On the use of the model-free way method for kinetic analysis of thermoanalytical data – advantages and limitations

Abstract

This paper analyzes the possibility of using the model-free way method (MFW method) to evaluate the kinetic triplet (activation energy (E), pre-exponential factor (A) and conversion function) of a single-step process by processing non-isothermal data recorded at several constant heating rates. For this purpose, simulated non-isothermal data and experimental thermogravimetric data obtained to investigate the thermo-oxidative degradation of a sort of polyvinyl chloride (PVC) have been kinetically analyzed. Two MFW procedures, one differential and one iterative, are presented and used to determine E and A, as well as the reconstruction of the conversion function corresponding to the investigated processes. It is highlighted the dependence of these parameters on the set of kinetic models considered for performing the application stage of the MFW procedure, which consists in evaluating the parameters of the compensation effect apparently due to the variation of reaction model. It has been shown that, for the whole range of the degree of conversion in which the value of the activation energy is constant, the values of numerically reconstructed conversion function are proportional to the real values of the conversion function. Therefore, the results obtained by the MFW procedures give indications regarding the real expression of the conversion function, but they cannot be used for the unambiguous determination of the real conversion function. To solve the problem of determining the real kinetic parameters, a modified MFW method in which the linear relation of the compensation effect is replaced by a polynomial regression of lnA vs. E has been suggested and verified. This method can lead to the real values of the conversion function even if its algebraic expression is not part of their known theoretical set.