Vadim Mamleev

Modelling of nonisothermal kinetics in thermogravimetry

A new, fast and simple numerical method is proposed for modelling data of thermogravimetric analysis under arbitrary temperature–time relationships. The algorithm searches for activation energies and rate constants by means of minimisation of the average square of deviation, Δ, between computed and experimental curves on a scale of the logarithm of reduced time that, in turn, is expressed as the integral of the Arrhenius exponential. The algorithm tests phenomenological relations by considering the process mechanism. Eighteen known models corresponding to different physical and chemical processes are included as the basic set in the algorithm. Sequential analysis of the 18 variants and arrangement of values of Δ1/2 in ascending order allow a selection of the best of models. The less Δ1/2 is, the nearer is the calculated activation energy to the correct value. Then one can detect that satisfactory models provide a good approximation of the original kinetic curves.

Thermal degradation of cotton under linear heating

Derivatives of mass loss of cotton under linear heating have two main peaks. For each of the two stages 18 known models were tested to select the best approximation of the kinetic curves. The model of nucleation and a nucleus growth for the first stage and the model of diffusion control for the second stage are found to be the most adequate ones. The derivatives of mass loss with an asymmetrical form and a short tail are typical for diffusion-controlled processes. One can expect derivatives of such a type in the final stage of decomposition of high-porosity carbonized polymers of different origin. The activation energy of the diffusion-controlled oxidative destruction is estimated as 120-140 kJ/mol.

Thermogravimetric analysis of multistage decomposition of materials

The numerical method developed for thermogravimetric analysis (TGA) of one-stage processes is generalised to systems with multistage decomposition. Eighteen different physicochemical models are used to describe kinetics at each of the stages. In a two-stage decomposition the algorithm realises sequential analysis of 18 × 18 = 324 variants and characterises a point separating the two stages on the TG curves. Thus, it provides the minimum of the average square of deviation between the computed and the experimental curves on the scale of the logarithm of the reduced time that is expressed as the integral of the Arrhenius exponential. The same approach is used for the approximation of three-stage decomposition. Three-stage decomposition of foamed polyurethane in air is considered as an illustrative example. The first and second stages both correspond to bimolecular reactions, while the third one has been found to be a diffusion controlled process.