Process for preparing vinyl chloride polymer

Abstract: A process for preparing a vinyl chloride polymer by polymerizing vinyl chloride, or vinyl monomers containing vinyl chloride, through a suspension polymerization in an aqueous medium; wherein an antioxidant dispersion comprising an antioxidant, a suspending agent and water is added to the above polymerization system. This process makes it possible to automatically charge the antioxidant to the polymerization system. The vinyl chloride polymer obtained is excellent in storage stability, thermal stability and plasticizer absorption performance.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a process for preparing a vinyl chloride polymer through a suspension polymerization in an aqueous medium. 2. Description of the Prior Art Conventionally, it is known to add an antioxidant to a polymerization system when preparing a vinyl chloride polymer, which is added for the purposes of (1) retarding or terminating the polymerization reaction, (2) homogeneously dispersing the antioxidant in a resulting vinyl chloride resin product to prevent the vinyl chloride resin from being deteriorated, etc. In any instances, the antioxidant is required to be homogeneously dispersed in the suspension polymerization system in order to bring the effect by the addition of the antioxidant into the maximum. The antioxidant conventionally includes phenol antioxidants, amine antioxidants, sulfur antioxidants, phosphorus antioxidants, etc., which, however, are powdery in general, making it difficult to automatically charge them in a predetermined amount. In particular, during polymerization, it is particularly difficult to pressing a desired amount of the powdery antioxidant into the system which is kept in a pressurized state. For this reason, it has been conventionally practiced to dissolve the antioxidant in a suitable organic solvent and add it as a solution. This method makes it possible to automatically charge the antioxidant, but has had the problems such that it becomes necessary to recover the solvent, the solvent remains in the resulting vinyl chloride resin to cause a poorness of the quality, the solvent mixes into the unreacted monomers to be recovered, and the COD in waste water increases in the instance of the suspension polymerization carried out in an aqueous medium. Now recently a process has been proposed in which a powdery antioxidant is dispersed in water by using an emulsifying agent and/or a suspending agent, to bring it into a state of emulsion, which is then added in the polymerization system (Japanese Unexamined Patent Publication No. 149608/1985). However, although according to said publication it is possible to use a suspending agent alone, all examples are directed to combinations of both emulsifying agent and suspending agent, showing that such a combined use is preferred. However, the above process in which the antioxidant is added in the form of an emulsion by use of an emulsifying agent may lower the thermal stability and storage stability of the resulting vinyl chloride resin because of the inclusion of the emulsifying agent therein. It also has the problems such that, when added for the purpose of terminating the polymerization, the reaction tends to be terminated with insufficient effect, and, when added at the time the polymerization is completed, it may follow that the plasticizer absorption performance of the vinyl chloride resin becomes poor after the resin is subjected to heat treatment. SUMMARY OF THE INVENTION Now, an object of this invention is to provide a process for preparing a vinyl chloride polymer, that makes it possible to automatically charge the antioxidant without using any solvent and any emulsifying agent either, and can sufficiently achieve the intended effect of adding the antioxidant to give a vinyl chloride resin of high quality. This invention provides a process for preparing a vinyl chloride polymer by polymerizing vinyl chloride, or a vinyl monomer mixture containing vinyl chloride, through a suspension polymerization in an aqueous medium, wherein an antioxidant dispersion consisting of an antioxidant, a polyvinyl alcohol having a saponification degree of from 60 to 79.5 mol% and a polymerization degree of 600 to 3,000 and water is previously heated to a temperature which is not lower than the melting point of the antioxidant, followed by cooling, and added to the above polymerization system. By using the specified partially saponified polyvinyl alcohol and heating the dispersion liquid once to above the melting point of the antioxidant, the partially saponified polyvinyl alcohol, dissolved in water, is changed into particulates which prevent the antioxidant particles from agglomerating. As a result, this makes it possible to obtain a liquid in which the antioxidant is dispersed homogeneously and stably without using an emulsifying agent. This invention makes it possible to automatically charge the antioxidant to a polymerization system. The resulting vinyl chloride polymer can be of high quality, e.g., high storage stability, high thermal stability and high plasticizer absorption performance. Also, the COD in waste water can be lowered, bringing about a great advantage in environmental sanitation. DETAILED DESCRIPTION OF THE INVENTION The antioxidant used in this invention may include, for example, phenol antioxidants, amine antioxidants, sulfur antioxidants, phosphorus antioxidants, etc., and particularly preferred are those having a melting point of not less than 30°C and less than 100°C. Examples of the antioxidant having such a melting point include p-methoxyphenol, t-butylhydroxyanisole (BHA), n-octadecyl-3-(4'-hydroxy-3',5'-di-t-butylphenyl)propionate, 3,5-di-t-butyl-4-hydroxytoluene (BHT), triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate], 1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-triazine, 2,2-thiodiethylene-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 2,2-thiobis-(4-methyl-6-t-butylphenol), etc., but are by no means limited to these. These antioxidants may be alone, or may be in combination of two or more. Among the above exemplary antioxidants, particularly preferred ones include triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate], n-octadecyl-3-(4'-hydroxy-3',5'-di-t-butylphenyl)propionate, t-butylhydroxylanisole, 3,5-di-t-butyl-4-hydroxytoluene, and 1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate. In the process of this invention, the suspending agent used for the preparation of the antioxidant dispersion is a water-soluble polyvinyl alcohol having a saponification degree of 60-79.5 mole % and a polymerization degree of 600 to 3,000. This is because those having a saponificition degree of less than 60 mole % can be dissolved in water with difficulty, and polyvinylalcohol having a saponification degree of more than 79.5 mole %, and in particular perfectly saponified polyvinyl-alcohols having a saponification degree of more than 98 mole %, tend to be gelled during storage. Polyethylene oxide, sodium polyacrylate, alginic acid, etc. may also be used in combination. The antioxidant dispersion used in this invention is prepared by feeding water to a dissolving tank equipped with a jacket for heating or cooling and a stirrer, adding the suspending agent and the powdery antioxidant, and dispersing them with stirring. In this occasion, after the antioxidant has been dispersed, the temperature is raised to a temperature which is not lower than the melting point of the antioxidant by heating with the jacket, followed by cooling when the dispersion was made sufficiently homogeneous. By virtue of this heating operation, the antioxidant dispersion can be obtained as an aqueous dispersion in which the antioxidant has been homogeneously dispersed. There is no particular limitation in the dispersing means used in the preparation of the above dispersion, and the stirring blade may have a general form such as a paddle, a propeller, a turbine, a Brumagin type, a Pfaudler type, etc. If necessary, it is also possible to use a homogenizer in combination, or there may be used a pressing high pressure pump, a colloid mill, high pressure ejection from a nozzle or orifice, ultrasonic wave, etc. Particles of the antioxidant dispersed in the dispersion may preferably have a particle size of 20 µm or less from the viewpoint of storage stability, and be in concentration of about 5 to 50% by weight. There is also no particular limitation in the concentration of the suspending agent, which may be suitably selected taking account of the storage stability and the readiness of being fed by a pump (in other words, the viscosity after formed as a dispersion), but usually may preferably be about 1 to 30 %. There is no particular limitation in the water used in preparing the dispersion, but preferred is to use distilled water or deionized water. The above antioxidant dispersion can be added at any time, i.e., before initiating the polymerization, during the polymerization or after completing the polymerization, depending on the purpose of the addition. Usually, the antioxidant dispersion may be added to the polymerization system before the initiation of polymerization for the purpose of controlling polymerization and thereby reducing fish eyes in resulting products, at the end of polymerization for the purpose of terminating the polymerization, and after completion of polymerization for the purpose of preventing degradation of resulting products. The antioxidant dispersion can be added once or more times to achieve one or more of the above purposes. In view of the readiness in operation, it may preferably be fed by means of a pump. The antioxidant dispersion may preferably be added in an amount (in terms of the antioxidant) of 0.001 to 1.0 % (by weight) based on the amount of the monomers to be fed. The amount less than 0.001 % makes it impossible to prevent the vinyl chloride resin from being deteriorated, and the amount more than 1.0 % can bring about no expectation of moreover improving the antioxidizing effect, resulting in not only an economical disadvantage but also an increase in COD in waste water. The process of this invention can be applied in the aqueous suspension polymerization of any of conventionally known vinyl chloride monomer or vinyl monomer mixtures containing vinyl chloride monomer. The vinyl monomers other than vinyl chloride may include, for example, α-olefins such as ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 1-undecene, 1-dodecene, 1-tridecene and 1-tetradecene, acrylic acid and esters thereof, methacrylic acid and esters thereof, maleic acid and esters thereof, vinyl compounds such as vinyl acetate, vinyl propionate and alkyl vinyl ethers, and a mixture of these. In the aqueous suspension polymerization, there is generally used 80 to 300 parts by weight of water based on 100 parts by weight of the monomers, and the polymerization is carried out usually at a temperature of 35 to 70°C. Usable polymerization initiators may include, for example, peroxydicarbonates such as diisopropyl peroxydicarbonate, dioctyl peroxydicarbonate, dilauryl peroxydicarbonate, dicetyl peroxydicarbonate, di-tert.-butyl peroxydicarbonate, di(2-ethoxyethyl)-peroxydicarbonate, di(2-methoxypropyl)peroxydicarbonate, dibenzyl peroxydicarbonate, dicyclohexyl peroxydicarbonate and di-tert.-butyl cyclohexyl peroxydicarbonate; azo compounds such as 2,2'-azobis-2,4-dimethylvaleronitrile and 2,2'-azobis-4-methoxy-2,4-dimethylvaleronitrile; peroxides such as tert.-butyl peroxyneodecanate, α-cumyl peroxyneodecanate, tert.-octyl peroxyneodecanate, tert.-butyl peroxypivalate, α-cumyl peroxypivalate, amyl proxypivalate, di-tert.-butyl oxalate and isobutyryl peroxide. These may be used in combination in order to make uniform the polymerization reaction rate. Dispersants used in the aqueous suspension polymerization may include, for example, polyvinyl alcohol, cellulose derivatives, a maleic anhydride/styrene copolymer, a maleic anhydride/methyl vinyl ether copolymer, gelatin, etc. Optionally, there may further be appropriately added polymerization regulators, chain transfer agents, gelation improvers, antistatic agents, pH adjustors, etc. EXAMPLES This invention will be described below in greater detail by way of Examples. Example 1 (1) Preparation of aqueous antioxidant dispersion: In a beaker of an internal volume of 300 cc, 210 g of deionized water, 15 g of polyvinyl alcohol having a saponification degree of 79.5 % and an average polymerization degree of 2,550 and 25 g of triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)-propionate] (m.p.: 80°C) were charged, and heated to 82°C with stirring, followed by cooling at the time they were homogeneously dispersed to obtain a homogeneous aqueous antioxidant dispersion. (2) Preparation of a vinyl chloride polymer by suspension polymerization: In a polymerization vessel made of stainless steel, having an internal volume of 100 lit., 60 kg of deionized water, 25 g of partially saponified polyvinyl alcohol and 15 g of di-2-ethylhexyl peroxydicarbonate were charged. After the inside of the vessel was evacuated up to 40 mmHg, 30 kg of vinyl chloride monomer were charged, and heated to 57°C with stirring, to carry out polymerization while keeping this temperature. At the time the pressure inside the vessel reached 6.5 kg/cm 2 G, 150 g of the aqueous antioxidant dispersion prepared in (1) were pressed into the vessel through a pump. Thereafter, while recovering the unreacted monomers, the temperature inside the vessel was raised, and maintained for 30 minutes at 80°C, followed by cooling. The resulting polymer slurry was dewatered and dried. Example 2 (1) Preparation of aqueous antioxidant dispersion: Example 1-(1) was repeated to prepare an aqueous dispersion, except that polyvinyl alcohol was used in an amount of 10 g and triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate] was used in an amount of 75 g. (2) Preparation of vinyl chloride polymer: Example 1-(2) was repeated to prepare a vinyl chloride polymer, except that used as an aqueous dispersion was 50 g of the one prepared in the above (1). Example 3 (1) Preparation of aqueous antioxidant dispersion: Example 1-(1) was repeated to prepare an aqueous dispersion, except that 35 g of polyvinyl alcohol having a saponification degree of 72.5 % and an average polymerization degree of 780 were used in place of 15 g of polyvinyl alcohol having a saponification degree of 79.5 % and an average polymerization degree of 2,550. (2) Preparation of vinyl chloride polymer: Example 1-(2) was repeated to prepare a vinyl chloride polymer, except that used as an aqueous dispersion was 162 g of the one prepared in the above (1). Comparative Example 1 (1) Preparation of antioxidant solution: In 100 g of methanol, 15 g of triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate] were dissolved. (2) Preparation of vinyl chloride polymer: Example 1-(2) was repeated to prepare a vinyl chloride polymer, except that the above methanol solution was used in place of the aqueous dispersion. Comparative Example 2 (1) Preparation of aqueous antioxidant dispersion: In a ball mill, 40 parts (by weight) of triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate], 57 parts of deionized water and 3 parts of sorbitan monolaurate were charged and ground to dispersion to prepare a homogeneous aqueous dispersion. (2) Preparation of vinyl chloride polymer: Example 1-(2) was repeated to prepare a vinyl chloride polymer, except that used as an aqueous dispersion was 37.5 g of the dispersion prepared in the above (1). The size of the dispersed particles, viscosity and storage stability of the antioxidant dispersions prepared in the above Examples and Comparative Examples were measured. The plasticizer absorption performance and initial color property of the resulting vinyl chloride polymers were measured. Also measured was COD in the waste water after polymerization. Results are shown in Table 1. The storage stability of the antioxidant dispersions and the initial color-free property of the vinyl chloride polymers were evaluated based on the standards shown below. In respect of the plasticizer absorption performance, the vinyl chloride polymers are left immersed in a prescribed amount of a plasticizer. Thereafter, the unrecovered plasticizer was separated by means of a centrifugal separator, and then the plasticizer absorption was determined from the increased weight of the vinyl chloride polymers to make evaluation. Storage stability of antioxidant dispersions: A: No settling occurs even after leaving for 24 hours. B: Settling occurs after leaving for 6 hours or more. C: Settling occurs within 1 hour. Initial color property of vinyl chloride polymers: Evaluation was made in comparison with a vinyl chloride polymer prepared in the same manner as in Example 1-(2) except that the aqueous dispersion of the antioxidant was not introduced. A: Superior B: Substantially equal C: Poorer

Claims: A process for preparing a vinyl chloride polymer by polymerizing vinyl chloride, or a vinyl monomer mixture containing vinyl chloride, through a suspension polymerization in an aqueous medium, wherein an antioxidant dispersion consisting of an antioxidant, a polyvinyl alcohol having a saponification degree of from 60 to 79.5 mol% and a polymerization degree of 600 to 3,000 and water is previously heated to a temperature which is not lower than the melting point of the antioxidant, followed by cooling, and added to the above polymerization system. The process according to claim 1, wherein the antioxidant has a melting point of not less than 30°C and less than 100°C. The process according to claim 1, wherein the antioxidant is at least one selected from triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate], n-octadecyl-3-(4'-hydroxy-3', 5'-di-t-butylphenyl)propionate, t-butylhydroxyanisole, 3,5-di-t-butyl-4-hydroxytoluene, and 1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate. The process according to claim 1, wherein the antioxidant in the dispersion has a concentration of about 5 to 50% by weight. The process according to claim 1, wherein said antioxidant dispersion is added to the system in an amount of 0.001 to 1.0% by weight in terms of antioxidant based on the amount of the monomers charged. The process according to claim 1, wherein said antioxidant dispersion is added to the polymerization system at at least one time of before initiating, during and after completing the polymerization. The process according to claim 1, wherein the vinyl monomers to be polymerized contain vinyl chloride and a vinyl monomer selected from the group consisting of α-olefins, acrylic acid, acrylates, methacrylic acid, methacrylates, maleic acid, maleates, vinyl acetate, vinyl propionate and alkyl vinyl ethers.

Inventors: JUNICHI WATANABE, TADASHI AMANO, JUNICHI, WATANABE, TADASHI, AMANO
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1988
Main classification: C08F14
IPC codes: C08F14, C08F2