GB2055861A

GB2055861A - Improvements in the polymerization of vinyl chloride

Info

Publication number: GB2055861A
Application number: GB8024150A
Authority: GB
Original assignee: Shin Etsu Chemical Co Ltd
Current assignee: Shin Etsu Chemical Co Ltd
Priority date: 1979-08-09
Filing date: 1980-07-23
Publication date: 1981-03-11
Also published as: DE3028753A1; PT71667A; PL226102A1; JPS5626908A; GR69338B; FR2463154A1; FR2463154B1; BR8004967A; DE3028753C2; JPS6042804B2; GB2055861B; MX156439A; IN153574B; PT71667B

Abstract

The invention relates the aqueous phase polymerization of vinyl chloride monomer and provides for eliminating the disadvantages caused by foams rising above the surface of the aqueous polymerization mixture by a mechanical foam destroying means during the polymerization and/or in the course of the recovery of the unreacted monomer after completion of the polymerization. The most convenient and effective mechanical foam breaking means is obtained with foam breaking blades rotating above the surface of the polymerization mixture and fixed to the upper part of the stirrer shaft.

Description

SPECIFICATION Improvements in the polymerization of vinyl chloride The present invention relates to an improvement in the polymerization of vinyl chloride monomer.

Needles to say, vinyl chloride-based resins, i.e. homopolymeric and copolymeric vinyl chloride resins, are mostly produced by polymerization in an aqueous polymerization medium, the polymerization being known as suspension polymerization or emulsion polymerization according to the state of dispersion of the monomer or monomers in the aqueous medium.

One of the difficult problems both in the suspension polymerization and in emulsion polymerization of vinyl chloride is foaming of the aqueous polymerization mixture during polymerization. When large volumes of foam are produced above the surface of the aqueous polymerization mixture, the foam rises in the polymerization reactor and eventually enters the reflux condenser installed in the upper part of the polymerization reactor or enters the pipes for the recovery of the unreacted monomer so that the operation of the condenser and monomer recovery through these pipes are substantially hindered. There is, in addition, the problem of increased polymer scale deposition on the upper part of the inner walls of the polymerization reactor, due to the foams.

The above mentioned foaming takes place most vigorously at three stages during the polymerization run, i.e. (1) at the stage of temperature elevation after introduction of the individual ingredients pertaining to the polymerization into the reactor to start the polymerization reaction, (2) in the period when the condensor is in operation and (3) at the stage of recovery of the unreacted monomer after completion of the polymerization reaction. When the foaming takes place violently in the first or temperature elevation stage, polymer scale deposits on the whole surface of the polymerization reactor in the subsequent polymerization process bringing about difficulties in temperature control of the polymerization mixture during polymerization or increased fish eyes intermingled in the resultant polymer product.Removal of the polymer scale from the reactor walls takes much labor and time and in addition there is the problem of safety arising from the toxicity of the monomer to workers.

When violent foaming takes place in the second and the third stages as above mentioned, the foam rising in the polymerization reactor eventually enters the reflux condenser and the pipes for monomer recovery to partly or totally destroy their functions.

A reflux condenser is particularly indispensable in a polymerization reactor of large volume as an auxiliary means for temperature control. Therefore, if the function of the reflux condenser is decreased or destroyed in the stage when effective temperature control by means of the condenser is essential, the polymerization reaction naturally runs out of control resulting in an extremely dangerous state.

Further, monomer recovery through the pipes is substantially retarded by the foam entering the pipes in the stage of the monomer recovery and, in addition, the polymer particles in the polymerizate slurry are carried over by the foam leading to a loss in the yield of the product. Thus the monomer recovery must be carried out at an undesirably low velocity to prevent the foams from reaching the pipes for monomer recovery. These occurrences are all detrimental to the overall efficiency of the polymerization reactor or the polymerization facilities as a whole.

Notwithstanding the importance of the problem of foaming, no effective method has yet been proposed in the art of vinyl chloride polymerization.

We have therefore sought ways of improving the efficiency of the vinyl chloride polymerization in an aqueous polymerization medium by reducing or destroying the foams rising above the polymerization mixture in the polymerization reactor during polymerization, and ways for preventing polymer scale deposition on the reactor walls caused largely by foam reaching the surface portions not in contact with the liquid polymerization mixture so contributing to an increase in the efficiency of temperature control as well as to the quality of the polymer products (smaller amounts of fish eyes).

Thus, the present invention provides an improvement, in a process of polymerizing vinyl chloride or a monomer mixture mainly composed of vinyl chloride in an aqueous polymerization medium as dispersed therein, in which foam rising above the surface of the polymerization mixture is destroyed by a mechanical means in the course of the polymerization or during the recovery of the unreacted monomer.

The most convenient and effective way for providing such a foam breaking means is the use of foam breaking blades fixed to the rotating shaft of the stirrer at the upper portion thereof exposed to the vapor phase above the polymerization mixture in the reactor in such a manner that the span of the foam breaking blades is at least two tenths of the inner diameter of the reactor and the foam breaking blades rotate at a speed which gives the wingtips a linear velocity of at least 1 m/second.

Further preferred features of the invention will now be described.

The above described serious problems caused by foaming in the course of polymerization or monomer recovery in the polymerization of vinyl chloride in an aqueous medium have come to the inventors' attention leading them to conduct extensive investigations to solve these problems. As a result, they have arrived at the conclusion that the most convenient and effective way to be freed from the difficulties caused by foaming is to mechanically destroy the foams in the reactor in the course of the polymerization or monomer recovery provided that the above described conditions are satisfied in case where the mechanical means is the provision of foam breaking blades rotating coaxially with the stirrer.

As is mentioned above, the present invention is applicable not only to the homopolymerization of vinyl chloride monomer alone but also to the copolymerization of a monomer mixture mainly, say 50% by weight or more, composed of vinyl chloride. The monomers to be copolymerized with vinyl chloride are exemplified by ethylene, propylene, vinyl acetate, vinyl propionate, vinyl lauryl ether, vinyl isobutyl ether, ethyl acrylate, butyl acrylate, acrylonitrile, vinylidene chloride and the like.

The polymerization process may be either suspension polymerization or emulsion polymerization both carried out in an aqueous medium. The polymerization initiator introduced into the polymerization mixture is well known in the art and includes monomer-soluble initiators and water-soluble initiators according to the type of the polymerization. Examples of the monomer-soluble initiators are azo compounds such as azobis-a,ce'-dimethylvaleronitrile, 2,2'-azobis-2,4-dimethyl-4-methoxyvaleronitrile and the like and organic peroxides such as diisopropylperoxy dicarbonate, di-2-ethylhexylperoxy dicarbonate, di-(2-ethoxyethyl)peroxy dicarbonate, tert-butylperoxy neodecanate, acetylcyclohexylsulfonyl peroxide, tert-butylperoxy pivalate, lauroyl peroxide and the like.The watersoluble polymerization initiators are exemplified by potassium persulfate, ammonium persulfate, hydrogen peroxide, cumene hydroperoxide and the like.

The dispersing agent used to disperse the vinyl chloride monomer or monomer mxiture in the aqueous medium can also be a conventional one selected according to the type of the polymerization process and chosen from known suspending agents for suspension polymerization and emulsifying agents for emulsion polymerization as exemplified by copolymers of styrene and maleic anhydride, partially saponified polyvinyl alcohols, methylcellulose, hydroxypropyl methylcellulose, ethylcellulose, gelatin, calcium carbonate, calcium phosphate, sodium laurylsulfate, sorbitan monostearate, sodium dedecylbenzenesulfonate and the like.

The polymerization mixture may contain, in addition to the above described polymerization initiator and dispersing agent, other additive ingredients conventionally used in the polymerization of vinyl chloride such as chain transfer agents, crosslinking agents, stabilizers, fillers, anti-oxidants, buffering agents, scale deposition inhibitors and the like according to need.

The procedure for carrying out the polymerization in the present invention is not different from conventional procedures (including the formulation of the polymerization mixture) except for the foam breaking means to be operated in the course of the polymerization or subsequent monomer recovery. In the process of the invention, the foams rising above the surface of the aqueous polymerization mixture in succession are destroyed by a mechanical means as far as possible to prevent them from reaching the upper part of the polymerization reactor.

The mechanical means for breaking the rising foam can be provided in various ways. For example, the foam is broken by the shearing force or impact force given by rotating foam breaking blades above the polymerization mixture or by a reciprocating up and down movement of a perforated disc having a somewhat smaller diameter than the polymerization reactor in the gaseous phase space in the reactor The most convenient and reliable means for the mechanical foam breaking is to provide the stirrer shaft with foam breaking blades in the upper part thereof which is exposed to the gaseous phase.

The most simple and convenient design for providing foam breaking blades is to have the blades integrally fixed to the shaft of the stirrer for agitating the polymerization mixture so that the foam breaking blades rotate in the gaseous phase coaxially and at the same speed as the stirrer blades in the liquid phase. If necessary, the foam breaking blades can be designed so that they can rotate coaxially with the stirrer blades but a different speed. The size of the foam breaking blades should be sufficiently large that their span, i.e. twice the radius of rotation, is at least two tenths or, preferably, at least three tenths of the inner diameter of the polymerization reactor. When the foam breaking blades are smaller than the above given lower limit, the rising foams can only be destroyed incompletely.Of course, the upper limit of the size of the foam breaking blades is determined by the inner diameter of the reactor or by the position of any auxiliary structure protruded into the reactor while must not be hit by the rotating foam breaking blades.

The shape of the foam breaking blades is not particularly significant and simple paddle type, turbine blade type, propeller wing type and Pfaudler type blades are suitable for the purpose.

In order to obtain a satisfactory foam breaking effect, it is desirable that the rotary speed of the foam breaking blades is sufficiently high that, usually, the linear velocity at the wingtip is at least 1 m/second.

The height at which the foam breaking blades are fixed to the stirrer shaft is not critical but it is preferable that the lower edges thereof are close to the surface of the aqueous polymerization mixture in the reactor. Care must be taken, however, that the foam breaking blades do not come into contact with the surface of the polymerization mixture in order to avoid possible adverse effects on the state of dispersion of the monomer and consequently on the particle size distribution of the polymer product caused by the foam breaking blades. Such an adverse effect is particularly remarkable at the initial stage of the polymerization, for example, up to a monomer conversion of about 5%. Accordingly, it is recommendable that the foam breaking blades are adjustable along the stirrer shaft during polymerization and the blades are kept high at the initial stage of the polymerization and are lowered after the monomer conversion has reached a predetermined percentage, e.g. 5%. Alternatively, similar effects can be obtained if the polymerization is started with the surface of the polymerization mixture somewhat lower than a final level which is achieved with subsequent addition of an additional volume of the aqueous medium so that the distance between the foam breaking blades and the surface of the polymerization mixture is reduced to the optimum for the highest foam breaking efficiency.

The process of the invention for the polymerization of vinyl chloride is most successfully applicable to suspension polymerization and hitherto known steps for preventing polymer scale deposition may be undertaken such as continuous washing down of the reactor walls with water or other liquid or providing a coating layer on the reactor walls with a material less susceptible to polymer scale deposition.

As will be understood from the above description, the process is effective in the industrial production of vinyl chloride polymers notwithstanding its very simple principle.

Following are Examples to illustrate the effectiveness of the process in further detail. In the following Examples, all of the polymerization experiments were carried out in a polymerization reactor of stainless steel having a capacity of 1.2 m3 and an inner diameter of 1.0 m and equipped with a stirrer having paddle type stirrer blades for agitation of the polymerization mixture on the lower part of the stirrer shaft.

EXAMPLE 1 (Experiments No.1 to No.8) The stirrer shaft of the polymerization reactor was provided with two paddle type blades of a span and width as indicated in Table 1 below for foam breaking at such a height that the lower edges of the foam breaking blades are just in contact with the surface of the water introduced into the reactor 800 liters (Experiments No.3 to No.7) or 900 liters (Experiment No.8). These heights of the foam breaking blades are called hereinafter the 800-liter level and the 900-liter level, respectively.

Into the above polymerization reactor were introduced 400 kg of deionized water, 250 g of a partially saponified polyvinyl alcohol and 100 g of di-2-ethylhexylperoxy dicarbonate and, after evacuation of the reactor to a pressure of 50 mmHg, 250 kg of vinyl chloride monomer were introduced thereinto to form a polymerization mixture which was heated up to a temperature of 570C to effect polymerization of the monomer dispersed in the aqueous medium with the stirrer rotating at various speeds as indicated in Table 1.

When the polymerization reaction came near to completion with the pressure inside the polymerization reactor having dropped to about 7 kg/cm2G, the reaction was stopped and the unreacted vinyl chloride monomer was recovered through a monomer recovery pipe at the upper part of the reactor at a rate of 1 minute or 2 m3/minute (calculated at 250C and under 1 atmosphere) with continued operation of the stirrer.

The stirrer was stopped after discharge of the polymerizate slurry from the reactor and the reactor was opened to examine the highest level on the reactor walls where adhering of the polymer particles was found indicating the maximum foaming height during polymerization and monomer recovery. The results are shown in Table 1 by the level of water charge in liters. In Experiments No.1 and No.2 in which no foam breaking blades were provided, this height was a 1200-liter level indicating that foams occupied the whole volume of the reactor at least once leaving the polymer particles adhering all over the surface of the reactor walls.

The reactor walls were then washed with a water jet at a pressure of 2 kg/cm2G to remove the polymer particles adhering thereon and the highest level of the polymer scale deposition was examined to give the results shown in Table 1 by the level of liters of water charge. In Experiments No.1 , No.2 and No.4, polymer scale deposition was found over the whole surface of the reactor walls. Further, the monomer recovery pipe was examined to find whether or not the polymer particles had been carried therein by the rising foam. The results are also shown in Table 1.

As will be understood from the results set out in Table 1, the results were quite unsatisactory in Experiments No.1 and No.2 because no mechanical means to destroy the foams was provided and the results in Experiments No.3 and No.4 were also not satisfactory by reason of the too small span of the foam breaking blades and the too small linear wingtip velocity, respectively.

TABLE 1

Experiment No. 1 2 3 4 5 6 7 8 Foam Span, cm - - 15 30 30 30 60 60 breaking Width, cm - - 5 5 5 5 5 5 blades Rotation, r.p.m. - - 200 60 130 200 200 200 Linear velocity at wingtip, m/second - - 1.57 0.94 2.04 3.14 6.28 6.28 Rate of monomer recovery, m /minute 1 2 1 1 1 2 2 2 Height of polymer particle adhesion, 1200 1200 980 1030 850 840 860 950 liter level Height of polymer scale deposition, 1200 1200 1150 1200 830 840 820 930 liter level Polymer particles in monomer recovery Yes Yes No Yes No No No No pipe EXAMPLE 2 (Experiments No.9 to No.11) The Experimental procedure was just the same as in Example 1 except that water was sprayed at the inside surface of the reactor walls above the surface of the polymerization mixture at a rate of 1 liter/minute from the beginning of temperature elevation of the complete polymerization mixture to 30 minutes after the temperature of the polymerization mixture had reached 570C. The total volume of the sprayed water was about 50 liters. The results of the experiments are shown in Table 2.In Experiments No.10 and No.11, the height of the foam breaking blades was at the 850-liter level, i.e. the lower edges of the blades were just in contact with the surface of water when 850 liters of water were introduced into the reactor.

TABLE 2

Experiment No. 9 10 11 Foam Span, cm - 60 60 breaking Width, cm 5 5 blades Rotation, r.p.m. ~ 200 200 Linear velocity at wingtip, m/second - 6.28 6.28 Rate of monomer recovery, minute 1 2 1 Height of polymer particle adhesion, 1200 870 800 liter level Height of polymer scale deposition, 960 750 760 liter level' EXAMPLE 3 (Experiments No.13 to No.15) The experimental procedure was just the same as in Example 1 except that the paddle type foam breaking blades in Example 1 were replaced with 4 turbine blades, 3 pfaudler type blades or 4 propeller wings having an angle of incidence of 600, respectively. The height of the foam breaking blades was at the 800-liter level. The results of the experiments are shown in Table 3 below.

TABLE 3

Experiment No. 13 14 15 Foam Type Turbine Pfaudler Propeller breaking Span, cm 60 60 60 blades Width, cm 5 5 5 Rotation, r.p.m ?00 200 150 Rate of monomer recovery, m3/minute 2 2 2 Height of polymer particle adhesion, 850 870 860 liter level Height of polymer scale deposition, 830- 820 830 liter level EXAMPLE 4 (Experiments No.16 to No.19) The experimental procedure was just the same as in Example 1 except that the polymerization reactor was equipped with a reflux condenser having an area of heat transfer of 0.5 m2 at the top thereof, which condenser was brought into operation at a moment 2 hours after the start of the polymerization reaction at a rate of heat removal of 7000 kilo-calories/hour. The height of the foam breaking blades, if provided, was at the 800-liter level.The results of the experiments are shown in Table 4 below including the data for the amounts of the polymer particles retained in the condenser after completion of the polymerization as determined by washing the disassembled condenser with water and collecting the washed out polymer particles followed by drying.

Table 4 also includes data for the number of fish eyes as the number of transparent particles counted in a 100 cm2 area of a 0.2 mm thick sheet prepared by milling 50 g of the powdery polymer product, 25 g of dioctyl phthalate, 0.3 g of tribasic lead sulfate, 1.0 g of lead stearate, 0.01 g of titanium dioxide and 0.05 g of carbon black on a roller at 1 400C for 7 minutes after keeping blended for 30 minutes and then taking the product from the roller in the form of a sheet of 0.2 mm thickness.

TABLE 4

Experiment No. 16 17 18 19 Foam Span, cm - 60 30 30 breaking Width, cm - 5 5 5 blades Rotation, r.p.m. 200 200 200 Rate of monomer recovery, m3/ recovery 1 I I I Height of polymer particle adhesion, 1200- 860 850 840 liter level Height of polymer scale deposition, 1200 830 840 840 liter level Polymer particles in condenser, g f 50 O Fish eyes in polymer product, pieces/100 cm2 500 13 17 10 EXAMPLE 5 (Experiments No.20 to No.22) The experimental procedure was about the same as in Example 1 except that the number of the paddle type foam breaking blades was increased to 4 and the polymerization was carried out at 580C with a charge composed of 400 kg of deionized water, 150 g of a partially saponified polyvinyl alcohol, 100 g of a methylcellulose, 150 g of diisopropylperoxy dicarbonate; 210 kg of vinyl chloride monomer and 40 kg of vinyl acetate monomer and the reaction was terminated at a moment when the pressure inside the reactor had dropped to 3 kg/cm2G.The height of the foam breaking blades was at-the 800liter level in Experiment No.21 and at the 850-liter level in Experiment No.22. The results of the experiments are shown in Table 5 below.

TABLE 5

Experiment No. 20 21 22 Foam Span, cm - 60 75 breaking Width, cm - 5 5 blades Rotation, r.p.m - 200 200 Linear velocity at wingtip, m/second - 6.28 7.85 Rate of monomer recovery, m /minute 1 1 1 Height of polymer particle adhesion, liter level 1200 910 880 Height of polymer scale deposition, liter level 1200 880 870 Polymer particles in monomer recovery pipe Yes No No EXAMPLE 6 (Experiments No.23 and No.24) The type of polymerization was emulsion polymerization but the experimental procedure was just the same as in Example 1 except that the partially saponified polyvinyl alcohol as the suspending agent was replaced with 1.3 kg of 2-ethylhexylsulfosuccinate (Rapisol B80, a product manufactured by Nippon Yushi Co.) and 1.0 kg of stearyl alcohol as the emulsifying agent. The height of the foam breaking blade was at the 800-liter level. The results of the experiments are shown in Table 6 below.

TABLE 6

Experiment No. 23 24 Foam Span, cm - 80- breaking Width, cm - 15 blades Rotation, r.p.m. - 150 - Linear velocity at wingtip, misecond - 6.28 Rate of monomer recovery, m3/minute 0.5 0.5 Height of polymer particle adhesion, liter level 1200- 880 Height of polymer scale deposition, liter level 1200 850 Polymer particles in monomer recovery pipe Yes No

Claims

1. A process for polymerizing vinyl chloride monomer, or a monomer mixture mainly composed of vinyl chloride, dispersed in an aqueous polymerization medium, which comprises destroying foam rising above the surface of the aqueous polymerization mixture by a mechanical foam breaking means in the course of the polymerization or in the course of the recovery of the unreacted monomer after completion of the polymerization.

2. A process as claimed in claim 1 wherein the mechanical foam breaking means is provided by foam breaking blades on the shaft of the stirrer in the polymerization reactor at the upper part thereof exposed to the vapor phase above the surface of the polymerization mixture.

3. A process as claimed in claim 2 wherein the span of the foam breaking blades is at least two tenths of the inner diameter of the polymerization reactor.

4. A process as claimed in claim 2 or 3 wherein the linear velocity of the wingtip of the foam breaking blades is at least 1 m/second.

5. A process as claimed in claim 2,3 or 4 wherein the foam breaking blades are not in contact with the aqueous polymerization mixture.

6. A process as claimed in claim 1, substantially as described in any of Experiments 3-8, 10-15, 17-19,21,22and24.

7. Vinyl chloride polymer when prepared a process as claimed in any preceding claim.