was carried out, and polymerization was further carried out for 1 hour. The polymerization rate at the end of the polymerization is shown in Table 1. Unpolymerized vinyl acetate was then removed by conventional methods, and the resulting vinyl ester polymer was flow dried at 40°C to obtain a dispersant D for suspension polymerization. In this example, no additives were added, and the vinyl ester polymer itself constituted the dispersant D for suspension polymerization. The obtained dispersing aid D for suspension polymerization was evaluated for viscosity average degree of polymerization, saponification degree, acid value and water solubility by the same method as in Example 1. The evaluation results are shown in Table 1.

(Comparative Example 1) Suspension polymerization of vinyl chloride was carried out under the same conditions as in Example 1 except that the dispersing aid for suspension polymerization was not used. The evaluation results of the vinyl chloride resin are shown in Table 1.

(Comparative Example 2) Except that the amount of butenoic acid added was changed to the conditions described in Table 1, a dispersing aid E for suspension polymerization was obtained in the same manner as in Example 1. The obtained dispersing aid E for suspension polymerization did not show water solubility. Then, suspension polymerization of vinyl chloride was carried out under the same conditions as in Example 1 except that the obtained dispersing aid E for suspension polymerization was used. The evaluation results of the dispersing aid E for suspension polymerization and the vinyl chloride resin obtained using the dispersing aid E for suspension polymerization are shown in Table 1.

(Comparative Example 3) Except that the amount of n-butyraldehyde and the amount of crotonic acid added were changed to the conditions described in Table 1, a dispersing aid F for suspension polymerization was obtained in the same manner as in Example 1. The obtained dispersing aid F for suspension polymerization showed water solubility. Subsequently, suspension polymerization of vinyl chloride was carried out under the same conditions as in Example 1 except that the obtained dispersing aid F for suspension polymerization was used, but vinyl chloride agglomerated in the autoclave and vinyl chloride particles could not be obtained. The evaluation results of the dispersing aid F for suspension polymerization are shown in Table 1.

(Comparative Example 4) 100 parts of vinyl acetate, 150 parts of methanol (solvent and chain transfer agent) and 0.03 parts of azobisisobutyronitrile (in the form of 1% methanol solution) were fed into a polymerization tank and polymerized at 60°C for 8 hours under a nitrogen atmosphere. The polymerization rate at the end of the polymerization is shown in Table 1. Unpolymerized vinyl acetate was then removed by conventional methods, and the solvent of the vinyl ester polymer was removed by drying to obtain a dispersing aid G for suspension polymerization. In this comparative example, no additives were added, and the vinyl ester polymer itself constituted the dispersing aid G for suspension polymerization. The obtained dispersing aid G for suspension polymerization did not show water solubility. Next, suspension polymerization of vinyl chloride was carried out under the same conditions as in Example 1 except that the obtained dispersing aid G for suspension polymerization was used. The evaluation results of the dispersing aid G for suspension polymerization and the vinyl chloride resin obtained by using the dispersing aid G for suspension polymerization are shown in Table 1.

(Comparative Example 5) 100 parts of vinyl acetate, 22.5 parts of methanol (solvent and chain transfer agent), 0.23 parts of 3-MPA (3-butylene propionic acid) as a chain transfer agent, and 0.03 parts of azobisisobutyronitrile (in the form of a 1% methanol solution) were fed into a polymerization tank and replaced with a nitrogen atmosphere. Heating was performed from the time point when the temperature reached 60°C, while 1.81 parts of a 10% methanol solution of butylene propionic acid was continuously added in a fixed manner so that the addition was completed in 6 hours, and polymerization was carried out for 6 hours. The polymerization rate at the end of the polymerization is shown in Table 1. Unpolymerized vinyl acetate was then removed by conventional methods, and the solvent of the vinyl ester polymer was removed by drying to obtain a modified vinyl ester polymer. The obtained modified vinyl ester polymer is dissolved in methanol to obtain a methanol solution having a polymer concentration of 35% in methanol. The modified vinyl ester polymer is saponified with sodium hydroxide at 35°C for 90 minutes. The amount of sodium hydroxide used is 7 millimoles equivalent relative to 1 mol of vinyl acetate unit of the obtained modified vinyl ester polymer. Dispersing aid H for suspension polymerization is obtained by filtration and drying. No additives are added in this comparative example, and the vinyl ester polymer itself constitutes the dispersing aid H for suspension polymerization. The obtained dispersing aid H for suspension polymerization has a saponification degree of 43 mol% as measured in accordance with JIS K6726:1994. Suspension polymerization of vinyl chloride was carried out under the same conditions as in Example 1 except that the obtained dispersing aid H for suspension polymerization was used. The evaluation results of the dispersing aid H for suspension polymerization and the vinyl chloride resin obtained by using the dispersing aid H for suspension polymerization are shown in Table 1.

From the results shown in Table 1, it can be seen that the dispersing aid for suspension polymerization of the present invention is water-soluble and therefore has excellent operability. The vinyl resin obtained by suspension polymerization using the dispersing aid for suspension polymerization has high plasticizer absorption and less formation of coarse particles. Furthermore, the reason why Comparative Example 5 is water-soluble despite its low acid value is that hydroxyl groups are introduced by saponification.

without.

without.

without.

Claims (5)

A dispersing aid for suspension polymerization of vinyl compounds, comprising a vinyl ester polymer having an acid value (KOH mg/g) of 5 or more and less than 100 and a viscosity average polymerization degree of 100 to 570; the aforementioned vinyl ester polymer is a vinyl acetate polymer. The dispersing aid for suspension polymerization as described in claim 1, wherein the saponification degree of the aforementioned vinyl ester polymer is less than 5 mol%. The dispersing aid for suspension polymerization described in claim 1 or 2 contains 60% by mass or more of the aforementioned vinyl ester polymer. The dispersing aid for suspension polymerization as described in claim 1 or 2, wherein the acid value (KOH mg/g) of the aforementioned vinyl ester polymer is greater than 10 and less than 40. A method for producing a vinyl resin comprises: using a dispersing aid for suspension polymerization as described in any one of claims 1 to 4 to disperse a vinyl compound monomer, or a mixture of a vinyl compound monomer and a monomer copolymerizable with the vinyl compound monomer, in water for suspension polymerization.