Vinyl chloride monomer bulk polymerization

Poly(vinyl chloride) (PVC) is by volume the second largest thermoplastic manufactured in the world [35]. The world PVC demand in 2003 was about 28 million tonnes and the predicted annual growth rate of world PVC demand is close to 4% [36]. The sustainable expansion of the PVC industry is due to the high versatility of PVC as a plastic raw material together with its low price. A review of the qualitative and quantitative aspects of PVC polymerization can be found in Smallwood [35], Burgess [37], Langsam [38], TorneU and Uustalu [39],Xie etal. [40,41] and Yuan etaZ. [42]. Four polymerization processes (i.e., suspension, bulk, emulsion and solution) are commercially employed in PVC manufacturing. 

Bulk polymerization is the simplest process for producing PVC, since there are no difficulties on the separation and recovery of the residual monomer, or on the finishing of the produced polymer. The most well-known bulk vinyl chloride monomer (VCM) polymerization process these days is the Rhone-Poulenc two-stage process. According to this process, only VCM and oil-soluble initiators are introduced into the reactor, since there is no need for water and suspending agents. Due to the absence of water, productivity is very high, with respect to the other VCM polymerization processes (i.e., suspension, emulsion, solution). 

However, problems relevant to the heat removal and temperature control, as well as to the extremely high viscosity of the polymerization system, established the suspension polymerization process much more attractive for PVC production. It is indicative that only 8% of the total PVC production is obtained by the bulk polymerization process. 

The polymerization of VCM is highly exothermic (i.e., 100 kj mol ). Thus, the efficient removal of the reaction heat is very critical for the operation of large-scale reactors [44]. When all of the free liquid monomer has been consumed, the pressure in the reactor starts to fall as a result of the monomer mass transfer from the vapor phase to the polymer phase due to subsaturation conditions. In industrial PVC production, the reaction is usually stopped when a certain pressure drop has been recorded. Because the polymer is effectively insoluble in its own monomer, once the polymer chains are first generated, they precipitate immediately to form a separate phase in the polymerizing mixture. Thus, from a kinetic point of view, the polymerization of VCM is considered to take place in three stages [45]. 

During the first stage, primary radicals formed by the thermal fragmentation of the initiator molecules rapidly react with monomer to produce the first polymer chains. During this early polymerization period, the polymer concentration is below its solubihty hmit in 

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The production of vinyl chloride monomer is only a part of PVC production. Polymerization of the monomer completes the process. Commercially, it is a batch operation by one of three methods suspension, emulsion, or bulk. In all three methods, the chemical reaction is a free radical-initiated chain reaction. Peroxides or redox systems generally are used to provide the initial free radicals. 

The diad fractions have been determined as a function of polymerization temperature for many vinyl and acryl polymers. Many values of (AH. — Aff ) and (AS. — A) can thus be calculated (see the compilation of Elias and Goeldi for 85 monomer/solvent pairs). It is interesting that many negative differences (AS. — ASf/8 ) can be found for a given system monomer/solvent (Table III), but only a few negative values of (AH, . — AHf/g). Most negative is the (A— AHf/g ) for the polymerization of vinyl chloride in bulk (see Table II). All other values of (AH. — AH /g) are higher than zero or at least nearly zero. 

PVC is produced by polymerization of vinyl chloride by free-radical mechanisms, mainly in suspension and emulsion, but bulk and solution processes are also employed to some extent [11-14]. (The control of vinyl chloride monomer escaping into the atmosphere in the PVC production plant has become important because cases of angiosarcoma, a rare type of liver cancer, were found among workers exposed to the monomer. This led to setting of stringent standards by governments and modification of manufacturing processes by the producers to comply with the standards.)... 

The earliest polymerization processes were either batch mode or semibatch. The semibatch method was used for products, where the two monomers differed greatly in reactivity, as in Union Carbide s early Dynel, acrylonitrile-vinyl chloride, process. Bulk, solution, and emulsion polymerization processes have also been developed for acrylonitrile and its copolymers. However, in recent years nearly every major acrylic fiber producer has used a continuous aqueous suspension process, employing a redox catalyst, followed by a series of steps, which includes slurry filtration and polymer drying. 

The polymeric PVC is insoluble in the monomer therefore, bulk polymerization of PVC is a heterogeneous process. Suspension PVC is synthesized by suspension polymerization. These are suspended droplets approximately 10 to 100 nm in diameter of vinyl chloride monomer in water. Suspension polymerizations allow control of particle size, shape, and size distribution by varying the dispersing agents and stirring rate. Emulsion polymerization results in much smaller particle sizes than snspension polymerized PVC, but soaps used in the emulsion polymerization process can affect the electrical and optical properties. 

PVC is manufactured by three routes bulk (or mass), suspension, and emulsion polymerization using free radical initiators (section 1.8.1). In the bulk polymerization using liquid vinyl chloride monomer (VCM), the polymerization is usually done in two stages at 60 °C. Pre-polymerization to about 10% conversion yields a viscous suspension (PVC is insoluble in VCM) which is then added to a second horizontal reactor (together with more monomer and peroxide catalyst) with slowly rotating agitator blades. The mixture at 25% conversion becomes a powder. 

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. 

Poly(vinyl chloride) (PVC) is one of the most widely produced polymeric materials in use today. It is commercially produced by four major processes suspension, bulk, emulsion and solution. An industrially important method of production of PVC is emulsion polymerization. There are a lot of data regarding the kinetics and mechanism of emulsion polymerization of vinyl monomers. However, relatively little work has been done on the kinetics of vinyl chloride emulsion polymerization and much less on the emulsion copolymerization. Concerning the preparation of copolymer latexes of vinyl chloride monomer, there are only patents [1-3]. 

In a precipitation polymerization, the system initially is in a homogeneous phase, because monomer and initiator are completely soluble in the initial reaction medium, but, upon initiation, the formed polymer precipitates as soon as it forms. After precipitation, the polymerization proceeds by absorption of monomer and initiator into the polymer particles. Bulk polymerization of vinyl chloride and solution polymerization of acrylonitrile in water are examples of precipitation polymerization. Precipitation polymerizations are often referred to as powder or granular polymerizations because of the forms in which the final polymer products are obtained. 

Poly(vinyl chloride). Poly(vinyl chloride) (PVC) [9002-86-2] is a thermoplastic for building products. It is prepared by either the bulk or the suspension polymerization process. In each process residual monomer is removed because it is carcinogenic. Oxygen must be avoided throughout the process (see Vinyl polymers). 

Suspension polymerization produces beads of plastic for styrene, methyl methacrviaie. viny l chloride, and vinyl acetate production. The monomer, in which the catalyst must be soluble, is maintained in droplet fonn suspended in water by agitation in the presence of a stabilizer such as gelatin each droplet of monomer undergoes bulk polymerization. In emulsion polymerization, ihe monomer is dispersed in water by means of a surfactant to form tiny particles held in suspension I micellcsK The monomer enters the hydrocarbon part of the micelles for polymerization by a... 

Polymerization of vinyl chloride occurs through a radical chain addition mechanism, which can be achieved through bulk, suspension, or emulsion polymerization processes. Radical initiators used in vinyl chloride polymerization fall into two classes water-soluble or monomer-soluble. The water-soluble initiators, such as hydrogen peroxide and alkali metal persulfates, are used in emulsion polymerization processes where polymerization begins in the aqueous phase. Monomer-soluble initiators include peroxides, such as dilauryl and benzoyl peroxide, and azo species, such as 1,1 -azobisisobutyrate, which are shown in Fig. 22.2. These initiators are used in emulsion and bulk polymerization processes. 

Monomer and initiator must be soluble in the liquid and the solvent must have the desired chain-transfer characteristics, boiling point (above the temperature necessary to carry out the polymerization and low enough to allow for ready removal if the polymer is recovered by solvent evaporation). The presence of the solvent assists in heat removal and control (as it also does for suspension and emulsion polymerization systems). Polymer yield per reaction volume is lower than for bulk reactions. Also, solvent recovery and removal (from the polymer) is necessary. Many free radical and ionic polymerizations are carried out utilizing solution polymerization including water-soluble polymers prepared in aqueous solution (namely poly(acrylic acid), polyacrylamide, and poly(A-vinylpyrrolidinone). Polystyrene, poly(methyl methacrylate), poly(vinyl chloride), and polybutadiene are prepared from organic solution polymerizations. 

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. 

Uses. The azobisnitriles have been used for bulk, solution, emulsion, and suspension polymerization of all of the common vinyl monomers, including ethylene, styrene vinyl chloride, vinyl acetate, acylonitrile, and methyl methacrylate. The polymerizations of unsaturated polyesters and copolymerizations of vinyl compounds also have been initiated by these compounds. 

In free-radical polymerization in bulk, solution, and aqueous suspension the initiator is dissolved in the monomer. Aqueous suspension polymerization is considered as bulk polymerization in droplets. Some polymers, like poly (vinyl chloride) or polyacrylonitrile, are insoluble in the monomers and precipitate during bulk polymerization. The growth of the precipitated chain, which depends on the number of trapped radicals, is... 

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... 

Each year, hundres of thousands of tons of vinyl chloride are polymerized in the world. Commensurate attention is thus paid to studies of its polymerization. Vinyl chloride is one of those monomers that are transformed to polymer by a complicated mechanism. Poly(vinyl chloride) is soluble neither in its own monomer nor in the common solvents. Its formation is therefore connected with the appearance of a solid phase the process has the character of precipitation polymerization. This greatly complicates the kinetics of solution and bulk (suspension) polymerization. 

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