The power of model-fitting kinetic analysis applied to complex thermal decomposition of explosives: reconciling the kinetics of bicyclo-HMX thermolysis in solid state and solution

Abstract

Thermal decomposition of cis-1,3,4,6-tetranitrooctahydroimidazo-[4,5-d]imidazole (Bicyclo-HMX) both in solid state and in solution has been studied with a set of thermal analysis methods. Differential scanning calorimetry data obtained at several heating rates were analyzed using the model-fitting kinetic technique using the free open-source THINKS thermokinetic software. The kinetic parameters for a noncatalytic stage of thermolysis in solution are found to be Ea = 168.7 ± 1.4 kJ mol−1, log (A, s−1) = 15.7 ± 0.2, to prove the activation energy agrees with the theoretical prediction of the barrier for N–NO2 bond rupture in BC-HMX molecule. Thermal decomposition of solid BC-HMX is a complex process, typical for explosives. A formal three-step kinetic scheme is proposed, which explains the numerous experimental findings (thermal behavior at various confinement degree, morphology changes under heating) and the previous literature results as well. The first step of the process obeys a third-order nucleation-growth model with the Ea = 186.6 ± 1.2 kJ mol−1 and log (A, s−1) = 16.7 ± 0.3. Overall, the power of the model-fitting kinetic analysis is demonstrated and the extension of the proposed methodology to other energetic materials with complex decomposition patterns is suggested.