First vinyl chloride monomer process

Technological advances in the production of the vinyl chloride monomer (VCM) have contributed to the declining price of the polymer. Figure 4 illustrates this statement the price of the vinyl chloride monomer (1) over a period of 20 years is plotted against two curves that represent the annual production of monomer made from two different bases, acetylene and ethylene. The classic acetylene route was the first to be exploited commercially, but its popularity has declined as more processes were developed that could utilize ethylene, a cheaper base. 

PVC, over the years, has been subjected to scrutiny on health and environmental grounds. One issue is the level of residual vinyl chloride monomer in the material that may migrate into food. Vinyl chloride monomer has been determined to be a carcinogen, at least under some conditions. In the polymerization process for PVC, less than 100% of the vinyl chloride monomer (VCM) is converted to polymer. This means that relatively high values of VCM may remain unreacted and trapped in the resin. To remove this residual monomer, the resin is subjected to repeated applications of vacuum. In this manner, VCM concentrations in the resin are reduced substantially. PVC packaging resins currently produced have much lower levels of residual vinyl chloride (under 10 ppb) than those used in containers in the mid-1970s when this concern first surfaced. 

Specifically for the PS bead suspension process, Villalobos et al. [9] reported that the end of the first stage occurred at approximately 30% monomer conversion, corresponding to a critical viscosity of about 0.1 Pa s. They also found that the second stage extended up to about a 70% monomer conversion. In the vinyl chloride monomer (VCM) powder polymerization, it has been shown that, at monomer conversions around 10-30%, a continuous polymer network is commonly formed inside the polymerizing monomer droplets that significantly reduces the drop/particle coalescence rate [10]. CeboUada etal. [11] reported that the PSD was essentially estabhshed at monomer conversions of about 35-40% (i.e., end of the second stage). 

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

Observations on the polymerization of readily polymerizable vinyl monomers such as styrene, vinyl chloride, and butadiene date back approximately to the first recorded isolation of the monomer in each case. Simon 2 reported in 1839 the conversion of styrene to a gelatinous mass, and Berthelot applied the term polymerization to the process in 1866. Bouchardat polymerized isoprene to a rubberlike substance. Depolymerization of a vinyl polymer to its monomer (and other products as well) by heating at elevated temperatures was frequently noted. Lemoine thought that these transformations of styrene could be likened to a reversible dissociation, a commonly held view. While the terms polymerization and depolymerization were quite generally applied in this sense, the constitution of the polymers was almost completely unknown. 

In addition to homopolymers of varying molecular and particle structure, copolymers are also available commercially in which vinyl chloride is the principal monomer. Comonomers used commercially include vinyl acetate, vinylidene chloride, propylene, acrylonitrile, vinyl isobutyl ether, and maleic, fumaric and acrylic esters. Of these the first three only are of importance to the plastics industry. The main function of introducing comonomer is to reduce the regularity of the polymer structure and thus lower the interchain forces. The polymers may therefore be processed at much lower temperatures and are useful in the manufacture of gramophone records and flooring compositions. 

Addition polymerization takes place for unsaturated monomers. In the presence of a catalyst, such as a free radical, a pi bond in the monomer is disturbed, and the resulting molecule is. itself, a chemically active free radical. This first step of the process is called initiation. The process may then continue, with the new molecule bonding with additional monomers in the same manner, thus forming a chain. Following this propagation step, free radicals may combine, thus forming a more stable polymer chain. This final step is called termination. Peroxides, such as benzoyl peroxide, are common agents that, when heat is applied, form free radicals that can initiate the polymerization process. An example of addition polymerization is shown below for the monomer vinyl chloride, which forms polyvinyl chloride. 

General Considerations. The terms addition and condensation polymers were first used by Carothers and are based on whether the repeating unit, mer, of a polymer chain contains the same atoms as the monomer Addition polymers have the same atoms as the monomer in the repeat unit, with the atoms in the backbone typically being only carbon. Condensation polymers typically contain fewer atoms within the repeat unit than the reactants because of the formation of byproducts during the polymerization process, and the polymer backbone typically contains atoms of more than one element. Polystyrene, poly(vinyl chloride), polyethylene, and poly(vinyl alcohol) are illustrative of addition polymers, and polyesters and polyamides (nylons) are illustrative of condensation polymers. The corresponding polymerizations are then called addition and condensation polymerizations. 

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacrylic fibers [14,15]. Dynel, a staple fiber, which was wet-spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus, acrylonitrile is mueh more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain transfer agent. 

Plastics waste can also serve as a source of chemical raw materials. The potential possibilities are considerable, here, since about 25%-30% of plastics consumed are thrown away as waste each year. The following process has proved to be useful hydrolyzable plastics are first hydrolyzed to their monomers below about 200° C the monomers are fractionally distilled off. Then, the poly(vinyl chloride) in the mixture is dehalogenated to poly(olefins) at about 350° C. The residues are then pyrolyzed at about 600-800° C in a sand-fluidized bed. The product fractions are very dependent on the composition of the pyrolyzed material. Generally, however, up to 40% fractions of the economically desirable aromatics are obtained by this high-temperature pyrolysis, and, indeed, when additional steam is blown into the system to reduce carbon char formation. Alternatively, what is known as a low-temperature pyrolysis can be carried out at about 400° C in poly(ethylene) wax as reaction medium. In this case, readily volatile oils of high olefin content are obtained together with waxes and carbon black. 

One process for bulk polymerization of vinyl chloride was developed in France where the initiator and monomer are heated at 60°C for approximately 12 h inside a rotating drum containing stainless steel balls. Typical initiators for this reaction are benzoyl peroxide or azobisisobutyronitrile. The speed of rotation of the drum controls the particle size of the final product. The process is also carried out in a two-reactor arrangement. In the first one approximately 10% of the monomer is converted. The material is then transferred to the second reactor where the polymerization is continued until it reaches 75-80% conversion. Special ribbon blenders are present in the second reactor. Control of the operation in the second reactor is quite critical [315]. 

Since poly(vinyl chloride) is insoluble in its own monomer, the bulk polymerization is a precipitation polymerization. It is carried out in a 12,000 dm cylindrical rotating autoclave to prevent the coagulation of precipitated polymer, which can occur if the heat of the polymerization is not dissipated. A modern process uses two polymerization stages. In the first stage, polymer is produced in relatively low yields. The polymer, which precipitates in the form of a low-density granular material, is then transferred to a second kettle, where further polymerization to high yields occurs on addition of more monomer. This process produces the polymer in particulate form, a form particularly suitable for the assimilation of plasticizers. In addition, this type of plasticizer takeup produces a more homogeneous system than is possible with the older, bulk polymerization method. 

The first process used to polymerize vinyl chloride was a heterogeneous bulk polymerization process where the monomer and initiator were reacted in a rotating cylindrical reactor with steel balls used to remove heat away from the product (the tumbling also served to ground up the resin product). 

Emulsion polymerization requires free-radical polymerizable monomers which form the structure of the polymer. The major monomers used in emulsion polymerization include butadiene, styrene, acrylonitrile, acrylate ester and methacrylate ester monomers, vinyl acetate, acrylic acid and methacrylic acid, and vinyl chloride. All these monomers have a different stmcture and, chemical and physical properties which can be considerable influence on the course of emulsion polymerization. The first classification of emulsion polymerization process is done with respect to the nature of monomers studied up to that time. This classification is based on data for the different solubilities of monomers in water and for the different initial rates of polymerization caused by the monomer solubilities in water. According to this classification, monomers are divided into three groups. The first group includes monomers which have good solubility in water such as acrylonitrile (solubility in water 8%). The second group includes monomers having 1-3 % solubility in water (methyl methacrylate and other acrylates). The third group includes monomers practically insoluble in water (butadiene, isoprene, styrene, vinyl chloride, etc.) [12]. 

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