Method for extraction of impurities in polymer

Abstract: There is disclosed a method for extraction of impurities in polymer dispersion which comprises bringing an extractant in contact with a polymer dispersion under a high pressure, wherein the weight ratio of the polymer to the extractant is in a limited range. According to the disclosure, a dispersion containing a polymer can be purified efficiently to the degree to meet the market demand without damaging the basic performance of the raw material.

Description: FIELD OF THE INVENTION The present invention relates to a method for purifying a polymer. BACKGROUND OF THE INVENTION As a process for removing volatile substances in a polymer, such as unreacted monomer and solvents, there are, for example, a process in which an aqueous dispersion of a polymer is subjected to stripping with air or an inert gas, and a process in which an extruder with a vent, a vacuum evaporator, or the like is used. Of the above prior techniques, as a process for purifying a polymer by stripping its aqueous dispersion, for example, a process described in Japanese Patent Application (OPI) No. 41387/1978 can be mentioned. This process can reduce the concentration of volatile substances remaining in polystyrene to about 350 to 650 ppm, and the volatile substances still remaining in the processed polystyrene are removed using an extruder with a vent. However, although even an extruder with a vent is used, there is such a limit that at most the concentration of volatile substances remaining in the polystyrene is reduced to about 240 to 350 ppm, and it is difficult for the process to satisfy the market demand for a product that has been purified to a higher level. BRIEF SUMMARY OF THE INVENTION The first object of the present invention is to provide a method for purifying efficiently a polymer. The second object of the present invention is to provide a method for removing impurities remaining in a polymer to an extent that can meet the market demand. The third object of the present invention is to provide a method for removing impurities to purify a polymer without damaging the basic performances of the polymer. Other and further objects, features, and advantages of the invention will appear more fully from the following description. DETAILED DESCRIPTION OF THE INVENTION The inventors have studied intensively to overcome various problems in the purification of polymers mentioned above, and have found that when a dispersion of a polymer is brought in contact with a prescribed amount of an extractant under a high pressure, the removal of volatile substances can be performed quite efficiently, and the concentration of volatile substances remaining in the dispersion containing the polymer can be reduced to a level of about 100 ppm or below, leading to the present invention. That is, the present invention provides a method for purifying a polymer, characterized in that an extractant is brought in contact with a dispersion containing a polymer, with the weight ratio of the polymer in the dispersion to the extractant being in the range of from 1 : 20 to 1 : 400 and especially 1 : 40 to 1 : 400, thereby carrying out an extraction under a pressure of from 20 to 500 kg/cm². The polymer dispersion to be processed by the method of the present invention may be either a homogeneous or heterogeneous dispersion of a polymer in a dispersion medium, but it preferably is a heterogeneous dispersion system containing a polymer dispersed phase in a liquid; for example a dispersion obtained from an emulsion polymerization process or from a suspension polymerization process. To be concrete, latex, coating compositions, adhesives, flocculants, processing agents, and the like, containing polystyrene, acrylonitrile/styrene copolymer, acrylonitrile/butadiene/styrene terpolymer, polymethacrylate, polyvinyl chloride, polyvinylidene chloride, styrene/butadiene, or polybutadiene can be mentioned. As a polymer, thermoplastic polymers are preferable. It is preferable to carry out the extraction process for the polymer dispersion after polymerization before preparation step of the final product. Examples of a dispersion medium include water, aliphatic hydrocarbons (e.g., pentane and hexane), aromatic hydrocarbons (e.g., benzene and toluene), and alcohols (e.g., methanol). In polymerization processes, dispersion medium is an indispensable material. For example, in a suspension polymerization process monomer dispersed in an aqueous phase is used for polymerization to obtain polymer in form of small particles. In the solution polymerization process for producing polypropylene, propylene as raw material is dissolved in n-hexane and mixed with catalyst, and thereafter polymerization reaction is proceeded. In the case of synthetic rubber, for example, butadiene is dissolved in toluene and mixed with dispersed catalyst for the polymerization reaction. Polymers obtained by polymerization processes are processed by the method of the present invention to extract the volatile substances in the dispersion. Although the mechanism of extraction according to the present invention has not been made clear yet, it is presumed as follows. After polymerization, substances such as unreacted monomer remain not only in the dispersion medium but also in the polymer in an equilibrium amount. When the residual monomer in the dispersion medium is removed by extraction, the residual monomer in the polymer diffuses to the dispersion medium to establish a new equilibrium state successively, resulting the decrease of residual monomer in the polymer. The dispersion medium is usually in a state of liquid. In case the evaporation of dispersion medium may affect to the quality of dispersion and/or polymer, for example, to the particle size of latex. The extraction condition and/or the extractant are usually selected so that the evaporation of dispersion medium is suppressed, or an additional dispersion medium is added to compensate the loss of evaporation in advance to the extraction. The particle size of the polymer dispersion changes depending on, for example, the use of polymer, and usually is in a range of 0.01 µm to 5 mm. In the case of latex, it is in a range of 0.1 µm to 5 µm. Although the extractant used in the present invention may be selected suitably for the dispersion containing a polymer that will be subjected to extraction, as specific examples of the extractant, (a) carbon dioxide, dinitrogen oxide, carbon disulfide, aliphatic hydrocarbons, such as ethane, ethylene, propane, butane, pentane, and hexane, halogenated hydrocarbons, aromatic hydrocarbons, such as benzene, toluene, and xylene, and alcohols, such as methanol and ethanol, and (b) a mixture of two or more of these, can be mentioned. For reference, critical constants of the mentioned typical extractants are shown in Table 1. Critical Constants Substance Critical Temperature Tc (°C) Critical Pressure Pc (atm) Boiling Point b.p. (°C) Carbon dioxide 31.1 73.0 -78.5 Dinitrogen oxide 36.5 71.7 -90.7 Carbon disulfide 273.0 76.0 46.3 Ethane 32.4 48.3 -88.6 Ethylene 9.7 50.5 -169 Propane 96.8 42.0 -187.7 Butane 152.0 37.5 -138.4 Pentane 196.6 33.3 -129.7 Hexane 234.7 29.9 -95.3 Benzene 288.5 47.7 80.1 Toluene 320.6 41.6 110.8 o-Xylene 358.4 36.9 144 Methanol 240.0 78.7 64.6 Ethanol 243.1 63.1 78.4 In the present invention, preferably the extractant is used in a supercritical state or in subcritical state, more preferably in a supercritical state. Although the pressure and temperature in the extractor may vary depending on the type of polymer dispersion to be purified and the type of extractant to be used, generally the pressure is preferably 40 ∼ 350 kg/cm². The temperature may be below or over the softening point of the polymer to be purified. If the pressure is too low, the extraction ability of the extractant for volatile substances remaining in the dispersion containing a polymer becomes low, and sometimes an adequate extraction efficiency cannot be secured. If the pressure is too high, great improvement of the extraction efficiency cannot be expected, and the cost of the pressure apparatus and the energy required for compressing the extractant increase, which is not economical. Volatile substances that are to be extracted in the present invention are volatile impurities mentioned above, such as the unreacted monomer, solvents, and oligomers contained in the dispersion containing the polymers. In the practice of the present invention, the extractor to be used is preferably a tank-type extractor, and, although, for example, a packed tower-type, tray tower-type, or spray tower-type extractor is suitable, there is no particular limitation if the extractor is one wherein there is good contact between the dispersion containing a polymer to be subjected to extraction and the extractant to be used. Plural extractors may be arranged in parallel to carry out the extraction continuously, or they may be arranged in series, thereby flowing the polymer dispersion and the extractant, for example, in a countercurrent manner, to effect extraction continuously. The extraction operation is carried out in the usual manner, except the above described points. The weight ratio of the polymer in the dispersion supplied to the extractor to the extractant is 1 : 20 to 1 : 400, and preferably 1 : 50 to 1 : 300. If the ratio of the polymer to the extractant is 1 : less than 20, an adequate extraction efficiency cannot be secured, whereas if the ratio is 1 : over 400, the improvement of the extraction efficiency is less, but since the amount of extractant to be used increases, the amount of energy used, for example, for compressing the extractant, increases. The effects of the present invention are listed below. (1) In comparison with the prior stripping process, or the prior process that uses an extruder with a vent, or an evaporator, a dispersion containing a polymer can be purified efficiently. (2) In order to reduce the concentration of residual volatile substances to the same degree as that of the present invention by using an extruder with a vent, it was necessary to operate for a longer time and at a higher temperature of melted resin which all affected to damage the basic properties of the polymer material. To the contrary, in the present invention the purification can be carried out without damaging the basic performances of the polymer material, such as the appearance, the color shade, and the physical properties. (3) A polymer including a quite small amount of residual volatile substances can be obtained, which can well meet the market demand, whose restriction on the concentration of remaining volatile substances is very severe. Now the present invention will be described in more detail with reference to Examples and a Comparative Example. Example 1 40g of an aqueous dispersion containing 50 wt.% of polystyrene, obtained by a usual suspension polymerization process (the concentration of volatile substances in the polystyrene being 2,300 ppm) was placed in an autoclave having a volume of 190 cm³. Carbon dioxide was supplied into the autoclave at a rate of 40 g/min, to increase the pressure in the autoclave, with the temperature being kept at 80°C. After the temperature and the pressure in the autoclave were kept at 80°C and 250 kg/cm² respectively for 2 hours, the supply of carbon dioxide was stopped. The weight ratio of the polystyrene in the aqueous dispersion supplied into the autoclave to the extractant was 1 : 265. After dehydrating, the polystyrene was analyzed, and it was found that the concentration of volatile substances was 10 ppm or less. Example 2 200 g of an aqueous dispersion containing 50 wt.% of acrylonitrile/styrene copolymer obtained by a usual emulsion polymerization process (the concentration of volatile substances being 5,000 ppm) was placed in an autoclave having a volume of 1 liter. Carbon dioxide was supplied into the autoclave at 20 g/min, to increase the pressure in the autoclave, with the temperature being kept at 60°C. After the temperature and the pressure in the autoclave were kept at 60°C and 250 kg/cm² respectively for 5 hours, the supply of carbon dioxide was stopped. The weight ratio of the acrylonitrile/styrene copolymer in the aqueous dispersion supplied into the autoclave to the extractant was 1 : 66. The aqueous dispersion containing the acrylonitrile/styrene copolymer in the autoclave was analyzed, and it was found that the concentration of volatile substances was 70 ppm. Comparative Example The experiment was carried in the same manner as Example 1 except that the ratio of extractant was changed changed as described below. 200 g of an aqueous dispersion containing 50 wt.% of polystyrene (the concentration of volatile substances being 2,300 ppm) was placed in an autoclave. Carbon dioxide was supplied into the autoclave at 10 g/min, to increase the pressure in the autoclave, with the temperature being kept at 80°C. After the temperature and the pressure in the autoclave were kept at 80°C and 250 kg/cm² respectively for 2 hours, the supply of carbon dioxide was stopped. The weight ratio of the polystyrene in the aqueous dispersion supplied into the autoclave to the extractant was 1 : 17. After dehydrating, the polystyrene was analyzed, and it was found that the concentration of volatile substances was 500 ppm.

Claims: A method for extracting of impurities in a polymer, the improvement comprising bringing an extractant in contact with a dispersion containing a polymer to be purified, with the weight ratio of the polymer in the dispersion to the extractant being in the range of from 1 : 20 to 1 : 400 under a pressure of from 20 to 500 kg/cm². The method as claimed in claim 1, wherein the polymer dispersion is a homogeneous dispersion system of a polymer in a medium. The method as claimed in claim 1 or 2, wherein the polymer dispersion is a heterogeneous dispersion system of a polymer in a medium. The method as claimed in claim 3, wherein the heterogeneous dispersion system of a polymer is obtained from an emulsion polymerization process or a suspension polymerization process. The method as claimed in any of the preceding claims, wherein the polymer is selected from polystyrene, acrylonitrile/styrene copolymer, acrylonitrile/butadien/styrene terpolymer, polymethacrylate, polyvinyl chloride, polyvinylidene chloride, styrene/butadiene, or polybutadiene. The method as claimed in any of the preceding claims, wherein the extractant is selected from the group consisting of carbon dioxide, dinitrogen oxide, carbon disulfide, an aliphatic hydrocarbon, a halogenated hydrocarbon, an aromatic hydrocarbon, and a mixture made up thereof. The method as claimed in any of the preceding claims, wherein the extractant is used in the supercritical state. The method as claimed in any of the preceding claims. wherein the temperature in the extractor is in a range of from below to over the softening point of the polymer to be purified. The method as claimed in any of the preceding claims, wherein the volatile material to be extracted is selected from unreacted monomers, solvents or oligomers. The method as claimed in any of the preceding claims. wherein as the extractor a tank-type extractor is used. The method as claimed in any of the preceding claims. wherein as the extractor plural extractors are arranged in parallel. The method as claimed in any of claims 1 to 10, wherein as the extractor plural extractors are arranged in series. The method as claimed in any of the preceding claims, wherein the weight ratio of the polymer in the dispersion to the extractant is in the range of from 1 : 50 to 1: 300.

Inventors: MAEDA TETSUO, MIYAKAWA HISASHI, SHINOHARA TAKANOBU, MAEDA, TETSUO, MIYAKAWA, HISASHI, SHINOHARA, TAKANOBU
Applicants: TOYO ENGINEERING CORP, TOYO ENGINEERING CORPORATION
Application year: 1989
Main classification: C08F6
IPC codes: B01D11, C08F6