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 x 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.