Effects of experimental conditions on the estimation of kinetic parameters of the thermal decomposition of azt using factorial design

The influence of experimental conditions on the kinetic parameters of solid-state reactions using thermogravimetry (TG) has been investigated for different materials. The evaluation of these thermal curves is used for the determination of the activation energy, Ea, the pre-exponential factor, A, and the function of the reaction extent,f(α) by the kinetic analysis for TG (Shimadzu software) based on the Ozawa method. Factorial designs are excellent tools for determining the effects of a wide variety of factors on the results of complex processes. Here a 24 factorial design was used to investigate the influence of four variables (sample mass, furnace environment, furnace atmosphere and sample container composition) on the estimation of kinetic parameters of the thermal decomposition of a zidovudine (AZT) pharmaceutical formulation. TG curves registered at five different heating rates showed that AZT has five stages of mass loss in the following temperature ranges: 25-130, 195-270, 270-350, 350-400 and 400-590°C. These events are characterized by the release of water molecules, thermal decomposition in three stages, followed by the elimination of carbonaceous materials. The average value obtained for Ea using the TG curves in different experimental conditions was 119 kj mol-1. The variability study of the influence of different experimental conditions of TG, using the factorial design, indicates with 95% probability that none of the analyzed effects were significant for the AZT sample. © 2005 Akadémiai Kiadó, Budapest.