Liquid vinyl chloride monomer process

In heterogeneous polymerizations in bulk, the formed polymer is insoluble in its monomer and the polyreaction is performed below the softening point of the polymer. On an industrial scale, this type of process is especially utilized for chain polymerizations, for example, the radical polymerization of liquid vinyl chloride, the polymerization of liquid propylene with Ziegler-Natta or with metallocene catalysts, and the polymerization of molten trioxane. 

This process is shown schematically in Figure 7. The ethylene part of the feed reacts with chlorine in the liquid phase to produce 1,2-di-chloroethane (EDC) by a simple addition reaction, in the presence of a ferric chloride catalyst (9). Thermal dehydrochlorination, or cracking, of the intermediate EDC then produces the vinyl chloride monomer and by-product HC1 (1). Acetylene is still needed as the other part of the over-all feed, to react with this by-product HC1 and produce VCM as in the all-acetylene route. 

Vinyl Chloride Monomer (VCM). Vinyl chloride monomer (VCM), bp -13.4°C, is a gas at room temperature and pressure. Therefore, it is handled as a compressed volatile liquid in all polymerization operations. Its vapor pressure over the typical polymerization temperature range of 50-70°C is 800-1250 kPa. As a result, PVC polymerization reactors are thick-walled jacketed steel vessels with a pressure rating of 1725 kPa. VCM is slightly soluble in water (0.11 wt% at 20°C). This has some influence on the suspension polymerization process and is critically important to the success of the emulsion polymerization process. The... 

Most vinyl chloride monomer today is made via a three-step process using ethylene oxyhydro-chlorination. A small amount is made by the reaction of acetylene and hydrogen chloride, either as liquids or gases, with a copper chloride catalyst in the liquid process and a mercury catalyst in the gas process. Vinyl chloride is also made by the heating of ethylene chloride with alcoholic alkali. 

Although vinyl chloride has been polymerized with organometallics [2], commercial poly(vinyl chloride) (PVC) is made by a free radical polymerization. The dominant process is the suspension process. Vinyl chloride (most commonly referred to as vinyl chloride monomer or VCM) is suspended as droplets in water and an initiator that is soluble in the VCM is added. VCM has a boiling point of -13 °C. It is reacted under pressure as a liquid. The resultant PVC powder is separated from the water and dried. PVC can also be prepared by a mass or bulk polymerization where the initiator is added to the liquid VCM. A third polymerization technique, an emulsion polymerization, employs liquid VCM, water, and an emulsifier. A water soluble initiator is added to the VCM/water emulsion. 

During the long period of development of poly(vinyl chloride) into one of the major plastics material, several basic processes for making PVC evolved. In all of these processes vinyl chloride was handled as a liquid under pressure. Despite the relative ease with which it could be polymerized by free radical initiators, the monomer, vinyl chloride, was regarded as an innocuous, relatively inert chemical. A number of producers of PVC resins were caught by total surprise in the 1970s when it was found that long-term (20-year) exposure to vinyl chloride monomer could cause rare forms of tumors. ... 

POLYVINYL CHLORIDE (PVC). [CAS 9002-86-2], The manufacture of polyvinyl chloride resins commences with the monomer, vinyl chloride, which is a gas, shipped and stored under pressure to keep it in a liquid state bp —14°C, fp —160°C, density (20°C), 0.91. The monomer is produced by the reaction of hydrochloric acid with acetylene. This reaction can be carried out in eidier a liquid or gaseous state. In another technique, ethylene is reacted with chlorine to produce ethylene dichloride. This is then cataiytically dehydrohalogcnatcd to produce vinyl chloride. The byproduct is hydrogen chloride. A later process, oxychlorination, permits the regeneration of chlorine from HC1 for recycle to the process. 

In other words, if any structural formations appear in the liquid, they may become a new reaction medium in which the polymerization may change substantially. The part of such a medium may be played, as we have seen, by fluctuational formations of monomer molecules but there is another possibility. The polymer molecules formed during polymerization are often more unevenly distributed in the monomer than in the solvent. They are much more likely to form structures than monomeric substances. The extreme case is the formation of a polymer which is insoluble in the monomer or in the solvent used for polymerization. In this case the polymer formed is evolved as a new phase, and if polymerization occurs inside or on the surface of the particles of this new phase, structural phenomena will naturally begin to play a major part. Perhaps the most vivid example of such a phenomenon is the bulk polymerization of vinyl chloride, whose structural features were studied recently (6, 7). The peculiarities of this process arise from the fact that... 

In this review dealing with recent advances in membrane science, the term membrane" will be used to indicate any medium which acts as a barrier to transport into or out of a region, provides selective transfer of one species over another or regelates the transport of a material to its environment at a controlled rate. In addition to the common usage of the word membrane" to indicate a dense polymer film, the above definition includes a variety of interesting cases such as highly porous ultrafiltration membranes and hydrophobic liquid membranes with selectivity properties which can be tailored by incorporation of materials which selectively complex with one of the species to be processed. The important topics of controlled release of chemicals from polymeric devices and removal of volatile monomers from addition polymers such as poly (vinyl chloride and poly (acrylonitrile are also treated here. 

Free radical polymerization of neat monomer in the absence of solvent and with only initiator present is called bulk or mass polymerization. Monomer in the liquid or vapor state is well mixed with initiator in a heated or cooled reactor as appropriate. The advantages of this method are that it is simple, and because of the few interacting components present, there is less possibility for contamination. However, vinyl-type polymerizations are highly exothermic so that control of the temperature of bulk polymerization may be difficult. Also, in the absence of a solvent viscosities may become very high toward the end of a polymerization, which could make stirring difficult, and add to the difficulty of heat removal from the system. The advantages of this system, however, are sufficiently attractive for this to be used commercially for the free radical polymerization of styrene, methyl methacrylate, vinyl chloride, and also for some of the polymerization processes of ethylene [7]. 

Chemical Applications. Fluidized bed processing has become widely used in the chemical industry. It is important in particular for the synthesis of polyethylene and polypropylene, key basic plastics used for packaging, textiles, and plastic components. Fluidized bed reactors are used also for the industrial production of monomers such as vinyl chloride or acrylonitrile, which are both used to make plastics. These reactors are also employed to produce polymers such as synthetic rubber and polystyrene. The advantages of uniform heat transfer, great surface interaction, and transportation as fluid, whether in liquid or gaseous form, have made fluidized bed processing very valuable for contemporary chemical industry processes. 

These factors alone may contribute significantly to variations in experimental results obtained in the polymerization of vinyl chloride. For example, at superatmospheric pressures, variation in the liquid volume of the monomer to the total free space of a closed reactor means a variation in the amount of gaseous monomer that can condense and/or diffuse into the polymer at some stage of the polymerization process. This vapor-phase monomer level may represent a monomer reservoir somewhat analogous to the monomer droplets postulated for the emulsion polymerization mechanism by Smith and Ewart. 

In Table VII, it will be noted that in a bulk polymerization, at a conversion of 10-20%, secondary particles form. That is, swollen polymer particles collide to form larger particles. As the process proceeds, virtually no free liquid monomer is present. Using a special autoclave, in which polymer lumps could be broken up, bulk polymerization of vinyl chloride could be carried out beyond this low conversion range [58]. This concept was improved upon in the basic patent for the Pechiney-Saint Gobain process [59]. Evidently Produits Chimique Pechiney-Saint Gobain was a... 

In the laboratory, residual monomer concentrations are typically measured by some kind of chromatographic analysis whether this is gas chromatography (GC) or liquid chromatography (LC) depends largely on the type(s) of monomers used in the polymerization process. Monomers that are gases or highly volatile liquids (for example, alkenes, vinyl chloride, acrylonitrile) or UV-inactive (for example, acrylates) are not readily compatible with LC. On the other hand, nonvolatile monomers (for example, diacids, diamines, bisphenol A) are not amenable to GC. 

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