Polymerization of polyvinyl chloride

Why is the reaction temperature of polymerization of polyvinyl chloride so important to its manufacture ... 

The family of dialkyl peroxides includes dicumyl peroxide, which accounts for one-third of the volume of dialkyls world-wide and is the workhorse of this family of peroxides. Dicumyl peroxide is commonly used as a catalyst in polyester resin systans and for cross-linking polyethylene. Benzoyl peroxide is the most common of the diacyl peroxides. It is also used as a catalyst for curing polyester resins. Hydroperoxides are generally used as a raw material to produce other organic peroxides. The most common peroxides in this family include cumene hydroperoxide and t-butyl hydroperoxide. Ketone peroxides are mixtures of peroxides and hydroperoxides that are commonly used for room-temperature curing of polyester resins. Methyl ethyl ketone peroxide (MEKP) is the major product in this family. Peroxydicarbonates are largely used to initiate polymerization of polyvinyl chloride (PVC). 

In the polymerization of polyvinyl chloride, unwanted gas is generated by side reactions, and some small amounts of air leak into the reactors. These inerts must be vented from the process. Because vinyl chloride monomer (VCM) is extremely volatile, the purge gas, although it is typically at 4-5 bars pressure, can contain as much as 50vol% monomer. As a consequence, the vented gas stream, although small, may contain several hundred thousand dollars worth of monomer values. A typical process flow scheme to recover VCM is shown in Figure 21.10 (Lahierc et al., 1993). 

The paper discusses the application of dynamic indentation method and apparatus for the evaluation of viscoelastic properties of polymeric materials. The three-element model of viscoelastic material has been used to calculate the rigidity and the viscosity. Using a measurements of the indentation as a function of a current velocity change on impact with the material under test, the contact force and the displacement diagrams as a function of time are plotted. Experimental results of the testing of polyvinyl chloride cable coating by dynamic indentation method and data of the static tensile test are presented. 

In the suspension polymerization of PVC, droplets of monomer 30—150 p.m in diameter are dispersed in water by agitation. A thin membrane is formed at the water—monomer interface by dispersants such as poly(vinyl alcohol) or methyl cellulose. This membrane, isolated by dissolving the PVC in tetrahydrofuran and measured at 0.01—0.02-p.m thick, has been found to be a graft copolymer of polyvinyl chloride and poly(vinyl alcohol) (4,5). Early in the polymerization, particles of PVC deposit onto the membrane from both the monomer and the water sides, forming a skin 0.5—5-p.m thick that can be observed on grains sectioned after polymerization (4,6). Primary particles, 1 p.m in diameter, deposit onto the membrane from the monomer side (Pig. 1), whereas water-phase polymer, 0.1 p.m in diameter, deposits onto the skin from the water side of the membrane (Pig. 2) (4). These domain-sized water-phase particles may be one source of the observed domain stmcture (7). 

The vinyl chloride monomer polymerizes via addition polymerization to form polyvinyl chloride. The final polymer has the chemical composition shown in Fig. 22.1. The polymer exhibits limited crystallinity, though this property is not often considered as important in defining its performance. It tends to be atactic or regionally syndiotactic, surrounded by extended atactic runs. When exposed to temperatures above 100 °C, polyvinyl chloride decomposes, creating free radicals that further attack the polymer chain, as we shall discuss in more detail later. For this reason, the degradation of polyvinyl chloride is autocatalytic... 

Figure 22.3 Example of weight average molecular weight of polyvinyl chloride as a function of polymerization temperature...

Vinyl chloride polymerization occurs via an exothermic radical reaction. In fact, the reaction is approximately 25% more exothermic than polyethylene polymerization. The highly exothermic nature of the reaction and the strong molecular weight dependence on temperature make heat transfer, and its control, critical to the manufacture of polyvinyl chloride. 

Vinyl chloride has been known for over a hundred years and its polymerization to polyvinyl chloride (PVC) was achieved in 1912. Industrial-scale production of this plastic began in 1927. PVC is still the most versatile plastic. One of the reasons for this is the numerous variations made possible by the method of manufacture of the polymer, namely by copolymerization with other monomers and their processing. Thus, PVC can be thermoformed on all conventional processing machines if the slight thermal damage is taken into consideration. Machining is easy and the material can be bonded, bent, welded, printed and thermoformed. 

Before the mechanism of vinyl polymerization was understood, the question of the structure of vinyl polymers was of considerable interest. Staudinger had written these polymers as having a head-to-tail arrangement of recurring units, but he had not really furnished evidence of the structure. As Carothers once said, Staudinger had assigned the structure by pronouncement. He was as usual correct, and chemical evidence was developed to establish such structures. For example, when monovinyl methyl ketone polymerized, it could produce by head-to-head, tail-to-tail reaction a 1,4-diketone. By head-to-tail polymerization it would give a 1,5-diketone. These two types have different reactions. The study of the polymer proper showed that the polymer was a 1,5-diketone. In the case of polyvinyl chloride, a head-to-head, tail-to-tail polymerization would lead to a 1,2-dihalide compound, and a head-to-tail polymerization would lead to a 1,3-dihalide. 

Addition of a molecule across a triple bond yields a molecule that stiU contains a double bond and is therefore reactive. Polymerization of vinyl chloride yields polyvinyl chloride... 

Carbon materials were obtained from polymeric precursors produced by chemical dehydrochlorination of polyvinyl chloride-polyvinyUdene chloride and chlorinated polyvinyl chloride in the presence of a strong base, followed by subsequent thermal treatment under relatively mild conditions. The sorbents obtained have three types of pores ultra-micropores, miaopores, and mesopores. hi this respect, they differ substantially from microporous activated carbons such as Saran, conventionally prepared from chlorinated polymers by thermal treatment without chemical dehydrochlorination. 

Ethynes are industrially used as a starting material for polymers, e.g. vinyl flooring, plastic piping. Teflon and acrylics. Polymers are large molecules, which are prepared hy linking many small monomers. Polyvinyl chloride, also commonly known as PVC, is a polymer produced from the polymerization of vinyl chloride. 

Polyvinyl chloride. Organic peroxides are used to catalyze the free radical polymerization of vinyl chloride monomer in water. The organic peroxide is selected to generate free radicals thermally at the temperature of polymerization. 

Write the reaction for the polymerization of vinyl chloride (chloroethene). Show the repeating unit of the resulting polymer (Polyvinyl chloride, PVC). 

Polyvinyl chloride is produced by the free-radical polymerization of vinyl chloride. Bulk, emulsion, solution, and suspension polymerization processes have been used. 

Thermal degradation and degradation kinetics of polyvinyl chloride (PVC) plastisols, plasticised by polymeric plasticiser, were examined using dynamic and... 

Molecular Weight of Polyvinyl Chloride Produced by Radiation Induced Emulsion Polymerization at 25°C... 

Another application in macromolecular chemistry is radiation-induced graft polymerization, by which favourable properties of two polymers can be combined. In this process, copolymers of A and B are produced by irradiation of the polymer A in the presence of the monomer B. Examples are graft polymers of polyethylene and acrylic acid or of polyvinyl chloride and styrene. The properties of textiles (cellulose, wool, natural silk, polyamides, polyesters) can also be modified by graft polymerization, for example for the production of weatherproof products. 

Manufacture. The monomer of polyvinyl chloride is gas at room temperature and has a boiling point of -13.9°C. The monomer is manufactured in two ways (1) Acetylene and HCl gas are allowed to pass over active carbon and to react with each other at 150 200 C (2) chlorine gas is allov/-ed to react on ethylene to produce C H CI CEDC) at 200 350 C to produce vinyl chloride and HCl gas. These are separated by distillation. The monomer is then polymerized in a pressure vessel the mixture of water and liquid monomer is stirred violently. Benzoyl peroxide is used as a catalyzer and polyvinyl alcohol as a dispersion agent for the reaction. 

In addition polymerization, monomers react to form a polymer chain without net loss of atoms. The most common type of addition polymerization involves the free-radical chain reaction of molecules that have C = C bonds. As in the chain reactions considered in Section 18.4, the overall process consists of three steps initiation, propagation (repeated many times to build up a long chain), and termination. As an example, consider the polymerization of vinyl chloride (chloro-ethene, CH2 = CHC1) to polyvinyl chloride (Fig. 23.1). This process can be initiated by a small concentration of molecules that have bonds weak enough to be broken by the action of light or heat, giving radicals. An example of such an initiator is a peroxide, which can be represented as R—O—O—R, where R and R represent alkyl groups. The weak 0—0 bonds break... 

Oxychlorination of hydrocarbons refers to a chemical reaction in which oxygen and hydrogen chloride react with a hydrocarbon in the vapor phase over a supported copper chloride catalyst to produce a chlorinated hydrocarbon and water. The oxychlorination of ethylene to produce 1, 2-dichloroethane (commonly, ethylene dichloride (EDC)) is of the greatest commercial importance. EDC is the precurser for vinyl chloride monomer, which when polymerized to polyvinyl chloride (PVC), becomes one of the most conunonly used commercial plastics. The overall oxychlorination reaction of ethane is given by... 

There are some other ways to apply coatings without the use of solvents. Plastisols315 involve the use of a paste of polyvinyl chloride particles in a plasticizer. Metal objects dipped in this can then be heated to harden the coating by dissolution of the plasticizer into the polymer. An example would be a wire rack for a dishwasher. Colored paint films can be laminated to plastics such as automobile parts and siding for houses by molding.316 Ideally, the dry paint film would be prepared by extrusion or other means that avoided the use of solvent. It is also possible to produce a coating from two monomer ingredients that polymerize in... 

The requirements for the quality of water used in polymerization processes (e.g. in the production of polyvinyl chloride, polystyrene) are particularly stringent. In this case, process water should have the quality of distilled water [13]. 

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