Poly thermal degradation rate

Milled rigid sheets of poly (vinyl chloride) on heating at 185°C. lose weight at a rate which increases with time. By polymer fractionation procedures, it was shown the rate of hydrogen chloride loss increases as the content of tetrahydro-furan-insoluble resin increases. The insoluble resin content accumulates at a rate which depends, in part, on the additive present. This insolubilization reaction is catalyzed by cadmium compounds. The increased dehydrochlorination rate of the insoluble crosslinked resins may result from the susceptibility of the crosslinked structures to oxidation and from the subsequent thermal degradation of the oxidation products. The effects of various common additives on the rates of insolubilization and weight loss are described. 

Thermal degradation does not occur until the temperature is so high that primary chemical bonds are separated. It begins typically at temperatures around 150-200 °C and the rate of degradation increases as the temperature increases. Pioneering work in this field was done by Madorsky and Straus (1954-1961), who found that some polymers (poly (methyl methacrylate), poly(oc-methylstyrene) and poly (tetrafluoroethylene)) mainly form back their monomers upon heating, while others (like polyethylene) yield a great many decomposition products. 

The same authors have also studied the effects of particle size on the rate of production of monomer during thermal degradation of poly(methyl methacrylate) and observed that the rate increases with particle size for particles smaller than about 50 mesh but it does fall away for larger particles. The study of the effect of layer thickness on the rate of mononrer production has also revealed the similar pattern of behavior of the polymer towards (tegradation. Several reasons for this behavior of the (tegradation are mentioned. "ITiese are ... 

The rate of degradation of poly(a-methylstyrene) increases linearly over time up to about 80% decomposition, after which it is sharply reduced. This linear dependence implies the reaction is of zero-order. The activation energy of the thermal degradation of poly(a-methylstyrene) is 187.3 kj/mol. 

Other workers state that the thermal degradation of poly(a-methylstyrene) is undoubtedly a first-order reaction with an activation energy of 271.7 kj/mol. The first-order kinetics are explained in terms of a mechanism of chain decay of the polymer proceeding randomly, which leads to the formation of free radicals. Then follows a rapid chain decay of chains having terminal free radicals to form monomer molecules. Since the rate of random cleavage of the chain is essentially lower than that of chain decay, the former predetermines the reaction rate as a whole, which therefore displays first-order behaviour. 

The activation energy of the thermal degradation of polyphenyl and poly(p-xylylene) as determined by their maximum decomposition rates are 209 and 305.1 kj/mol, respectively. 

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