Thermal degradation of PVC

Guyot et al. [87] studied the influence of regular structures on thermal degradation of PVC in an inert... 

As discussed in Section III.G, the possibility of simultaneous initiation of degradation from normal units in the PVC chain is receiving increased attention. The work of Millan and coworkers who showed that the normal secondary chlorines in isotactic triads are also labile and can act as points of initiation for thermal degradation was also discussed. This favors simultaneous participation of normal chlorines along with defect sites in the thermal degradation of PVC. The content of nondefect labile structures in PVC is much higher than that of the defect structures [125]. 

Thus, the role of the defect labile structures in the thermal degradation of PVC, which is the key to the Frye and Horst mechanism of stabilization, is itself debatable. The Frye and Horst mechanism—the substitution of labile chlorines by more stable groups—has also been criticized. 

Mechanisms of thermal degradation of PVC, the structure of PVC and the stabilization of PVC have been the subject of many reviews. Those by Starnes,44 Endo45 and Ivan46 are some of the more recent. Defect structures in PVC arise during the propagation and chain transfer steps. As with PMMA, PVC formed by... 

Another relevant example is the alternating bond weight model. Here, we consider the repeat units as linked by two different bonds that alternate throughout the molecule. This case would apply to the thermal degradation of PVC after dehydrochlorination, where an alternating bond structure of the form C-C=C-C= exists along the polymer backbone.1 2 5 Let the ratio of the individual weights for the two bond types be X. Then for an i-mer... 

Orlando, Fl 7th-llth May, 2000, paper 86 PREDICTING THERMAL DEGRADATION OF PVC COMPOUNDS DURING INJECTION MOLDING... 

The influence of dicarboxylic acid ester plasticisers on the thermal degradation of PVC significantly depends on the physical state of the PVC-plasticiser system. If PVC retains the structure formed in the stage of suspension polymerisation, the additive produces inhibition of the process of thermal dehydrochlorination. In the case of true diluted PVC solutions in ester plasticisers, the polymer exhibits accelerated degradation, in accordance with a high value of the solvent basicity. 7 refs. 

Another type of argument to support a free radical mechanism was advanced by Palma and Carenza [172]. Thermal and 7-initiated dehydrochlorination between 80 and 130°C were compared. In view of the resemblance shown by the kinetic data for polyene formation, the same mechanism was thought to be operative in both cases. Since according to the authors, a free radical mechanism is clearly established for 7-initiated processes, this is also operative in thermal degradation. Salovey and Bair [171] reported that the thermal degradation of PVC at 155°C is enhanced by irradiation with 1 MeV electrons. Since later stages of isothermal weight loss for thermal and radiolytic decomposition follow 3/2 order kinetics, a free radical mechanism is also postulated by these workers. 

It is quite clear that free radicals are formed during the thermal degradation of PVC. On the other hand, the existence of molecular elimination also seems probable. There are two ways of correlating the data. 

De Rosa, M. I., and C. D. Litton. 1986. Oxidative thermal degradation of PVC-derived, fiberglass, cotton, and jute brattices, and other mine materials. A comparison of toxic gas and liquid concentrations and smoke-particle characterization. U.S. Bureau of Mines Rep. 9058 cited in Chem. Abstr. CA 106(6) 37827h. 

The chemical processes occurring in the thermal degradation of PVC are summarized in Scheme 5.4, which is designed to accommodate the two distinct processes that may cause the evolution of aromatic compoxmds and the mechanism of metal halide catalysis. 

Spectral analysis of volatile products shows that pure plasticizer evaporates before thermal degradation of PVC is recorded.Kinetic models of PVC thermal decomposition for interpretation of thermogravimetric data were proposed and verified by experimental data. ... 

Studies of the effect of plasticizers on PVC film stability were conducted using samples cast from tetrahydrofuran, THF. Samples which contained THF degraded more rapidly which is well known in weathering studies. THF exposed to UV radiation forms radicals which accelerate thermal degradation of PVC. Based on this example, it shonldbe also noted that plasticizer performance in any other aspect than weathering is not affected by admixtures but admixtures may be very critical for the outdoor performance of maity products. It is thus important to check the source and quality of the plasticizer. 

The thermal degradation of PVC, being essentially a dehydrochlorination up to about 277 °C, has a reaction order (n) of 1.5 and follows the equation ... 

The initiation of dehydrochlorination via formation of free chlorine atoms, which are capable of interacting with any atom in the chain, is hardly possible, since the release of chlorine due to chain termination by the reaction Cl + Cl Cl should also be expected. However, chlorine is not observed in the products of the thermal degradation of PVC over a wide temperature range [1]. 

---