Thermogravimetric Analysis of Nonisothermal Decomposition

Kinetics of Substances

Vadim Mamleev^{1}, Serge Bourbigot^{2}, Michel Le Bras^{3}, Sophie Duquesne^{3},

Jaroslav Šesták 4

^{1Institute of Polymer Materials and Technology, Kazakh-American University, Satpaev Str., 18a, Almaty, 480013, Kazakhstan.
2Laboratoire de Génie et Matériaux Textiles (GEMTEX), UPRES EA2161, Ecole Nationale Supérieure des Arts et Industries Textiles (ENSAIT), BP 30329, 59056 Roubaix Cedex 01, France
3Laboratoire de Génie des Procédés d’Interactions Fluides Réactifs-Matériaux (GEPIFREM), UPRES EA2698, Ecole Nationale Supérieure de Chimie de Lille (ENSCL), Université des Sciences et Technologies de Lille (USTL), BP 108, 59652 Villeneuve d’Ascq Cedex, France
4Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 18040 Prague 8, Czech Republic
Abstract
A new, fast and simple numerical method is proposed for modeling data of thermogravimetric analysis under arbitrary temperature-time relationships. The algorithm searches for activation energies and rate constants by means of minimization of the average square of deviation, D, 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 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 18 variants and arrangement of values of D1/2 in ascending order allow a selection of the best of models. The less D1/2 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.
The method is generalized to systems with multistage decomposition. In a two-stage decomposition the algorithm realizes sequential analysis of 18´18 = 324 variants and characterizes a point separating the two stages on the TG curves. The same approach is used for the approximation of a three-stage decomposition. Three-stage decomposition of foamed polyurethane in air is considered as an illustrative example. The first stage and the second one both correspond to bimolecular reactions, while the third one has been found to be diffusion controlled process.
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