GB2242434A

GB2242434A - Purifying a polymer melt using high pressure

Info

Publication number: GB2242434A
Application number: GB9106008A
Authority: GB
Inventors: Hisashi Miyakawa; Takanobu Shinohara; Tetsuo Maeda
Original assignee: Toyo Engineering Corp
Current assignee: Toyo Engineering Corp
Priority date: 1990-03-22
Filing date: 1991-03-21
Publication date: 1991-10-02
Anticipated expiration: 2011-03-21
Also published as: KR910016783A; GB2242434B; DE4109527A1; GB9106008D0; JPH03273006A

Abstract

A method for purifying a polymer by treating the polymer with an extractant under high pressure to remove volatile materials contained therein comprises contacting the polymer in a molten state with the extractant in a ratio of polymer to extractant by weight in a range of from 1 : 0.1 to 1: a value less than 20. Exemplified is the purification of polystyrene using carbon dioxide.

Description

j A METHOD FOR PURIFYING A 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.

As methods for removing volatile materials in polymers there have been proposed, for example, in Japanese Patent Publications Nos. 29245/1986 and 52163/1986, methods wherein a molten thermoplastic resin is processed in a vent extruder, and in Japanese Patent Application (OPI) No. 166506/1984, a method wherein a polymeric liquid composition is 15 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 which 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 oxide polymers, or tetrahydrofuran/alkylene oxide copolymers, a method wherein such a polymer is brought into contact with a gas which is in the supercritical state.

However, in the methods described in the above Japanese Patent Publications 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 of 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 polymer 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, a first object of the present invention is to provide a method for purifying effectively a polymer in the molten state.

A 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 wherein the degree of extraction is very high.

Other and further objects, features, and advantages of the invention will appear more fully from the following detailed description taken in conjunction with the accompanying drawing.

The inventors of the present Patent Application 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 into contact with an extractant, the removal of volatile materials can be performed effectively to reduce the concentration of volatile materials remaining in the polymer to or below approximately 100 ppm.

The present invention provides a method for purifying a polymer by treating the polymer with an extractant under high pressure, thereby removing volatile materials contained therein which method comprises contacting the polymer in a molten state with the extractant in a ratio of polymer to extractant by weight in a range of from 1: 0.1 to 1: a value less than 20.

1 PI 1 11 In carrying out the method of the present invention, preferred polymers are thermoplastic molding resins which can be melted and fluidized by heating but are solidified at room temperature, for example, polyethylene, polypropylene, polystyrene, acrylonitrile/styrene copolymer, acrylonitrile/ butadiene/styrene terpolymer, polyvinyl acetate, polyacrylate, polymethylacrylate, polyvinyl chloride, polyvinylidene chloride, fluoroplastic, polyacrylonitrile, polyvinyl ethers, polyvinyl ketones, polyethers, thermoplastic polyesters, polyamides, diene-type plastics, and polyurethane plastics, and as heat resistant polymers polyxylylene, polycarbonate, polyphenylene oxide, and polysulfone.

Herein by llpolymer 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 disulfidet 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. For reference, critical constants of typical extractants are shown in Table 1.

1 Table 1 Critical Constants Critical Critical Boiling Substance Temperature Pressure Point Tc (OK) Pc (atm) b.p.(OK) Carbon dioxide 304.2 73.0 194.7 Dinitrogen oxide 309.8 71.7 184.2 Carbon disulfide 549.4 76.0 319.4 Ethane 305.4 48.3 184.6 Ethylene 282.8 50.5 169.4 Propane 369.7 42.0 231.1 Butane 425.6 37.5 273.0 Pentane 470.0 33.3 309.2 Hexane 507.9 29.9 341.9 Benzene 562.0 47.7 353.3 Toluene 593.1 41.6 383.7 0-Xylene 631.1 36.9 417.5 Methanol 513.0 78.7 337.8 Ethanol 515.8 63.1 351.5 Note: sublimation 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: 0.1 to 1: a value less than 20. In the case of a continuous method in which usually a counter-current mode is employed, the ratio is preferably 1: 1 to 1: a value less than 20, 2 i 1 particularly preferably 1: 1 to 1: 15. If too little extraptant is used, sufficient efficiency of extraction cannot be attained. If too much extractant is used the energy consumed in compression of the extractant increases greatly due to the increased volume of extractant, to the detriment of extraction efficiency.

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 1500C 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 0.8 to 10 times, more preferably 1.0 to 5 times, the critical pressure of the extractant; and is preferably 20 to 500 kg/CM2G, more preferably 100 to 350 kg/CM2G. The temperature is usually 0.9 to 2.0 times the critical temperature of the extractant or the melting temperature of the polymer to be purified and is preferably 80 to 2500C, more preferably, 150 to 2200C. The upper limit temperature is that of causing deterioration of polymer. 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.

Volatile materials that are subject to extraction in the present invention are volatile impurities contained in the polymer in the molten 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 a polymer obtained by bulk polymerization or solution polymerization that has been roughly devolatilized (freed from the solvents), or the melt of a 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 firstly solidified to pellets or the like and charged into the extraction tank directly, or after melting the solid. An embodiment of the invention will be described with reference to the drawing in which: Fig. 1 is a flow diagram showing an embodiment of the invention which utilizes a packedtower type extractor. 35 In Fig. 1, reference numeral 1 indicates a polymerization reactor provided with an agitator.

- i Monomer to be polymerized is fed into the reactor 1 through line 10 and polymerized therein continuously by a known method. The polymer product is continuously drawn out of the reactor with accompanying volatile matter, passes through line 12 and heat exchanger 2 to elevate the temperature of the polymer suitable for devolatilization and is then fed to a devolatilizer 3 through a line 13 to evaporate the volatile material contained in the polymer by a conventional method. If the amount of volatile matter in the polymer is low enough taking into account the concentration in the extracted product,the devolatilizer 3 may be omitted or bypassed.

Generally, the concentration of volatile impurity in the thus devolatilized polymer ranges from about 400 ppm to a few thousand ppin.

Referring again to Fig. 1, conventionally devolatilized polymer is fed, through line 15, to pump 4 to elevate the pressure of the molten polymer, and then through line 16, to an extractor 5, whose pressure has been boosted to a pressure suitable for extraction by a pressure booster 7.

The pressure booster 7 is a compressor when the extractant is in the gas phase or a pump when the extractant is in the liquid phase. Molten polymer is fed continuously, through line 16, to the upper part of the extractor 5, which is kept at a preselected pressure and temperature, and flows down therethrough towards the bottom thereof and the extractant is fed continuously through line 19 to the lower part of the extractor by the booster 7, flows up, counter currently and in contact with the molten polymer flowing down, to the top of the extractor and is drawn out continuously therefrom through line 17.

During contact with the extractant, volatile is impurities are extracted from the molten polymer, which, after contact for extraction with the extractant, is taken out of the bottom of the extractor through line 20 and fed to the following 5 process.

Through line 17 ext ractant containing the extracted volatile impurities is fed to a separator 6 wherein volatile impurities in the extractant are separated therefrom by applying a pressure reduction to and/or elevating the temperature of the extractant (not shown). The method of separation applied may be a conventional method, for example, liquidifying separation, separation by a distillation column, or separation by an absorption tower.

The extractant thus purified in the separator 6 is recycled to the bottom of the extractor 5 through line 18, line 26, pressure booster 7 and line 19. If needed, make-up extractant is fed to the extractor through line 26, pressure booster 6 and line 19.

Volatile matter separated in the separator 6 which may accompany the extractant in the gas phase is drawn out through line 21, and, in this example, flows into line 14 for the volatile matter from the devolatilizer 3 and the volatile material from both are fed to a process, not shown, through line 21, and, in this example, flows into line 14 for the volatile matter from the devolatilizer 3 and the volatile material from both are fed to a process, not shown, through line 11 for recovery of unpolymerized monomers.

According to the present invention 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 meet well the severe demand of the market that i 11 restricts the concentration of remaining volatile materials.

Embodiments of the present invention will now be described in more detail with reference to the Examples and the Comparative Example.

Example 1

Molten polystyrene, at a temperature of 2200C, having a concentration of volatile materials of 2,500 ppm was pressurized to 245 kg/cm2G by a gear pump and fed to the upper part of a packed tower type extractor, kept at 245 kg/cm2G, continuously at a rate of 1 kg/hr. The molten polystyrene flowed down towards the bottom of the tower therethrough fed from a shower head provided at the upper part of the tower and put in contact contercurrently, with carbon dioxide as the extractant. The height of the extractor (contact zone) was 5 metres.

Carbon dioxide was fed to the bottom of the extractor at et rate of 5 NM3/hr which amount corresponded to 10 times by weight of the molten polystyrene fed as mentioned above.

Molten polystyrene and carbon dioxide were drawn out of the extractor at the bottom and top thereof, respectively, at the same rates as the supply rates.

The concentration of volatile materials in the polystyrene discharged from the extractor tower as above was 40 ppm.

Comparative Example 1 Molten polystyrene the same as that in Example 1 was processed in the same way as that of Example 1 except that the feed rate of carbon dioxide was changed. That is, a molten polystyrene having a temperature of 2200C was charged from the top of extractor (height: 5 metres) kept at 245 kg/cm2 of pressure, continuously at a rate of 1 kg/h, and carbon dioxide gas was fed from the lower part of the extractor at a rate of 25 1/h. herein the weight ratio of polymer to extractant corresponded to 1: 0.05.

The flow out of the lower part of the extractor was analyzed and it was found that the concentration of 5 volatile materials was 2,100 ppm. Example 2 Polystyrene, in the form of pellets having a concentration of volatile material of 400 ppm, was heated at 2200C and the molten polystyrene was pressurized to 200 kg/cm2G by a gear pump and fed to the upper part of a packed tower type extractor, kept at 200 kg/cm2G, continuously at a rate of 1 kg/hr. The molten polystyrene flowed down through the tower towards the bottom fed from a shower head provided at the upper part of the tower and put in contact counter-currently, with carbon dioxide as the extractant. The height of the extractor (contact zone) was 5 metres.

Carbon dioxide was fed to the bottom of the extractorant a rate of 250 N1/hr which amount corresponded to 0.5 times by weight of the molten polystyrene feed as mentioned above.

Molten polystyrene and carbon dioxide were drawn out of the extractor at the bottom and top thereof, respectively, at the same rate as the supply.

The concentration of volatile materials in the polystyrene discharged from the extractor tower as above was 100 ppm.

Having described the invention in relation to specific embodiments, it is the Applicant's 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.

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Claims

1. A method for purifying a polymer by treating the polymer with an extractant under high pressure to remove volatile materials contained therein which method comprises contacting the polymer in a molten state with the extractant in a ratio of polymer to extractant by weight in a range of from 1: 0.1 to 1 a value less than 20.

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

3. A method as claimed in claim 1 or claim 2 is 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 thereof.

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

5. A method as claimed in any one of the preceding claims wherein the extraction is carried ou in a counter-current manner of polymer flow and 30 extractant flow.

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

7. A method as claimed in any one of the preceding claims wherein the extractant is used in the supercritical state.

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

9 A method as claimed in any one of the preceding claims wherein the pressure in the extractor is in the range of 0.8 to 10 times the critical pressure of the extractant.

10. A method as claimed in any one of the preceding claims wherein the weight ratio of the polymer to the extractant is in a range of 1: 1 to 1 a value less than 20 in a continuous process in which a counter-current mode is employed.

11. 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.

12. A method as claimed in any one of the preceding claims wherein the temperature in the extractor is in the range of 80 to 2500C.

13. 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.

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

1 Y

15. A method as claimed in any one of claims 1 to 13 wherein as the extractor plural extractors are arranged in series.

16. A method as claimed in claim 1 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.

17. A method as claimed in claim 1 substantially 10 as hereinbefore described in Example 1 or Example 2.

Published 1991 at The Patent Office. Concept House. Cardiff Road. Newport. Gwent NP9 IRE. Further copies may be obtained from Sales Branch. Unit 6. Nine Mile Point. Cxxmfelinfach. Cross Keys. Newport. NPI 7HZ. Printed by Mukiplex techniques ltd, St Mary Cray. Kent-