Sur la dégradation thermique du polychlorure de vinyle. VI. Etapes initiales—etude générale préliminaire

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Abstract
A differential conductimetric apparatus allowing the continuous titration of HCl carried by a stream of gas with a limit sensitivity of 40 nanograms is described and applied to study the initial steps of poly(vinyl chloride) thermal degradation. The powdered sample is placed in a quartz cell which may be closed and detached and placed at a chosen time interval in a reflexion diffuse UV spectrometer. The dehydrochlorination process begins just above Tg; the isotherms under argon show, after a variable time, a steady state with a zero order and an activation energy of 22 kcal. For a dehydrochlorination ratio (DR) less than 1000 ppm, the shape of the UV visible spectrum does not depend on the DR, and for the same DR it does not depend on the degradation temperature. A plot of the reflectance ratio versus the DR shows a coloration process in two steps: a rapid step followed by a slower step. The phenomenon, which does not depend on the physical state of the sample, tends to disappear if the amount of polymer is lowered and if the rate of flow of the gas is increased. It suggests that the HCl formed upon the dehydrochlorination process remains partly dissolved in the polymer. Further proof of the dissolution of HCl in the polymer is given, and a through method of titration is proposed. It is suggested that the initial steps of the dehydrochlorination follows an ionic mechanism, and might be partially reversible and diffusion controlled, although the autocatalysis which involves a rather high dehydrochlorination ratio and the presence of a large amount of dissolved HCl might initiate the radical process.

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