GB2245273A

GB2245273A - Method for extracting impurities from a polymer dispersion

Info

Publication number: GB2245273A
Application number: GB9112792A
Authority: GB
Inventors: Hisashi Miyakawa; Takanobu Shinohara; Tetsuo Maeda
Original assignee: Toyo Engineering Corp
Current assignee: Toyo Engineering Corp
Priority date: 1990-06-21
Filing date: 1991-06-13
Publication date: 1992-01-02
Anticipated expiration: 2011-06-13
Also published as: KR0163354B1; GB2245273B; DE4120437A1; JPH0453804A; KR920000800A; GB9112792D0

Abstract

A method for extraction of impurities from a polymer dispersion comprises bringing into contact under high pressure an extractant and a polymer dispersion wherein the weight ratio of the polymer to the extractant is in a limited range (1:01 to 20), whereby the dispersion containing the polymer is purified efficiently to a degree sufficient to meet the market demand without damaging the basic characteristics of the polymer. Exemplified are the purification of polystyrene and polyvinyl alcohol using carbon dioxide as the extractant.

Description

METHOD FOR EXTRACTING IMPURITIES FROM A POLYMER DISPERSION The present

invention relates to a method for purifying a polymer dispersion.

Known processes for removing volatile substances from polymers, such as unreacted monomer and solvents, include, for example, a process in which an aqueous dispersion of a polymer is subjected to stripping with air and/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 poor techniques, a process for purifying a polymer by stripping its aqueous dispersion is described, for example, in Japanese Patent Application (OPI) No. 41387/1978. 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 a limit such that at most the concentration of volatile substances remaining in the polystyrene is reduced to about 240 to 350 ppm and denaturation and color change of the product occurs during purification. Thus it is difficult to satisfy the market demand for a product that has been purified to a higher level.

The present invention provides a method for purifying efficiently a polymer in a dispersion.

The present invention also provides a method for removing impurities remaining in a polymer to an extent that can meet the market demand.

The present invention additionally provides a method of removing impurities to purify a polymer without damaging the basic characteristics of the polymer.

other features and advantages of the invention will appear more fully from the following description, taken in conjunction with the accompanying drawing in which:

Fig. 1 is a flow sheet showing an embodiment of the present invention.

The inventors of the present Application have found that when a dispersion of a polymer is brought into contact with a prescribed amount of an extractant under high pressure, the removal of volatile substances can be performed 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.

The present invention provides a method for extracting impurities from a polymer which method comprises bringing into contact under high pressure an extractant and a dispersion containing a polymer to be purified, the weight ratio of the polymer in the dispersion to the extractant being in the range of 1: 0.1 to 1: 9 value less than 20.

The polymer dispersion to be processed by the method of the present invention may be either a homogenous or heterogeneous dispersion of a polymer in a dispersion medium, but it is preferably 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 specific, latex, coating compositions, adhesives, flocculants, processing agents, and the like, containing polystyrene, acrylonitrile/styrene copolymer, acrylonitrile/butadiene/styrene terpolymer, polymethacrylate, poly(vinyl chloride), poly(vinylidene chloride), styrene/butadiene, or 4 polybutadiene can be mentioned. As the polymer, thermoplastic polymers are preferred. It is preferable to carry out the extraction process on the polymer dispersion after polymerization but before the preparative step for the final polymer product.

Since the present invention is most preferably effected in combination with a polymerization process, the present invention is particularly suited to polymer production processes utilizing a dispersion medium (or solvent). Usually, according to the conventional processes, a polymer may be provided in a solid state or in a dispersed state, such as an emulsion or latex, as the final product.

In the present invention the extraction can be carried out before the separation of the dispersion medium. This is advantageous as described below. That is, when extraction is performed before the separation of the medium, it is possible to feed the polymer dispersion into an extractor kept continuously at a high pressure by pressurizing means such as a pump. on the other hand, when a solid state polymer is extracted after separating the dispersion medium, as in a conventional method, it is necessary firstly to lower the pressure of the extractor from high pressure to normal pressure in order to charge and take out the polymer, into and from the extractor, respectively. Therefore, it not only takes much time but also requires much energy repeatedly to raise and lower the pressure in the extractor.

Thus, the extraction of impurities from a polymer in a dispersion not only makes it possible to operate the process continuously and easily but also brings advantages. For example the cost of construction is lowered by making the apparatus of small size and energy consumption is reduced for the same quantity of product polymer.

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 a 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 polymers in the form of small particles. In solution polymerization processes for producing polypropylene, propylene as raw material is dissolved in n-hexane and mixed with a catalyst, and thereafter the polymerization reaction is carried out. In the case of synthetic rubber, for example, butadiene is dissolved in toluene and mixed with a dispersed catalyst for the polymerization reaction.

Polymers obtained by polymerization processes are processed by themethod of the present invention to extract the volatile substances in the dispersion. Although the mechanism of extraction according to the present invention is not yet clearly understood, it is presumed to be 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 then diffuses to the dispersion medium to establish a new equilibrium state, resulting in the decrease of residual monomer in the polymer.

The dispersion medium is usually in the state of a liquid. In case the evaporation of dispersion medium should affect the quality of the dispersion and/or the polymer, for example the particle size of a latex, the extraction conditions and/or the extractant are usually selected so that the evaporation of dispersion medium is suppressed, or additional dispersion medium is added to compensate for the loss by evaporation in advance of the 5 extraction.

The particle size of the polymer dispersion varies depending on, for example, the polymer used, and usually is in a range of 0.01 pm to 5 mm. In the case of latex it is in a range of 0.1 pm to 5 Am.

Although the extractant used in the present invention may be selected to suit the dispersion containing the polymer which is to be subjected to extraction, specific examples of the extractant are, 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 mixtures of two or more of these compounds. For reference, critical constants of the above-mentioned typical extractants are shown in Table 1.

Table 1 Critical Constants Substance Critical Critical Boiling Temperature Pressure Point Tc (OC) Pc (a tm) 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 -103.9 Propane 96.8 42.0 -42.1 Butane 152.0 37.5 -0.5 Pentane l6.6 33.3 36.1 Hexane 234.7 29.9 68.7 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 In the present invention, preferably the extractant is used in a supercritical state or in a 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 20 to 500 kg/cm2G, more preferably 4 to 350 kg/cm2G. The temperature may be below or above the softening point of the polymer to be purified. If the pressure is too low the ability of the extractant to extract volatile substances remaining in the dispersion containing the polymer becomes low, and sometimes an adequate extraction efficiency cannot be achieved. If the pressure is too high significant 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 the volatile impurities mentioned above, such as the unreacted monomer, solvents, and oligomers contained in the dispersion of the polymers.

In the practice of the present invention the extractor to be used is preferably a stirring vessel-type or a tower-type extractor, and, although, for example, a RDC-type, 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 the polymer to be subjected to extraction and the extractant to be used. A plurality of extractors may be arranged in parallel to carry out the extraction continuously, or they may be arranged in series, thereby processing 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 for the above described features.

The ratio of polymer and medium to be used in the present invention basically depends on that of the polymer dispersion after the polymerization reaction rather than that in the composition charged initially. However, when a medium in the dispersion is removed by mixture with the extractant in a supercritical state in the extraction process the equivalent of the amount to be removed may be added before extraction. In particular, when it is necesary to maintain a certain quantity of the medium for the stability of the dispersion, such as an emulsion and a latex, wherein the required property as a product is lost by reducing the amount of the dispersion medium, the extraction is effected after adding dispersion medium.

Though there is no particular limitation with respect to the amount of dispersion medium in the dispersion, the amount of medium is preferably 20 to 90 wt.%, more preferably 30 to 70 wt., in the dispersion. When the amount of medium is too small it is hard to handle the dispersion as a slurry because its fluidity is poor, although there are no problems for the extraction itself. When the amount of medium is in excess, the total volume for extraction becomes too large with the content of polymer being small and thus the size of the extraction apparatus becomes too large and much energy is required.

The weight ratio of the polymer in the dispersion supplied to the extractor to the extractant is 1: 0.1 to 1: a value less than 20, and preferably 1: 1 to 1: 10. If the amount of the extractant is less than the lower limit, an adequate extraction efficiency cannot be achieved, whereas if the ratio if 1: over 20 the improvement in 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.

As previously mentioned, Fig. 1 is a flow diagram showing an embodiment of the invention.

In Fig. 1, reference numeral 1 indicates a slurry tank provided with an agitator. The dispersion of polymer which is produced in a polymerization reactor (not shown) is fed and stored in the slurry tank 1. Stripping in the slurry tank 1 using an inert gas may be carried out in a conventional manner. The polymer dispersion in the slurry tank 1 is drawn out through a line 2 and fed into an extractor 5 through a line 4 after increasing its pressure by a pump 3. The temperature of the polymer dispersion, if it differs from the operating temperature of the extractor, may be regulated suitably by a heat-exchanger (not shown). An extractant is fed to the lower part of extractor 5 through a circulating compressor 9 and a line 10. Inside the extractor 5, the polymer dispersion descending from the upper part comes into contact countercurrently with the extractant rising from the lower part. The extractant extracts impurities (volatile substances, etc.) contained in the dispersion during ascent in the extractor and is drawn out from the upper part of the extractor 5 and is fed to a separation tower 7 through a line 6. On the other hand, the polymer dispersion from which impurities (volatile substances, etc.) are removed during descent through the extractor 5, is removed from the bottom of the extractor 5 through a line 11 and fed to the following process. If the extractant dissolved in the polymer dispersion affects the following process, a separating tank 12 may be provided after the line 11, if needed. Extractant containing the extracted volatile impurities is fed to a separation tower 7 through the line 6 after reducing the pressure and/or regulating the temperature (by means not shown) and the impurities are removed in the separation tower 7. Separating is performed by a conventional method, for example separation by liquefaction, separation in a distilling column and separation in an absorbing tower. Extractant separated from volatile impurities is fed to a circulating compressor 9 through a line 8 from the separator 7. On the other hand, separated volatile substances are fed to a recovery process, not shown, through a line 13 or are discharged. Reference numeral 14 indicates a feed line for the make-up extractant.

Advantages of the method 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) The extraction of polymer in a dispersion not only makes it possible to operate the process continously and easily but also brings advantages such as reduction in the cost of construction by making the apparatus of small size, and the amount of energy required is reduced (for the same amount of product).

(3) In order to reduce the concentration of residual volatile substances to the same degree as that achieved by the present invention by using an 1 extruder with a vent, it was necessary to operate for a longer time and at a higher temperature of melted resin, which all had the effect of damaging the basic properties of the polymer material. On the other hand, in the method of the present invention the purification can be carried out without damaging the basic characteristics of the polymer material, such as its appearance, color shade, and other physical properties.

(4) A polymer containing a relatively small amount of residual volatile substances can be obtained, which can well meet the market demand, which scarcely restricts the concentration of remaining volatile substances.

The method of the present invention will now be described in more detail with reference to the Examples and a Comparative Example. Example 1 40 g of an aqueous dispersion containing 50 wtA of polystyrene, obtained by a conventional suspension polymerization process (the concentration of volatile substances in the polystyrene being 1,500 ppm) was placed in an autoclave having a volume of 190 cn3. 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 800C. After the temperature and the pressure in the autoclave were kept at 800C and 250 kg/cm2G 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: 5. After dehydrating, the polystyrene was analyzed, and it was found that the concentration of volatile substances was 60 ppm.

Example 2

A slurry of poly(vinyl chloride) obtained by a conventional suspension polymerization (the concentration of the slurry being 30 wtA and the concentration of vinyl chloride monomer being 10,000 ppm) (on a dry basis) was treated in accordance with the flow sheet shown in Fig. 1. Carbon dioxide was used as the extractant. The separation tower was filled with active carbon.

Material balance and conditions such as the operating temperature and pressure on each line are shown in Table 2. The concentration of vinyl chloride nonomer in the poly(vinyl chloride) of the poly(vinyl chloride) slurry obtained was 100 ppm (on 15 a dry basis).

Table 2

Unit: g/h Line No. 2 6 11 10 8 13 14 1) PVC 297 0 297 0 0 0 0 vcm 2) 3 3 0 0 0 3 0 Water 700 5 695 0 0 5 0 CO 2 3) 0 2,950 50 3,000 2,950 0 50 Total 1,000 2,958 1,042 3,000 2,950 8 50 Temperature (C0) 60 60 60 60 30 30 30 2 Pressure (kg/cm G) 0 100 100 100 60 60 60 Remarks 4) Note; 1) PVC: Poly(vinyl chloride) 2) VCM: Vinyl chloride monomer 3) C02: Carbon dioxide 4) Regenerated after end of experiment.

Comparative Example The experiment was carried out in the same manner as Example 1 except that the ratio of extractant was changed as described below.

g of an aqueous dispersion containing so wt.% of polystyrene (the concentration of volatile substances being 1,500 ppm) was placed in an autoclave. Carbon dioxide was supplied to the autoclave at 10 g/min, to increase the pressure in the autoclave, with the temperature being kept at SOOC. After the temperature and the pressure in the autoclave were kept at 800C and 250 kg/CM2G 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: 0.06. Conditions in the autoclave were kept one hour and then the pressure was released. After dehydrating, the polystrene was analyzed, and it was found that the concentration of volatile substances was 600 ppm.

Having described the invention with reference to the above examples and preferred features it is not intended that the invention be limited by any of the details of the description, unless otherwise specified, but rather be construed broadly within the scope of the accompanying claims.

Claims

CLAIMS:

1. A method for extracting impurities from a polymer which method comprises bringing into contact under high pressure an extractant and a dispersion containing a polymer to be purified, the weight ratio of the polymer in the dispersion to the extractant being in the range of 1: 0.1 to 1: a value less than 20.

2. A method as claimed in claim 1 wherein the polymer dispersion is a homogeneous dispersion of the polymer in a dispersion medium.

3. A method as claimed in claim 1 wherein the polymer dispersion is a heterogeneous dispersion of the polymer in a dispersion medium.

4. A method as claimed in claim 3 wherein the heterogeneous dispersion of the polymer is obtained from an emulsion polymerization process or a suspension polymerization process.

5. A method as claimed in claim 3 or claim 4 wherein the polymer is selected from polystyrene, acrylonitrile/styrene copolymer, acrylonitrile/ butadiene terpolymer, polymethacrylate, polyvinyl chloride, polyvinylidene chloride, styrene/butadiene, and polybutadiene.

6. 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, and a mixture of two or more of said compounds.

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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 from 20 to 500 kg/CM2G.

9. A method as claimed in any one of the preceding claims wherein the temperature in the extractor is at or above the softening point of the polymer to be purified.

10. A method as claimed in any one of the preceding claims wherein the volatile material to be extracted is selected from unreacted monomers, solvents, and oligomers.

11. A method as claimed in any one of the preceding claims wherein as the extractor a tank-type extractor is used.

12. A method as claimed in any one of claims 1 to 10 wherein as the extractor plural extractors are arranged in parallel.

13. A method as claimed in any one of claims 1 to 10 wherein as the extractor plural extractors are arranged in series.

14. A method as claimed in any one of the preceding claims wherein the weight ratio of the polymer in the dispersion to the extractant is in the range of 1: 1 to 1: 10.

15. A method as claimed in claim 1 - 16 substantially as hereinbefore described in Example 1 or Example 2.

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

is 1 Published 1991 at The Patent Office. Concept House, Cardiff Road. Newport. Gwent NP9 I RH. Further copies may be obtained from Sales Branch, Unit 6. Nine Mile Point. Cwmfelinfach. Cross Keys. Newport. NP I 7HZ. Printed by Multiplex techniques ltd, St Mary Cray. Kent.