GB2078237A

GB2078237A - Suspension polymerization of vinyl chloride

Info

Publication number: GB2078237A
Application number: GB8118207A
Authority: GB
Original assignee: Shin Etsu Chemical Co Ltd
Current assignee: Shin Etsu Chemical Co Ltd
Priority date: 1980-06-16
Filing date: 1981-06-12
Publication date: 1982-01-06
Also published as: JPS575704A; IT8122256A0; FR2484420A1; BE889218A; FR2484420B1; DE3123015C2; DE3123015A1; GB2078237B; JPS6239601B2; IT1137251B

Abstract

The productivity of the suspension polymerization of vinyl chloride in an aqueous medium, especially in a large polymerization reactor, is improved by shortening the time taken for completing polymerization run. In this method, the polymerization initiator is dissolved in the monomer to form a uniform mixture and the mixture is introduced into the polymerization reactor simultaneously with pre-heated water to form a hot polymerization mixture in the reactor at a temperature substantially equal to the prescribed polymerization temperature so that the time otherwise necessary for the temperature elevation of the cold polymerization mixture to the prescribed polymerization temperature can be mostly saved.

Description

SPECIFICATION Method for the suspension polymerization of vinyl chloride The present invention concerns an improvement in the suspension polymerization of vinyl chloride in an aqueous medium.

As is well known, suspension polymerization of vinyl chloride monomer is usually carried out in a batch-wise process. In the conventional method for the suspension polymerization of vinyl chloride, water as the aqueous polymerization medium, dispersing agent or suspending agent, polymerization initiator and the vinyl chloride monomer are introduced into the polymerization reactor with addition of optional additives to form a polymerization mixture which is then heated to a prescribed temperature to start the polymerization reaction. The polymerization temperature is usually maintained constant throughout the time to complete a polymerization run.

When improvement in the productivity of the suspension polymerization is desired by shortening the overall time taken for a polymerization run, therefore, it is necessary to shorten the time for the introduction of the ingredients of the polymerization mixture into the polymerization reactor and the reaction time as well as the time necessary for elevating the temperature of the cold polymerization mixture to the prescribed polymerization temperature.The temperature elevation of the polymerization mixture contained in the polymerization reactor is carried out usually solely by means of the heating jacket surrounding the reactor, but a large sized polymerization reactor sometimes has an insufficient heating capacity because the surface area, i.e. the area for heat transfer, of such a large reactor is small relative to the volume thereof so that it has been desired to develop an efficient means for the temperature elevation of the polymerization mixture contained in a large polymerization reactor.

An alternative method for the solution of the above problem is the introduction of water or vinyl chloride monomer heated in advance. When the polymerization initiator is introduced together with the water at an elevated temperature, however, the initiator is rapidly decomposed before the introduction of the vinyl chloride monomer so that certain difficulties are encountered in the control of the polymerization reaction. When the vinyl chloride monomer and the polymerization initiator are introduced into the polymerization reactor already containing the water heated in advance, on the other hand, the polymerization of the monomer begins during introduction of the monomer so that a large amount of polymer scale deposits on the walls of the polymerization reactor resulting in inferior quality of the polymer product which has an extremely large number of fish-eyes.

Further, the amount of fish-eyes is increased by adding the polymerization initiator after introduCtion of the pre-heated water and the vinyl chloride monomer into the reactor. Similar disadvantages are unavoidable in the case of the introduction of pre-heated monomer.

The problem from which the present invention arose, was therefore to provide a method for the improvement of productivity in the suspension polymerization of vinyl chloride monomer by shortening the overall time taken for completing a polymerization run.

The method of the present invention is based on the principle that the most effective way for shortening the time before the start of the polymerization reaction is the simultaneous introduction of pre-heated water and a uniform mixture composed of the vinyl chloride monomer and the polymerization initiator together into the polymerization reactor to form a hot polymerization mixture in which the polymerization reaction can immediately start. It is desirable that the polymerization mixture formed in the reactor during and at the end of the introduction of the pre-heated water and the uniform mixture of the monomer and the initiator is at a temperature substantially equal to the prescribed polymerization temperature. In particular, the temperature of the polymerization mixture should not exceed the prescribed polymerization temperature by 20C or more.

Preferred embodiments of the invention will now be described.

In practicing the method of the present invention, vinyl chloride monomer and the polymerization initiator are first mixed together to form a uniform mixture before they are introduced into the polymerization reactor. There are several ways for forming the uniform mixture of the monomer and the initiator in advance. For example, they are mixed together in advance in a separate vessel to form a uniform mixture and the mixture is then introduced into the polymerization reactor. Alternatively, they are mixed together on the way to the polymerization reactor in a continuous line mixer so that they arrive at the reactor as a uniform mixture.

As will be readily understood from the above requirement that the monomer and the polymerization initiator are introduced into the polymerization reactor as a uniform mixture, the initiator naturally should be selected from monomer-soluble ones. In this regard, conventional initiators used for the suspension polymerization may be used in the present invention such as organic peroxides and azo compounds as exemplified by diisopropylperoxy dicarbonate, diethoxyethylperoxy dicarbonate, tertbutylperoxy neodecanoate, br-cumylperoxy neodecanoate, tert-butylperoxy neodecanoate, acetyl cyclohexylsulfonyl peroxide, 2, 4, 4-trimethylpentyl 2-peroxyphenoxy acetate and the like as examples of the former class and azobis-2, 4-dimethylvaleronitrile, azobis (4-methoxy-2, 4-dimethyívaleronitrile) and the like as the examples of the latter class.These initiators may be used either alone or as a combination of two or more according to need. The amount of the polymerization initiator relative to the monomer is conventional.

When the monomer and the initiator are mixed together to form a uniform mixture and introduced into the polymerization reactor simultaneously with the pre-heated water, the temperature of the mixture should be kept sufficiently low before contacting with the pre-heated water so as not to cause premature decomposition of the initiator before the polymerization mixture is formed in the reactor. As a result of investigations to establish the effect of this temperature, it was noted that the temperature of the mixture of the monomer and the initiator should be such that the conversion of the monomer in the mixture is 5% or less or, preferably, 3% or less after 2 hours at the temperature. If the above condition is not satisfied, deposition of polymer scale on the reactor walls is remarkably increased and the polymer product contains a large amount of fish-eyes.

As the uniform mixture of the monomer and the polymerization initiator as introduced into the polymerization reactor, pre-heated water is simultaneously introduced into the reactor to form a polymerization mixture at an elevated temperature so that the polymerization reaction may start. In this case, it is essential that the temperature of the thus formed polymerization mixture at the end of the introduction of both of the monomer-initiator mixture and the pre-heated water is substantially equal or as close as possible to the prescribed polymerization temperature. In particular, the temperature of the polymerization mixture should not be higher than the polymerization temperature by 20C or more since an excessively high temperature of the polymerization mixture causes a danger of a violent polymerization reaction only controllabie with difficulty.On the other hand, a lower temperature of the polymerization mixture than the polymerization temperature involves no problems of safety but the advantages of the present invention, viz improved productivity, are reduced in proportion to the temperature difference since the temperature of the polymerization mixture must be elevated by an outer heating means so that a time elapses before the start of the polymerization reaction. A temperature difference of 50C or smaller may be practically tolerable.Though not limited thereto, it is practically convenient that the temperatures of the monomer-initiator mixture and the pre-heated water are both kept constant throughout their period of introduction into the reactor and their introduction is begun at the same time and continued at constant rates in proportion to the ratio of their total amounts so that their introduction comes to an end at approximately the same time. In this manner, the temperature of the polymerization mixture is kept approximately constant during and at the end of the introduction of the monomer-initiator mixture and the pre-heated water.

The simultaneous introduction of the monomer-initiator mixture and the pre-heated water should be carried out under agitation and completed within a time as short as possible or, usually, within 30 minutes or less. If their introduction is prolonged, deposition of the polymer scale on the reactor walls is increased resulting in inferior quality of the polyvinyl chloride resin product with remarkably increased fish-eyes.

Needles to say, the suspension polymerization of vinyl chloride monomer is performed by dispersing the monomer in the aqueous medium with the aid of a dispersing agent or a suspending agent. When the dispersing agent is soluble in the monomer, it can be introduced into the polymerization reactor together with the monomer and dissolved therein. When the dispersing agent is soluble in water, on the other hand, it may be introduced into the reactor together with the pre-heated water and dissolved therein or, alternatively, it may be introduced into the reactor separately as an aqueous solution of a relatively high concentration, preferably, at the initial stage of or prior to the introduction of the pre-heated water.If the introduction of the dispersing agent is too iate the amount of the polymer scale deposition is increased resulting in inferior quality of the polymer product with increased fish-eyes.

The above mentioned dispersing agent or suspending agent may be a conventional one used in the suspension polymerization of vinyl chloride exemplified by water-soluble polymeric materials such as water-soluble cellulose ethers, e.g. methylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, hydroxypropyl methylcellulose and the like, partially saponified polyvinyl alcohols, polymers of acrylic acid, gelatin and the like, monomer-soluble emulsifying agents such as sorbitan monolaurate, sorbitan trioleate, block copolymers of ethyleneoxide and propyleneoxide and water-soluble emulsifying agents such as polyoxyethylene sorbitan monolaurate, polyoxyethylene glycerin oleate, sodium laurate and the like. These suspending agents may be used either alone or as a combination of two or more.

Furthermore, the suspension polymerization according to the present invention may be carried out with addition of various additives conventionally used in the prior art polymerization of vinyl chloride such as polymerization controlling agents, chain transfer agents, gelation improvers, anti-static agents, pH controlling agents and the like.

The method of the present invention is applicable not only to the homopolymerization of vinyl chloride but also to the copolymerization of a monomer mixture of which the main component, say, 50% by weight or more, is vinyl chloride. The comonomers copolymerizable with vinyl chloride are exemplified by vinyl esters such as vinyl acetate, vinyl propionate and the like, esters of acrylic acid and methacrylic acid such as methyl acrylate, ethyl acrylate, methyl methacrylate and the like, olefins such as ethylene, propylene and the like, maleic anhydride, acrylonitrile, styrene and vinylidene chloride.

In practicing the method of the present invention, the weight ratios of the individual ingredients.

polymerization temperature and other conditions may be determined in accordance with the conditions conventionally undertaken in the prior art methods of the suspension polymerization of vinyl chloride.

Using the method of the present invention, an advantage can be obtained in that the time taken for completion of a polymerization run can be greatly shortened so that the method is particularly effective in giving high productivity, especially in the case of polymerization with a large polymerization reactor having a relatively small heating capacity in relation to the volume capacity thereof, such that, in a conventional method, a long time is taken for the temperature elevation of the cold polymerization mixture up to the prescribed polymerization temperature.

EXAMPLE 1 (Experiments No. 1 to No. 8) In each of the experiments, the polymerization of vinyl chloride was performed in a polymerization reactor of 1.2 m3 capacity equipped with a stirrer and provided with a heating jacket.

In the first place, a mixture of the monomer and the polymerization initiator was prepared by dissolving 200 g of di-2-ethylhexylperoxy dicarbonate in 350 kg of vinyl chloride monomer without heating. The temperature of the mixture in each of the runs before introduction into the reactor was as indicated in Table 1 below together with the estimated conversion of the monomer in % to the polymer after 2 hours in the mixture at the temperature.

Separately, an aqueous solution of a suspending agent was prepared by dissolving 1 50 g of a partially saponified polyvinyl alcohol and 50 g of a hydroxypropyl methylcellulose in 480 kg of preheated water. The temperature of the thus prepared aqueous solution was as indicated in the table.

The above prepared cold monomer-initiator mixture and the pre-heated aqueous solution of the suspending agent were introduced separately but simultaneously at constant rates into the polymerization reactor through separate inlets to form a hot polymerization mixture with continuous agitation. The temperatures of the polymerization mixture during and at the end of the introduction of the monomer-initiator mixture and the aqueous solution in the reactor were as indicated in the table.

When the above given temperature of the polymerization mixture at the end of the introduction of the monomer-initiator mixture and the aqueous solution was lower than the prescribed polymerization temperature of 570C, the temperature of the polymerization mixture was further increased up to 57CC by means of the heating jacket taking a time shown in Table 1 in minutes and the polymerization reaction was started with continuous agitation.

When the polymerization reaction came near to the end and the pressure inside the reactor had dropped to 7 kg/cm2G, the unreacted monomer was recovered and the aqueous polymerizate slurry discharged out of the reactor was dehydrated and dried to give a polyvinyl chloride resin product.

In Experiment No. 8 shown in Table 1, the monomer initiator mixture was introduced into the polymerization reactor simultaneously with the pre-heated water containing no suspending agent dissolved therein followed by the separate addition of the suspending agent as an aqueous solution of a relatively high concentration.

In each of the experiments, the state of the polymer scale deposition on the reactor walls andthe number of fish-eyes in the polymer product were recorded according to the following criteria or the testing procedure.

Conversion of the monomer in the monomer-initiator mixture to the polymer after 2 hours (in%): 20 ml of the uniform mixture of the monomer and the polymerization initiator were sealed in a glass ampoule and the conversion was determined after 2 hours of standing of the glass ampoule at the temperature indicated.

Number of fish-eyes (pieces): a resin compound was prepared by blending 100 parts by weight of the resin product, 1 part by weight of tribasic lead sulfate, 1.5 parts by weight of lead stearate, 0.2 part by weight of titanium dioxide, 0.1 part by weight of carbon black and 50 parts by weight of dioctyl phthalate and 25 g of the compound were milled in a roller mill at 1 450C for 5 minutes and shaped into a sheet of 0.2 mm thickness. The number of translucent spots in 100 cm2 of this sheet was counted and recorded as the number of pieces of fish-eyes.

State of polymer scale deposition: A... almost no polymer scale deposition B... polymer scale deposition on the vapor-liquid interfacial region C . . . polymer scale deposition all over the walls of the polymerization reactor TABLE 1

Experiment No. 1 2 3 4 5 6 7 | 8 Monomerinitiator mixture Temperature, "C 25 12 18 23 22 36 30 25 Monomer conversion after 2 hours, % 1.5 < 1 < 1 1.5 1.5 7 4.5 1.5 Temperature of aqueous solution, "C 65.5 68 67 61.3 55.3 62.5 63 65 Charge time, minutes 10 20 40 15 15 10 5 15 Temperature of polymerization mixture, "C During charging 56-57 55-57 55-57 52-53 48-4955-57 55-56 55-56 At the end of charging 57 56 57 53 48 57 56 56 Time for temperature elevation to 57"C, minutes 0 5 0 20 45 0 5 5 Fish-eyes, pieces 9 12 > 500 23 > 500 > 500 28 > 500 Polymer scale deposition A A C A C C B C EXAMPLE 2 (Experiments No. 9 to No. 13) Into a polymerization reactor of 40 m3 capacity equipped with a condenser having an area for heat transfer of 40 m2 were introduced simultaneously 13 tons of vinyl chloride monomer and 1 8 m3 of pre-heated water at constant rates of 0.65 ton/minute and 0.9 m3/minute, respectively, with agitation taking 20 minutes.

In the piping line for the transfer of the vinyl chloride monomer to the reactor, 14 kg of a toluene solution containing 50% by weight of di(ethoxyethyl)peroxy dicarbonate were injected into the stream of the monomer at a rate of 0.7 kg/minute with agitation by a static mixer so that the initiator was uniformly blended with the monomer when arriving at the reactor.

Similarly, 100 liters of an aqueous solution containing 3.0 kg of a partially saponified polyvinyl alcohol and 2.5 kg of a hydroxypropyl methylcellulose dissolved therein were injected into the preheated water in the piping line at a rate of 5 liters/minute.

When the temperature of the polymerization mixture thus formed in the polymerization reactor was lower than 57"C, the temperature was further increased up to 570C by means of the heating jacket and the polymerization reaction was started with continuous agitation.

The polymerization reaction was terminated when the pressure inside the reactor had dropped to 7 kg/cm2G by recovering the unreacted monomer, and the aqueous polymerizate slurry discharged out of the reactor was dehydrated and dried to give a polyvinyl chloride resin product.

Table 2 given below shows the temperature of the monomer-initiator mixture at the inlet to the reactor, the monomer conversion in the monomer-initiator mixture at such a temperature after 2 hours, the temperature of the pre-heated water at the inlet to the reactor after admixing of the solution of the suspending agents, the temperature of the polymerization mixture at the end of the introduction of the monomer and the pre-heated water, the time taken for the temperature elevation of the polymerization mixture up to 570C and the polymerization time as well as the state of polymer scale deposition on the reactor walls and the number of fish-eyes in the polymer product in each of the experiments. In Table 2, Experiment No. 12 was undertaken for comparative purpose according to the conventional procedure in which the water as the aqueous polymerization medium containing the suspending agents, polymerization initiator and vinyl chloride monomer were introduced into the reactor successively one by one.

For further comparison, substantially the same procedure was repeated as in Experiment No. 9 except that the vinyl chloride monomer and the toluene solution of the polymerization initiator were introduced into the reactor not as a uniform mixture but separately through separate inlets omitting the static mixer in the piping. The increased number of fish-eyes in the resin product obtained in this experiment as shown in Table 2 clearly indicates the significance of the introduction of the monomer and the initiator as a uniform mixture (Experiment No. 1 3).

TABLE 2

Experiment No. 9 10 ' 11 12 13 Monomerbinitiator mixture Temperature, 1C 18 8 16 22 18 Monomer conversion after 2 hours, % 1 1 1 - I Temperature of pre-heated water, "C 67 67.5 19 25 67 Temperature of polymerization mixture at the end of charging, C 57 55 18 24 57 Time for temperature elevation to 57 .C, minutes 0 10 55 50 O Polymerization time, hours 4.8 5.0 5.7 6.2 4.8 Fish-eyes, pieces 8 9 5 8 > 500 Polymer scale deposition A A A A A

Claims

1. A method for the suspension polymerization of vinyl chloride monomer or a monomer mixture mainly composed of vinyl chloride in an aqueous medium containing a suspending agent in a polymerization reactor in the resence of a polymerization initiator at a prescribed elevated polymerization temperature, which comprises introducing the monomer or monomer mixture and the polymerization initiator as a uniform mixture simultaneously with the introduction of pre-heated water into the polymerization reactor to form a polymerization mixture therein to start the polymerization reaction, the temperature of the polymerization mixture during and at the end of the introduction of the uniform mixture of the monomer and the polymerization initiator and the pre-heated water being substantially equal to the prescribed polymerization temperature.

2. The method as set forth in claim 1 wherein the temperature of the polymerization mixture at the end of the introduction of the uniform mixture of the monomer and the polymerization initiator and the pre-heated water is not higher than the prescribed polymerization temperature by 20C or more.

3. The method as set forth in claim 1 or 2 wherein the uniform mixture of the monomer and the polymerization initiator before introduction into the polymerization reactor is kept at such a temperature that, when the mixture is kept at this temperature, the conversion of the monomer to polymer does not exceed 5% after 2 hours.

4. The method as set forth in any preceding claim wherein the introduction of the uniform mixture of the monomer and the polymerization reactor is completed within a time not exceeding 30 minutes.

5. The method as set forth in claim 1, 2, 3 or 4 wherein the suspending agent is dissolved in the pre-heated water prior to the introduction of the pre-heated water into the polymerization reactor.

6. The method as set forth in claim 1, 2, 3 or 4 wherein the suspending agent is introduced into the polymerization reactor in a form of an aqueous solution prior to the introduction or the uniform mixture of the monomer and the polymerization initiator and the pre-heated water into the polymerization reactor.

7. The method as set forth in any preceding claim wherein the introduction of the uniform mixture of the monomer and the polymerization initiator and the introduction of the pre-heated water are performed at constant rates beginning and ending at substantially the same time.

8. The method as set forth in Claim 1, substantially as described in any of Experiments 1,2,4,7,9,10,11 or 12.

9. A vinyl chloride polymer prepared by a method as set forth in any preceding claim.