Applicability of Non-isothermal DSC and Ozawa Method for Studying Kinetics of Double Base Propellant Decomposition

In order to determine Arrhenius kinetic constants various experimental techniques and testing conditions have been used. Also, various kinetic approaches and data treatment procedures have been applied, resulting sometimes in considerable disagreement in the values of the kinetic parameters reported in literature. The non-isothermal differential scanning calorimetry (DSC) measurements and isoconversional Ozawa kinetic method are very often used to study kinetics of energetic materials. However, in some cases the Ozawa method is used uncritically, i.e. not taking into account some limitations of the method and possible dependence of experimental data on testing conditions. In our previous studies on double base and single base propellants we have shown that testing conditions (sample mass, heating rate, type of sample pan, etc.) may considerably affect kinetic results. An unusual behaviour that manifests in existence of a discontinuity and slope change of the Ozawa plot has been observed in the case of double base propellants. We have explained such behaviour by the sample self-heating effects. In this paper we have studied kinetics of decomposition of double base propellants from non-isothermal DSC experiments using unhermetically closed sample pans, and effect of nitroglycerine evaporation on the kinetic results. Kinetics of nitroglycerine evaporation has been studied by isothermal thermogravimetry. It has been shown by experiments and numerical simulation that at slower heating rates and smaller sample mass nitroglycerine may completely evaporate before DSC peak maximum, resulting in a higher values of the activation energy (173 kJ/mol). At faster heating rates and larger sample masses certain amount of nitroglycerine still exists in the propellant at the peak maximum temperature, resulting in lower values of the activation energy (142 kJ/mol). The discontinuity point on the Ozawa plot is connected with the presence of nitroglycerine in the propellant at DSC peak maximum temperature. This implies that the activation energy obtained using small samples and slow heating rates (173 kJ/mol) corresponds to the activation energy of decomposition of nitrocellulose from double base propellant