The effect of crystal structure on the thermal reactivity of CL-20 and its C4 bonded explosives (I): Thermodynamic properties and decomposition kinetics

The crystal structure, thermal behavior, and decomposition kinetics of ε-CL-20, RS-ε-CL-20, α-CL-20, ε-CL-20/C4, and RS-ε-CL-20/C4 were investigated by nonisothermal FTIR, TG, and DSC techniques. It was found that the thermal decomposition of α-CL-20, ε-CL-20/C4, and RS-ε-CL-20/C4 could be considered as a two-step process and the initial step is partly controlled by crystal structure. However, the crystal structure could only affect the initial step of decomposition and the total heat release, and the heat release of RS-ε-CL-20 is the highest compared with α- and normal ε-CL-20. In addition, the activation energy of studied materials was calculated by Kissinger method and modified KAS method, which was compared with the results obtained by other researchers. It was indicated that the obtained activation energy of ε-CL-20 by Kissinger method is about 176.0 kJ mol^-1, which is almost the same with the results from the literatures by STABIL and Noniso-TG methods. It was noticed that the crystal structure has significant effect on the initial activation energy distribution of CL-20, while in case of second stage (α = 0.30-0.85) this effect is relatively small, resulting in identical decomposition mechanism. Moreover, the kinetic compensation effects show that the studied materials could be divided into two groups, one including ε-CL-20, RS-ε-CL-20, α-CL-20, and ε-CL-20/C4 which decompose at solid state and another including ε-CL-20/Formex and RS-ε-CL-20/C4 which decompose at partial liquid state, resulting in different kinetic compensation effects. It reveals that the C4 base could affect the distribution of activation energy of ε-CL-20 and RS-ε-CL-20 in a totally different way.