Comparative investigation of thermal decomposition of various modifications of hexanitrohexaazaisowurtzitane (CL-20)

The thermal decomposition kinetics of different polymorphs of CL-20 (alpha, gamma and epsilon) has been investigated by thermogravimetry, IR spectroscopy and optical and electronic microscopy. The reactions proceed with self-acceleration and can be described by a kinetic law of first order with autocatalysis. Already at the earliest stages of decomposition (less than or equal to1%) phase transitions take place from alpha --> gamma and from epsilon --> gamma. For this reason the observed decomposition is related to the decomposition of gamma -CL-20. On the other hand, the kinetics of decomposition depends on the initial polymorphic state, so that the thermal decomposition increases in the series: alpha < <gamma> < <epsilon>. Experiments with different samples of alpha -CL-20 demonstrate that different rates of decomposition are observed for the same polymorph depending on the mean size and the size distribution of the crystals and their morphological features. In some cases the thermal stability of alpha -CL-20 can be increased by previous annealing. It is concluded that the thermal decomposition of CL-20 is purely a solid-state process. Microscopical and spectroscopical analysis of the condensed CL-20 decomposition product (formed after prolonged heating at high temperature) show that it has a network structure and consists mainly of carbon and nitrogen.