GB2223022A

GB2223022A - Method for purifying polymer

Info

Publication number: GB2223022A
Application number: GB8920786A
Authority: GB
Inventors: Hisashi Miyakawa; Takanobu Shinohara; Tetsuo Maeda
Original assignee: Toyo Engineering Corp
Current assignee: Toyo Engineering Corp
Priority date: 1988-09-22
Filing date: 1989-09-14
Publication date: 1990-03-28
Anticipated expiration: 2009-09-14
Also published as: GB2223022B; KR950008562B1; KR900004774A; JPH0286604A; GB8920786D0; DE3931498A1

Description

4 1 1 ' X 2 223 0 2 2 METHOD FOR PURIFYING POLYMER The present invention

relates to a method for purifying a polymer, and more particularly a method for extracting volatile materials contained in a polymer in the molten state under high pressure, thereby removing them.

is As methods for removing volatile materials in a polymer, there are proposed, for example, in Japanese Patent PublicatiorsNos. 29245/1986 and 52163/1986, methods wherein a melted thermoplastic resin is processed in a vent extruder, or in Japanese Patent Application (OPI) No. 166506/1984, a method wherein a polymeric liquid composition is continuously devolatilized by a volatilization separator, to which a vertical bubbling-type preheater and a vacuum tank are directly connected. In recent years, a high-pressure extraction method that uses a supercritical fluid has been proposed, and Japanese Patent Publication No.

46972/1984 discloses, as a method for reducing oligomeric cyclic ethers contained in tetrahydrofuran polymers, alkylene oxides polymers, or tetrahydrofuran/alkylene oxides copolymers, a method wherein such a polymer is brought in contact with a gas that is in the supercritical state.

However, in the methods described in the above Japanese Patent Publicatiom Nos. 29245/1986 and 52163/1986 and Japanese Patent Application (OPI) No.

166506/1984, the reduction in the concentration of.

volatile materials remaining in the processed polymer is approximately to the order of, at the most, 400 ppm, and these methods cannot meet the market demand where the concentration of volatile materials is required to be lower than 400 ppm. Further, although the method described in Japanese Patent Publication No. 46972/1984 is a method for extracting impurities from a polyme.r in the liquid state at ordinary temperatures under high pressure, thereby removing them, the method cannot be applied to high-degree extraction wherein the concentration of impurities remaining in a polymer is required to be reduced to or below a certain concentration (thousands ppm).

Therefore, the first object of the present invention is to provide a method for purifying effectively a polymer in the molten state.

2 z. 1 The second object of the present invention is to provide a method for purifying a polymer by extracting volatile materials from the polymer in the molten state with the degree of extraction being very high.

Other and further objects, features, and advantages of the invention will appear more fully from the following description.

The inventors have studied intensively to overcome the above problems related to the purification of polymers, and have found that by bringing a polymer in the molten state in contact with an extractant, the removal of volatile materials can be performed effectively to reduce the cpncentration of volatile materials remaining in the polymer to or below approximately 100 ppm, leading to completion of the invention.

The present invention provides a method for purifying a polymer comprising treating the polymer with an extractant under high pressure, to remove volatile materials contained in the polymer, the polymer being brought in to contact in the molten state with the extractant.

is In carrying out the present method, preferable polymers are thermoplastic molding resins that will be melt ed by heating but will be solidified at room temperature, for example, polyethylene, polypropylene, polystyrene, acrylonitrile/styrene copolymer, acrylonitrile/butadiene/styrene terpolymer, polyvinyl acetate, polyacrylate, polymethacrylate, polyvinyl chloride, polyvinylidene chloride, fluoroplastic, polyacrylonitrile, polyvinyl ethers, polyvinyl ketones, polyethers, thermoplastic polyesters, polyamides, dienetype plastics, and polyurethane plastics, and as heat resistant polymers polyxylylene, polycarbonate, polyphenylene oxide, and polysulfone.

Herein by "polymer in the molten state" is meant, for example, a bulk polymeric liquid composition of the above polymer, a melt obtained by heating a polymer in a powder or pellet form to be melted, or a mixture of a bulk polymeric liquid composition with a melt obtained by heating a polymer in the form of a powder or pellets to be melted.

The extractant to be used in the present invention is selected suitably for the polymer in the molten state that will be subjected to extraction, and as specific examples can be mentioned (a) carbon dioxide, dinitrogen oxide, carbon disulfide, aliphatic hydrocarbons such as ethaner 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. For reference, critical constants, etc. of the mentioned typical extractants are shown in Table 1.

r_ Table 1 Critical Constants Substance Critical Critical Boiling Temperature Pressure Point Tc (OC) Pc (atm) b.p. (OC) 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 Note: sublimation 1 1;. G_ 1 The extractant used in the present invention is in the liquid state or supercritical state, and it is preferable to use it in the supercritical state. When it is taken into consideration that the extractant is to be separated from the purified polymer, it is preferable to use an extractant that will be gaseous at normal temperatures and pressures or an extractant that has a boiling point of 1000C or below at normal pressures, and particularly preferably the extractant is used in the supercritical state. When the extractant is used in the supercritical state, although the pressure and temperature in the extractor may vary depending on the type of polymer to be purified and the type of extractant, generally the pressure is preferably 20 to 500 kg/CM2, and the temperature is preferably at or over the melting temperature of the polymer to be purified. In this case, although the temperature in the extractor may be at or over the melting temperature of the polymer and below the critical temperature of the extractant, more preferably the temperature in the extractor is at or over the critical temperature of the extractant.

In the present invention the ratio of the molten polymer in the extractor to the extractant to be introduced into the extractor in weight ratio is in a range of 1:20 to 1:400 preferably, and 1:50 to 1:300 r7 1 more preferably. If the extractant is too little, the sufficient efficiency of extraction cannot be attained. If the extractant is too much, the energy consumed for the compression of extractant or the like increases so much due to increased volume of extractant, with less enhancement of the extraction efficiency.

Volatile materials that are subject to extraction in the present invention are volatile impurities contained in the polymer in the melted state, such as unreacted monomers, solvents, and oligomers.

In carrying out the present invention, although the extractor to be used is suitably of a packed tower type, tray tower type, spray tower type, or tank type, there is no particular restriction on the type of the extractor if the extractor secures good contact of the polymer in the molten state that will be subjected to extraction with the extractant to be used. Multiple extractors may be arranged in parallel to effect the extraction continually, or they may be arranged in series, thereby flowing the polymer and the extractant, for example, in a countercurrent manner, to effect extraction continually.

Further, the melt of the polymer obtained by bulk polymerization or solution polymerization that has been roughly devolatilized (freed from the solvents), or is the melt of the polymer obtained by emulsion polymerization or suspension polymerization that has been dehydrated (freed from the liquid medium) may be directly charged into the extraction tank, or it may be once solidified to pellets or the like and charged into the extraction tank directly, or after melting the solid.

According to the present method, a polymer in the molten state can be purified effectively, and a polymer in the molten state can be extracted of volatile materials to a very high degree. The molded product of the polymer obtained by the present method can well meet the severe demand of the market that restricts the concentration of remaining volatile materials.

Now the present invention will be described in more detail with reference to Examples and a Comparative Example. Example 1 g of polystyrene, in the form of pellets having a concentration of volatile materials of 450 ppm, was charged into an extractor having a cylindrical space with an inner diameter of about 46 mm and a height of about 87 mm (with an inner volume of 145 me), and heated to 1500C to melt the pellets, and then, as an extractant, carbon dioxide was continuously supplied q from the bottom of the extractor, to increase gradually the pressure in the extractor. Aftert ultimately, the pressure in the extractor was brought to 245 kg/CM2G and the temperature was brought to 1500C, with these conditions kept, carbon dioxide was supplied 1 continuously at a rate of 80 g/min, and at the same time carbon dioxide and the extract were released from the top of the extractor over 1 hour. Then the supply of the carbon dioxide and the heating were stopped. The release was continued, and then after the pressure in the extractor reached atmospheric pressure, the heating was stopped, and after the extractor was cooled to allow the temperature in the extractor to reach 300C or below, the polystyrene was removed for analysis. The result of the analysis showed that the concentration of volatile materials decreased to 50 ppm. Comparative Example 1 This example was carried out using the same polystyrene in the form of pellets as in Example 1.

That is, 20 g of polystyrene, in the form of pellets having a concentration of volatile materials of 450 ppm, was charged into an extractor, carbon dioxide was supplied from the bottom as an extractant, and the pressure in the extractor was increased gradually. After, ultimately, the pressure in the extractor was 1 1 brought to 245 kg/CM2G and the temperature was brought to 50C, with these conditions kept, carbon dioxide was supplied continuously at a rate of 80 g/min, and at the same time carbon dioxide and the extract were released from the top of the extractor over 4 hours. Then the supply of the carbon dioxide and the heating were stopped. The release was continued, and then after the pressure in the extractor reached atmospheric pressure and the temperature in the extractor reached 300C or below, the polystyrene was removed for analysis. The result of the analysis showed that the concentration of volatile materials was 410 ppm. Example 2 g of a polystyrene polymeric liquid in the melted state having a concentration of volatile materials of 25,000 ppm was charged into an extractor similar to that in Example 1, then carbon dioxide was supplied as an extractant, and thereafter, similarly to Example 1, extraction was effected, except that the pressure and the temperature in the extractor were kept at 245 kg/CM2G and 2200C, respectively. After extraction the temperature in the extractor was brought to 300C or below and the polystyrene was removed and analyzed. The result of the analysis showed that the concentration of volatile material was 90 ppm.

11 4 Though the devolatilized polymers were taken out of the extractor in solidified state in the above examples, it may be practical to make the devolatilized molten polymer into pellet form in an industrial application, by passing the molten polymer through an extruder for pelletization. In such a case it is often recommended to.pass the molten polymer through a flash tank or the like to degas the polymer before it is extruded for a good operation of the extrusion or further processes. Alternatively, it may be practical to take out after the extraction the devolatilized molten polymer in a molten state for the next processing step.

Having described our invention as related to the embodiment, it is our intention that the invention be not limited by any of the details of the description, unless otherwise specified, but rather be construed broadly within its spirit and scope as set out in the accompanying claims.

12 A 4 1

Claims

CLAIMS:

1. A method for purifying a polymer comprising treating the polymer with an extractant under high pressure to remove volatile materials contained therein, the polymer being brought into contact in a molten state with the extractant.

2. A method as claimed in claim 1, wherein the 10 polymer is a thermoplastic resin.

3. A method as claimed in claim 1 or claim 2 wherein the polymer in the molten state is a bulk polymeric liquid composition of a polymer, a melt obtained by heating a polymer in a powder or pellet form or a mixture made up thereof.

4. A method as claimed in any one of the preceding claims wherein the extractant is selected 20 from carbon dioxide, dinitrogen oxide, carbon disulfide, an aliphatic hydrocarbon, a halogenated hydrocarbon, an aromatic hydrocarbon, and a mixture of two or more of said extractants.

5. A method as claimed in any one of the preceding claims, wherein the extractant is used in the liquid state.

6. A method as claimed in any one of claims to 4 wherein the extractant is used in the supercritical state.

7. A method as claimed in any one of the preceding claims, wherein the pressure in the extractor is in a range of 20 to 500 kg/cm2G.

a 1 1

8. A method as claimed in any one of the preceding claims, wherein the temperature in the extractor is the critical temperature of the extractant or higher than the critical temperature.

9. A method as claimed in any one of the preceding claims, wherein the volatile material to be extracted is an impurity such as an unreacted monomer, solvent, or,an oligomer.

10. A method as claimed in any one of the preceding claims, wherein as the extractor plural extractors are arranged in parallel.

11. A method as claimed in any one of claims 1 to 9, wherein as the extractor plural extractors are arranged in a series.

12. A method as claimed in any one of the preceding claims, wherein the extraction is carried out in a countercurrent manner of the polymer flow and the extractant flow.

13. A method as claimed in claim 1 substantially as hereinbefore described in Example 1 or Example 2.

14. A polymer when purified by the method as claimed in any one of the preceding claims.

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