The study of competitive thermal degradation and evaporation phenomena by thermal analysis techniques

Thermal analysis techniques are commonly used to study thermal degradation reactions of solid or liquid substances. The degradative phenomena are often accompanied by contemporary evaporation, which may heavily influence the decomposition pathways and consequently the products formed in the decomposition process. Hence the importance of focusing on the relevance of the volatility of a substance during a thermal degradation process. The purpose of the present work is the validation of a method for the estimation of the vapour pressure of a substance. Thermogravimetric (TG) devices can be used in the evaluation of the weight loss of a liquid at constant temperature. The evaporation rate of a liquid below its boiling point in the TG analyser is controlled by mass transfer through a stagnant film between the evaporating surface and the crucible edge. If perturbations due to the nitrogen purge flow around the crucible are limited, a monodimensional analysis, based on Fick’s law, may be applied to the problem. This approach allowed the estimation of the apparent values of the vapour pressure from TG data through a simplified method based on the use of crucibles with different surface areas available for evaporation. The method was validated through experiments on several organic substances with known vapor pressure. In case of substances undergoing competitive thermal degradation and evaporation, the TG technique alone is not able to separate the two phenomena. It must be remarked that in this case also the use of conventional techniques for vapour pressure measurement is not possible, due to the influence of thermal degradation. Thus integrated approaches, based on the use of combined calorimetric techniques were developed. An experimental strategy based on simultaneous TG-DSC-FTIR analysis over different temperature ranges was used to provide information on the relative contributions of decomposition and thermal degradation. The use of a fixed bed reactor (FBR) for volatile product recovery allowed the estimation of the overall apparent kinetics of the thermal degradation process. Thus, it was possible to separate the contribution due to evaporation from the total weight loss in the TG runs, obtaining data on the apparent vapour pressure of the substance. Compared to conventional techniques for the measurement of vapour pressure, the methodologies proposed are able to provide only rough preliminary data. However, if a preliminary estimate of vapour pressure is of interest, the calorimetric techniques have important advantages. Conventional techniques need the use of specific equipment for vapour pressure measurement and require relevant quantities of substance (usually about 1000ml). The techniques developed in the present study are based on the use of standard devices for thermal analysis and require the use of small amount of substance (about 300l). Moreover, data may be obtained also in the presence of thermal decomposition processes by the use of specific experimental strategies.