Assessment of isoconversional methods and peak functions for the kinetic analysis of thermogravimetric data and its application to degradation processes of organic phase change materials

Abstract

In this work, theoretical kinetic curves of both single- and multi-step reaction mechanisms were simulated by using different sets of kinetic parameters. Various isoconversional methods were applied for the kinetic analysis of these curves so that the corresponding activation energy vs. conversion degree curves were obtained and then compared with the energy values used in the simulations. For single-step reaction mechanisms Friedman method resulted to be the most accurate while for multi-step reaction mechanisms, Kissinger–Akahira–Sunose and Coats–Redfern methods led to the most accurate estimation of the activation energy. On the other hand, conversion rate curves of different single-step reaction mechanisms were fitted with two kinds of peak functions (normalized Fraser–Suzuki and generalized logistic) so that the relationships between the parameters of these functions and the kinetic parameters used in the simulations were obtained. These relationships were then used in the mathematical deconvolution analysis of conversion rate curves simulated for multi-step reaction mechanisms. In general, the curves resulting from deconvolution fitted quite well the simulated conversion rate curves and the analysis of the resulting single-step reaction curves with Kissinger method led the kinetic parameters close to the ones used in the simulations. Finally, a similar kinetic analysis was applied to experimental thermogravimetric measurements taken both under N₂ and air for two phase change materials (PCMs) based on polyethylene glycol, PEG6000 and PEG12000. Activation energy values obtained with isoconversional methods for the measurements under N₂, varied from 40 kJ mol⁻¹ at low conversions up to 150 kJ mol⁻¹ at high conversions, whereas for the measurements under air the energy values remained almost constant in the range of 50–75 kJ mol⁻¹. The lower activation energies obtained for the measurements under air are clearly associated with the polymer combustion. The experimental conversion rate curves were deconvoluted with the most appropriate peak functions so that the possible single-step reaction mechanisms occurring in these PCMs were separated and further analyzed with Kissinger method. The activation energies obtained with this method were in good agreement with the values resulting from the isoconversional methods.