GB2309698A - Process for removing volatile components from low molecular weight copolymer solutions - Google Patents

Abstract

A method of removing volatile components from a low molecular weight copolymer solution prepared from a vinyl aromatic monomer and an ethylenically unsaturated dicarboxylic acid anhydride in the presence of at least one solvent comprises: (a) feeding the solution into an extruding means 1 to remove the volatile components, means 1 having a feed zone 10, a rear vent 12 upstream of said zone 10, and at least two downstream vents including first and second vents 13, 14; (b) preheating the solution to about 150-300{C; (c) feeding the preheated solution, under a pressure higher than the boiling pressure of volatile components, into zone 10, (d) controlling the pressure at vent 12 to be higher than 400 torr and removing 50-90 wt% of the total volatile components; (e) controlling the pressure at vent 13 to be 400-760 torr and removing 1-25 wt% of the total volatile components; (f) controlling the pressure at the vent 14 to be 20-450 torr and removing 0.1-25 wt% of the total volatile components; and (g) recovering the copolymer with a total volatile residue below 1 wt%. The extruding means may be a continuous kneader.

Description

PROCESS FOR REMOVING VOLATILE COMPONENTS FROM LOW MOLECULAR WEIGHT COPOLYMER SOLUTIONS The invention relates to a process for removing volatile components from a low molecular weight copolymer and, particularly, to a process for removing volatile components from a solution of a low molecular weight copolymer derived from a vinyl aromatic monomer and an ethylenically unsaturated dicarboxylic acid anhydride, the process offering products with good color appearance and low residual volatile components even when the product is mass produced.

Low molecular weight copolymers derived from vinyl aromatic monomers and ethylenically unsaturated dicarboxylic acid anhydrides have been commonly used, owing to their particular characteristics, for the following purposes: (1) Since these copolymers have high compatibility with metallic powder particles and contain considerable amount of oil-compatible groups, they are used as dispersants for pigments in oil-based paints.

(2) The copolymers contain carboxylic acid anhydride groups which are readily soluble in alkaline solutions and therefore can be used in water-based paints. Addition of suitable amounts of such copolymers to a water-based paint improves the hiding power, color development, scrubability and durability.

(3) Since the copolymers have high heat sensitivity, low melt viscosity and good flowability, they have been used as a dispersing agent for carbon black and magnetic powders in dry printing so as to improve the color development thereof.

(4) Due to the readily melting property and the good flowability of the copolymers when subjected to heat, powders such as titanium oxide, calcium carbonate and coloring powders can be dispersed in a plastic material by using the copolymer so that the color development and the hiding power of the plastic materials can be improved.

In order to remove volatile components from solutions of such copolymers, the following method has been generally used in the art: In the method, such copolymers are precipitated by adding suitable non-solvent materials to the solutions thereof, followed by filtration and drying.

The method is zomplicated and not suitable for mass production. Moreover, it is difficult to recover and reuse the non-solvent materials as well as to reduce the residual volatile components to a desired low level.

In view of the unsatisfactory results of the conventional methods, it is desirable to develop a method that can alleviate the problems existing in the conventional methods.

An object of the invention is to provide a process of removing volatile components from low molecular weight copolymers derived from 48 wt% - 75wt% of vinyl aromatic monomers, 52 wt - 25 wtt of ethylenically unsaturated dicarboxylic acids anhydrides, and 0-30 wt% of copolymerizabie monomers such as methyl acrylate, ethyl methacrylate and dimethyl fumarate, whereby residual volatile components can be reduced to a minimum level with good product color appearance.

According to the invention, a method of removing volatile components from a low molecular weight copolymer solution which is prepared from a vinyl aromatic monomer and an ethylenically unsaturated dicarboxylic acid anhydride in the presence of at least one solvent, is characterized by: (a) feeding the copolymer solution into an extruding means to remove the volatile components, the extruding means having a feed zone, a rear vent upstream of said feed zone, and at least two downstream vents including first and second downstream vents downstream of said feed zone; (b) preheating the copolymer solution to a temperature of about 150-300 C; (c) feeding the preheated copolymer solution, under a pressure higher than the boiling pressure of volatile components, into the feed zone; (d) controlling the pressure at the rear vent to be higher than 400 torr and removing 50-90 wt% of the total volatile components; (e) controlling the pressure at the first downstream vent to be 400-760 torr and removing 1-25 wt% of the total volatile components; (f) controlling the pressure at the second downstream vent to be 20-450 torr and removing 0.1-25 wt% of the total volatile components; and (g) recovering the copolymer with a total volatile residue below 1 wt% of the copolymer.

Examples of the extruding means of the present invention are extruders and continuous kneaders.

The term "low molecular weight copolymer solution" as used herein refers to a copolymer solution having a solid content of at least 33%, containing a copolymer derived from a vinyl aromatic and containing a copolymer derived monomer and an ethylenically unsaturated dicarboxylic acid anhydride and having a weight average molecular weight lower than 30,000, preferably lower than 15,000, more preferably lower than 12,000. If the molecular weight of the copolymer is higher than 30,000, the dispersing effect of the copolymer for the pigment is unsatisfactory.

Examples of vinyl aromatic monomers suitable for the copolymer are styrene, a-methyl styrene, a-chloro styrene, p-t-butyl styrene, p-methyl styrene, a-chloro styrene, p-chloro styrene, 2,5-dichloro styrene, 3,4dichloro styrene, 2,4,6,-tribromo styrene, 2,5-dibromo styrene. More preferable monomers are styrene and amethyl styrene.

Examples of ethylenically unsaturated dicarboxylic acid anhydride suitable for the copolymer are maleic anhydride, citraconic anhydride, itaconic anhydride, aconitic anhydride. Maleic ahhydride is preferable.

The solvents suitable for the copolymer solution in the present invention are aromatic hydrocarbons, ketones, tetrahydrofuran, dimethyl formamide, dimethyl sulfoxide, and dimethyl pyrrolidone. Examples of aromatic hydrocarbons are benzene, toluene, ethyl benzene, cumene, p-xylene, o-xylene, m-xylene, 1,2,4trimethyl benzene, and 1,3,5-trimethyl benzene.

Examples of ketones are propyl ketone, methyl ethyl ketone, and methyl isobutyl ketone.

The term "volatile components" as used herein denotes those including the solvent used in the copolymer solution and the monomers of the copolymer such as vinyl aromatic monomer and dicarboxylic acid anhydride.

According to the process of the invention, an antioxidant may be added during or before the devolatilization process of the copolymer solution in order to improve the heat stability of the copolymer, particularly, to obtain a better yellow index of the copolymer. The amount of the antioxidant added into the copolymer solution is 0.01-5% by weight, preferably 0.12% by weight, based on the weight of the copolymer.

Examples of antioxidants suitable for the copolymer solution are dilauryl-thio-di-propionate (DLTDP), distearyl-thio-di-propionate (DSTDP) , distearyl pentaerythritol diphosphite (PEP-8 Asahi-Denka Co.), octadecyl-3- (3, 5-ditertbutyl-4-hydroxyphenyl) -propionate (Irganox-1076, Ciba-Geigy), and 2,2'-oxamido bis-ethyl3(3,5-di-tert-butyl-4-hydroxy phenyl) propionate (Naugard XL-1, Uniroyal Chemical Co.) Dilauryl-thio-dipropionate (DLTDP) and distearyl-thio-di-propionate (DSTDP) are preferable.

Figure 1 is a schematic view showing an extruder having different operating regions according to the present invention.

As shown in Figure 1, an extruder 1 to carry out the process of the present invention comprises a feed port 11, a feed zone 10, a rear vent 12 upstream of the feed zone 10, and first, second and third downstream vents 13, 14 and 15 downstream of the feed zone 10. A pair of twin screws (not shown) is provided in the extruder 1.

While three downstream vents are shown in this embodiment, the downstream vents may be two or more than three vents according to the volatile level intended to be achieved.

According to a preferred embodiment of the process of the invention, a low molecular weight copolymer solution is pumped to a shell-tube heat exchanger via a gear pump (not shown) and is preheated to 150-3000C under a pressure higher than the boiling pressure of volatile components. The preheated copolymer solution is fed into feed port 11 through which the copolymer solution enters the feed zone 10. When the preheating temperature is lower than 1500C, the effect of removing volatile components is unsatisfactory. If the temperature is higher than 3000C, substantial decomposition will occur and the quality of the products will be poor.

The apparatus for preheating the copolymer solution may be a shell-tube heat exchanger, a double-pipe heat exchanger, a plate-type heat exchanger, a static type heat exchanger (such as SMX, SMR, SMX1 produced by Sulzer Co.), an evaporator or an electric heater. The heat source required for the heat exchanger is supplied by means of a gaseous or liquid heating medium such as steam or Dowtherm A (Product of Dow Chemical Co.).

The revolutional speed of the screw of the extruder 1 is controlled to be 150-300 rpm, preferably 150-250 rpm, in order to achieve satisfactory devolatilization.

The pressure in the rear vent 12 of the extruder 1 is controlled to be higher than 400 torr, and 50-90% of the total volatile components are removed through this vent. When the pressure at the rear vent 12 is lower than 400 torr, clogging will result at the vent 12.

Since the extruder screw is of a twin screw type, the copolymer solution flows forwardly and the gaseousvolatile components can easily move backward through the rear part of the screw without obstruction so that more than of 50% of the volatile components can be removed through the rear vent 12. Furthermore, because the copolymer solution is preheated to 150-3000C under pressure and is passed through the depressurized feed port 11, the preheated copolymer solution is in a flushing state when it reaches the feed zone 10. The depressurization can be controlled by means of a valve 17 provided upstream of the feed port 11.

The low molecular weight copolymer solution advanced by the two screws sequentially moves downstream to the first, second and third vents 13, 14 and 15.

The barrel of the extruder 1 is heated to 180-2500C. If the temperature is lower than 1800C, the devolatilization effect will be unsatisfactory. If the temperature is higher than 2500C, the color of the copolymer product will be poor.

The pressure in the first downstream vent 13 is controlled between 400-760 torr, preferably 550-760 torr. 1-25t of the total volatile components are removed through this vent. If the pressure of downstream vent 13 is higher than 760 torr, the devolatilization will be inefficient. If the pressure is lower than 400 torr, a large amount of fine copolymer particles will be carried outward by gaseous volatile components, thereby clogging passages of the vents and causing difficulty in the operation of the extruder.

The above-mentioned clogging situation is usually a serious problem for devolatilizing the low molecular weight copolymer solution, especially when the solution of low molecular weight styrene-maleic anhydrice copolymer has a wegith average molecular weight lower than 15,000.

The pressure at the second vent 14 is controlled between 20-450 torr wherein 0.1-25% of the volatile components are removed. If the pressure at the vent 14 is lower or higher than this pressure range, the vent 14 will encounter the same problems as the vent 13.

At the third downstream vent 15, the pressure is controlled to be below 50 torr so as to remove an additional amount of the volatile components.

According to the invention, the pressures in the first, second and third downstream vents are lower than or equal to the atmospheric pressure. As the copolymer moves forward, the vacuum level in the extruder increases from the first downstream vent 13 to the third downstream vent 15.

The number of the downstream vents may be 2 to 5 according to the amount of the volatile components to be removed. The third downstream vent 15 is not absolutely necessary. Addition of water to the extruder at locations upstream of the downstream vents, aids in the devolatilizing of a certain amount of the volatile components. The amount of the added water may be 0.1-5% by weight of the low molecular weight copolymer solution.

According to the invention, merely with a single stage process, a substantial amount of volatile components can be removed from the copolymer solution without causing the problems of clogging of the vents and difficult operation.

The effects and results of the invention will be more clear from the following examples and comparative examples.

The yellow indexes of the examples and comparative examples were obtained by dissolving each extruded product in methyl ethyl ketone to form a solution of 20% solid content and by determining the color of the solution with a colorimeter.

EXAMPLE (1) A solution of styrene-maleic anhydride copolymer (SMA, average molecular weight10,000) containing 50% solid content was pumped to a shell-tube heat exchanger and was heated therein to about 1800C. The solvent contained in the solution was methyl isobutyl ketone.

The copolymer solution was introduced at a rate of 25 kg/hr into a feed port of a twin screw extruder ( ZSK25, W & P COMPANY). The pressure of the solution before being fed into the feed port was maintained at about 6.2 kg/sq.cm, which pressure was controlled by a valve (designated at 17 in Figure 1). The revolutional speed of the extruder screw was 250 rpm, and the temperature of the barrel of the extruder was kept at about 2300C.

The extruder had a feed zone below the feed port, a rear vent, and first and second downstream vents. The operating conditions at the vents are stated in Table 1.

TABLE 1 Conditions Rear First down- Second down vent stream vent stream vent Pressure 500 torr 760 torr 250 torr Devolatilized Amount (%) *1 72 16 11.7 Clogging *2 no no no *1 Devolatilized percentages were calculated based on the total volatile component amount.

*2 The clogging conditions of the vents were inspected after a continuous operation time of 10 hours.

The residual volatile components contained in the extruded copolymer was found to be 0.3 % by weight, and the yellow index thereof was 20. No clogging condition was found at the vents.

EXAMPLE 2 An experiment was conducted substantially in the same manner as that of Example 1 except that the extruder used in this example had three downstream vents and that water injection ports were provided at locations between the second and third downstream vents.

The rate of water injection was 0.03 kg/hr. The operating conditions of this example are stated in Table 2.

The residual volatile components contained in the extruded copolymer was found to be 0.16% by weight and the yellow index thereof was 16. No clogging condition was found at the vents.

TABLE 2 Conditions Rear First down- Second down- Third down Vent stream vent stream vent stream vent Pressure 500 torr 750 torr 300 torr 50 torr Devolatilized Amount (%) *1 73 19 7.5 0.34 Clogging *2 no no no no *1 Devolatilized perventages were calculated based on the total volatile component amount.

*2 The clogging condition at the vents was inspected after a continuous operation time of 10 hours.

COMPARATIVE EXAMPLE 1 A solution of styrene-maleic anhydride copolymer which is the same as that of Example 1 was pumped to a preheater heated at 2200C. The solvent contained therein is methyl isobutyl ketone. The preheated copolymer solution was evaporated in a tank devolatilizer which is kept at 2500C under a pressure of 100 torr. The bottom of the devolatilizer was connected to a gear pump. The devolatilized styrene-maleic anhydride copolymer melt was pumped out through a gear pump and pulverized into fine particles. The product contained 0.5% by weight of volatile residue and the measured yellow index was 55.

COMPARATIVE EXAMPLE 2 An experiment was carried out in the same manner as that of Example 1 except that a pressure of 300 torr was maintained at the first downstream vent. The amount of the resulting residual volatile components was found to be 0.28%. However, after two hours of operation, clogging occurred in the first downstream vent due to the fine powder of the copolymer.

EXAMPLE 3 An experiment was carried out by following the procedure and the conditions of Example 1 except that DSTDP (distearylthiopropionate) was added in an amount of 0.6 % by weight of the copolymer to the copolymer solution before preheating the copolymer solution. The extruded product had a yellow index of 12.

EXAMPLE 4 Experiments were carried out by following the procedure and the conditions of Example 3 except that the antioxidant used in Example 3 (DSTDP) was replaced by the following three antioxidants respectively: 1. Octadecyl-3- (3, 5-ditertbutyl-4-hydroxyphenyl (Irganox-1076, Ciba Geigy) 2. 2,2'-oxamido bis-(ethyl-3(3,5-di-tert-butyl-4-hydroxy phenyl) propionate (Naugard XL-1, Uniroyal Chemical Co.); and 3. Distearyl pentaerythritol diphosphite (PEP-8 Asahi Denka Co.).

The yellow indexes of the products were 15, 18 and 19 respectively.

COMPARATIVE EXAMPLE 3 The procedure and the materials employed in this example were substantially the same as those of Example 1, except that the styrene-maleic anhydride copolymer solution was preheated to a temperature of 1200C, the speed of the extruder screw was 360 rpm, and the temperature of the extruder barrel was 280"C. In addition, the operating condition at each vent is as stated in Table 3.

TABLE 3 Condition Rear First down- Second down Vent stream Vent stream Vent Pressure 500 torr 420 torr 250 torr Devolatilized Amount (%) *1 46 36 16 Clogging *2 no serious serious *1 Devolatilized percentages were calculated based on the total volatile component amount.

*2 The clogging conditions of the vents were inspected after a continuous operation time of 10 hours.

The product obtained in this example contained 2% by weight of volatile residue and the yellow index of the product was 35.

From a comparison of Example 1 and Comparative Example 1 which illustrates a conventional process, it can be noted that the color appearance (yellow index) of the products obtained in the present invention is better than that of the conventional devolatilizer process.

Furthermore, in view of Comparative Example 2, it can be understood that, when the pressure of the first downstream vent is lower than 400 torr, a serious clogging problem occurs. In view of Comparative Example 3, when the preheating temperature is lower than 1500C, the devolatilization effect is unsatisfactory and the clogging problem is serious. Moreover, Examples 3 and 4 manifest that the yellow index of the products can be improved by adding an antioxidant and that DSTDP is the most preferable antioxidant.

In view of the examples and comparative examples, it can be appreciated that the process according to the present invention provides advantages, such as good color appearance of the products, less degradation, low level of volatile residue, and smoothness of the continuous operation.

Claims (10)

CLAIMS:

1. A method of removing volatile Components from a low molecular weight copolymer solution which is prepared from a vinyl aromatic monomer and an ethylenically unsaturated dicarboxylinic acid anhydride in the presence of at least one solvent, characterized by: (a) feeding said copolymer solution into an extruding means to remove the volatile components, said extruding means having a feed zone, a rear vent upstream of said feed zone, and at least two downstream vents including first and second downstream vents downstream of said feed zone; (b) preheating said copolymer solution to a temperature of about 150-300 C; (c) feeding said preheated copolymer solution, under a pressure higher than the boiling pressure of the volatile components, into the feed zone; (d) controlling the pressure at said rear vent to be higher than 400 torr and removing 50-90 wt% of the total volatile components; (e) controlling the pressure at said first downstream vent to be 400-760 torr and removinq 1-25 wtt of the total volatile components; (f) controlling the pressure at said second downstream vent to be 20-450 torr and removing 0.1-25 8 of the total volatile components; and (g) recovering said copolymer with a total volatile residue below 1 was of said copolymer.

2. A method as claimed in Claim 1, further characterized by controlling the screw of the extruding means to operate at a revolutional speed of about 150-300 rpm.

3. A method as claimed in Claim 1, characterized in that the copolymer solution has a solid content of at least 33% by weight.

4. A method as claimed in Claim 1, characterized in that said extruding means is further provided with a third downstream vent downstream of said second downstream vent.

5. A method as claimed in Claim 1, characterized in that the pressure at said first downstream vent is 550760 torr.

6. A method as claimed in Claim 1, further characterized by adding 0.1-5t by weight of water into said extruding means before said downstream vents.

7. A method as claimed in Claim 1, characterized in that said copolymer solution comprises styrene-maleic anhydride copolymer.

8. A method as claimed in Claim 1, further characterized by adding 0.01-5î by weight of an antioxidant into said copolymer solution.

9. A method as claimed in Claim 9, characteri2ed in that said antioxidant is distearyl-thio-di-propionate.

10. A method of removing volatile components from a low molecular weight copolymer solution cubstantially as hereinbefore described with reference to and as illustrated in the acompanying drawings.