Making and Using Quantitative Measurements of Thermal Degradation

Abstract

Experimental approaches: Thermal degradation studies of polymers present problems because at elevated temperatures the rates may be so high that the reaction times may be measured in seconds. This means that degradation temperatures of several hundred degrees must not only be attained in milliseconds, but then precisely maintained. Moreover, even if a pyrolysis filament can be made to have these characteristics, the polymer coated on it will not follow the temperature profile unless it is in the form of a very thin film and in practice this means sample sizes must be of the order of micrograms. Finally the products must be swept away, characterised and measured, with equipment of adequate sensitivity, before there has been any opportunity for secondary reactions. Considerable progress has been made in overcoming all of these difficulties by the pyrolysis-g.c. technique and reliable quantitative data can now be obtained on most polymeric systems. What data should be obtained & how is the information utilised? Data of different types may be measured with certain kinetic or mechanistic objectives in mind: (1) measurement of overall rate constants and activation energies; (2) verifying whether random scission is occurring; (3) studying chain depropagation, initiation and termination mechanisms; (4) verifying chain transfer mechanisms and (5) testing for secondary (consecutive) reactions. More complex examples and recent developments: (1) effects of blending on thermal stability; (2) studying the thermal stability of cross-linked samples; (3) thermal stability of low-MW oils; (4) predicting yields and structures of degradation products; (5) direct sample comparisons.