Reassessment of the Kinetics of the Thermal Decomposition of the High Explosive, HMX, in the Range 508 K to 524 K, as Studied by Isothermal Gravimetry

摘要

In 1986, Bulusu et al. reported on the kinetics of the thermal decomposition of HMX at atmospheric pressure from 508 K to 555 K, using isothermal gravimetric analysis. Their work implied that the decomposition kinetics followed a simple Arrhenius temperature rate law dependency but their analysis did not describe the actual kinetic pathway in any detail. In the present study, some key experiments were repeated. As a result, the following points, most of which are pertinent to any mechanistic discussion but were not considered in Bulusu's paper, can be made: (1) Over the entire temperature range of the Bulusu study, HMX exists not in its beta polymorphic form but in its delta form. (2) The thermal decomposition of HMX follows a reaction rate law which fits a sigmoidal alpha-t relationship of the form d (alpha) / d t = k' alpha (1 - alpha), where k' is the kinetic rate constant, alpha is the fractional extent of the reaction and t is the time. (3) This reaction proceeds with the complete loss of mass in the form of volatile products. (4) Over the temperature range 508 K to 524 K, the kinetics follow an Arrhenius relationship, the Arrhenius pre-exponential, In A (min-1), over this temperature range having a value of 31.07 and the energy of activation, Ea, a value of 144 kJ mol-1. However, somewhere beyond 524 K, a decomposition reaction with a different Arrhenius dependency probably comes into play the rate of reaction observed at 539 K being much greater than that calculated using the lower temperature range Arrhenius parameters. The mechanistic implications of this study are placed in the context of information from other studies.