Abstract
In this tutorial overview article the authors share their original experience in studying the kinetics of thermally stimulated reactions under the conditions of continuous cooling. It is stressed that the kinetics measured on heating is similar to that measured on cooling only for single-step reactions. For multi-step reactions the respective kinetics can differ dramatically. The application of an isoconversional method to thermogravimetry (TGA) or differential scanning calorimetry (DSC) data allows one to recognize multi-step kinetics in the form of the activation energy that varies with conversion. Authors’ argument is supported by theoretical considerations as well as by experimental examples that include the reactions of thermal decomposition and crosslinking polymerization (curing). The observed differences in the kinetics measured on heating and cooling ultimately manifest themselves in the Arrhenius plots of the opposite curvatures, which means that the heating kinetics cannot be used to predict the kinetics on cooling. The article provides important background knowledge necessary for conducting successful kinetic studies on cooling. It includes a practical advice on optimizing the parameters of cooling experiments as well as on proper usage of kinetic methods for analysis of obtained data.
Highlights
This article summarizes our experience in studying the kinetics of thermally stimulated reactions taking place during continuous cooling
We have studied the kinetics of thermal decomposition and thermal polymerization by using two methods of thermal analysis: differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA)
The plot obtained for heating is concave down (E decreases with T), whereas for cooling it is concave up (E increases with T)
Summary
This article summarizes our experience in studying the kinetics of thermally stimulated reactions taking place during continuous cooling. DSC and TGA are used broadly for kinetic studies of these types of reactions [1,2,3] These studies are conducted routinely on heating, i.e., by continuously raising temperature. Molecules 2019, 24, 1918 modulated DSC and TGA as well as of controlled rate thermal analysis In these techniques the cooling segments are invariably combined with heating segments. Its objective is to create a single reference that covers all basics needed for one to successfully conduct a kinetic study of thermally stimulated reactions occurring on continuous cooling. To accomplish this objective, we discuss scholastic and pragmatic motivation, the theory and praxis of measurements, kinetic computations, and representative examples
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