Chapter 21 - Thermal Decomposition

Abstract

The heat resistance of a polymer may be characterized by its temperatures of initial and of half decomposition. For many polymers, thermal degradation is characterized by the breaking of the weakest bond and is consequently determined by bond dissociation energy. Since the change in entropy is of the same order of magnitude in almost all dissociation reactions, it is assumed that also the activation entropy is approximately the same. This means that, in principle, the bond dissociation energy determines the phenomenon,so it is expected that the temperature at which the same degree of conversion is reached will be virtually proportional to this bond dissociation energy. This chapter summarizes the most important data about the thermal degradation of polymers. Following this, it describes the phenomology of thermal decomposition. The process of thermal decomposition or pyrolysis is characterized by a number of experimental indicators— the temperature of initial decomposition, the temperature of half decomposition, the temperature of the maximum rate of decomposition, the average energy of activation, and the amount of char residue at the end of the pyrolysis. These indicators, especially the characteristic temperatures, are dependent on the rate of heating (rise of temperature) applied during the pyrolysis. At increasing rate of heating, the characteristic temperature shifts to a higher value.