Vinyl chloride thermal decomposition

From the results obtained by thermal decomposition of both low-molecular weight vicinal dichlorides in the gas phase [74,75] and of the copolymers of vinyl chloride and /rthermal instability of PVC to the individual head-to-head structures. Crawley and McNeill [76] chlorinated m-1,4-polybutadiene in methylene chloride, leading to a head-to-head, and a tail-to-tail PVC. They found, for powder samples under programmed heating conditions, that head-to-head polymers had a lower threshold temperature of degradation than normal PVC, but reached its maximum rate of degradation at higher temperatures. 

Thermal decomposition of this product yields vinyl chloride and HCl ... 

Homogeneous gas phase reactors will always be operated continuously whereas liquid phase reactors may be batch or continuous. Tubular (pipe-line) reactors are normally used for homogeneous gas-phase reactions for example, in the thermal cracking of petroleum crude oil fractions to ethylene, and the thermal decomposition of dichloroethane to vinyl chloride. Both tubular and stirred tank reactors are used for homogeneous liquid-phase reactions. 

The gaseous products formed on thermal decomposition of ethylene-platinous chloride are ethylene, hydrogen chloride, vinyl chloride, ethyl chloride, ethylene dichloride and ethylidine dichloride. The half life for the decomposition at 130° is 4.5 days, at 172° it is 1.7 hours 98). The hydrolysis of Zeisc s salt K[PtCl3(C2H4)] by water and dilute acids has been studied ... 

Pyrolysis. Vinyl chloride is more stable than saturated ehloroalkanes to thermal pyrolysis. That is why nearly all vmyl chlonde made commercially comes from thermal clehydrochlorination of ethylene dichloride (EDC). When vinyl chloride is heated to 450°C, only small amounts of acetylene form. Decomposition of vinyl chlonde via a free-radical chain process begins at approximately 550°C, and increases with increasing temperature. Acetylene, HC1. chloropiene, and vinylacetylene are formed in about 35% total yield at 680°C. At higher temperatures, tar and soot formation becomes increasingly important. When dry and in contact with metals, vinyl chloride does not decompose below 450°C. However, if water is present, vinyl chloride can corrode iron, steel, and alum in 11m because ofthe presence of trace amounts of HC1. This HC1 may result from the hydrolysis of the peroxide formed between oxygen and vinyl chlonde. 

The thermal and photochemical dehydrochlorination of the vinyl chloride—CO copolymer have been studied by two different groups56,57). The decomposition rate for the copolymer was significantly higher than that for poly(vinyl chloride), the rate increasing with increasing CO content of the copolymer. In addition, the thermal decomposition of the copolymer was accelerated in the presence of molecular 02 while the photodegradation was slowed down 57). As with poly(vinyl chloride), the dehydrochlorination of the copolymer resulted in the formation of polyene sequences. There was no appreciable decrease in molecular weight. 

The thermal stability of poly(vinyl chloride) is improved greatly by the in situ polymerization of butadiene or by reaction with preformed cis-1,4-polybutadiene using a diethyl-aluminum chloride-cobalt compound catalyst system. The improved thermal stability at 3-10% add-on is manifested by greatly reduced discoloration when the modified poly-(vinyl chloride) is compression molded at 200°C in air in the absence of a stabilizer, hydrogen chloride evolution at 180°C is retarded, and the temperature for the onset of HCl evolution and the peak decomposition temperature (DTA) increase, i.e. 260°-280°C and 290°-325° C, respectively, compared with 240°-260°C and 260°-280°C for the unmodified homopolymer, in the absence of stabilizer. The grafting reaction may be carried out on suspension, emulsion, or bulk polymerized poly(vinyl chloride) with little or no change in the glass transition temperature. 

INFLUENCE OF POLYMER ADDITIVES ON THERMAL DECOMPOSITION AND SMOKE EMISSION OF POLY (VINYL CHLORIDE)... 

Polymer Degradation and Stability 78, No.2, 2002, p.349-56 A STUDY OF THE THERMAL DECOMPOSITION AND SMOKE SUPPRESSION OF POLY(VINYL CHLORIDE) TREATED WITH METAL OXIDES USING A CONE CALORIMETER AT A HIGH INCIDENT HEAT FLUX Bin Li... 

Thermal decomposition of —N=N— groups in the presence of monomers (for example vinyl chloride) leads to the formation of block copolymers [107], When the decomposing —N=N— groups is bound to a polymeric chain as a substituent, graft copolymers are produced. An example of such an initiator may be the copolymer of styrene with (4-vinylphenylazo)-2-methylmalononitrile [108]... 

Important in combustion is not so much the thermal stability of the material itself but rather the amount and nature of the decomposition products. It is sufficient to compare the LOI of poly(vinyl chloride), whose thermal decomposition begins at 160-175 °C with that of heat resistant phenol-formaldehyde fibers (Kynol). The thermodynamic approach to the problem seems to be most reasonable. It allows to consider the polymer structure to explain the details of the combustion reactions and to estimate the heat of combustion of polymers. 

M. Blazso and E. Jakab, Effect of metals, metal oxides, and carboxylates on the thermal decomposition processes of poly-(vinyl chloride), J. Anal. Appl. Pyrolysis 49, 125 (1999). 

Elimination of carbon dioxide from carboxyl, water from alcoholic hydroxyl, carboxylic acid from alkanoate, and hydrogen chloride from chlorine side groups or chain ends are typical thermal decomposition reactions in the temperature range 250-350°C. Hydrogen chloride is an important product of poly(vinyl chloride) because every second carbon atom of the hydrocarbon polymer chain is chlorine substituted. But hydroxyl, alkanoate and free carboxylic acid groups normally occur only at the ends of the macromolecular chains in customary plastics, thus the contribution of their elimination to the volatile pyrolysis products is negligible. 

The majority of packaging plastic materials consists of polyolefins and vinyl polymers, namely polyethylene (PE), polypropylene (PP), polystyrene (PS) and poly(vinyl chloride) (PVC). Obviously, these polymers have many other applications not only as packaging materials. Chemically they are all composed of saturated hydrocarbon chains of macro-molecular size their typical thermal decomposition pathway is free radical one initiated by the homolytic scission of a backbone carbon-carbon bond. In spite of the basic similarity of the initial cleavage, the decomposition of the hydrocarbon macroradicals is strongly influenced by fhe nafure of the side groups of the main chain. 

PVC is a vinyl polymer where X is a chlorine atom in the general formula of Scheme 12.3, but its thermal decomposition follows a quite differeut pathway from that of PS, starting with division of the side groups from the main chain. When a chlorine atom is cut off from a defect point (tertiary carbon atom or allyl bond), it abstracts the hydrogen atom from the next carbon atom, then hydrogen chloride is eliminated, together with the... 

Z. Czegeny, E. Jakab, M. Blazso, Thermal decomposition of Polymer mixtures containing Poly(vinyl chloride), Macromol. Mater. Eng., 287, 277-284 (2002). 

Z. Czegeny and M. Blazso, Thermal decomposition of polyamides in the presence of poly(vinyl chloride), J. Anal. Appl Pyrol, 58-59, 95-104 (2001). 

Some studies show that pyrolysis of certain polymer blends can be influenced by the migration of a small molecule or a small radical formed from one type of polymer and affecting the other type. For example, poly(methyl methacrylate) (PMMA) in blends with poly(vinyl chloride) (PVC) shows higher resistance to heat. The thermal decomposition of PVC generates HCI, which interacts with the PMMA forming anhydride units in the middle of PMMA chains, as shown below ... 

Thermal properties of some copolymers of poly(a-methylstyrene) are reported in literature, an example being poly(a-methylstyrene-co-methyl methacrylate) [100]. Thermal decomposition of poly(vinyl chloride)-/b/end-poly(a-methylstyrene) when heated between ambient to 500° C also is reported [106], In these conditions, the polymer generates mainly HCI and a-methylstyrene. Other pyrolysis studies are also available [100, 123-125],... 

Table 6.3.1. Summary regarding literature information on thermal decomposition of poly(vinyl chloride), CAS 9002-86-2.

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