Thermal decomposition models for HMX-based plastic bonded explosives

Global multistep chemical kinetic models for the thermal decomposition of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazine (HMX)-based plastic bonded explosives (PBXs) using endothermic or exothermic binders are developed for calculation of the times to thermal explosion as functions of heating rate and geometry in the Chemical TOPAZ heat-transfer computer code. The decomposition mechanisms of the binder materials are treated separately from that of HMX, and the chemical reactions of each constituent are assumed to occur independently. Experimental data and theoretical predictions of the thermal properties, decomposition pathways, and chemical kinetic reaction rate constants are used to develop reaction sequences for each of the components present in the PBX at various weight percentages. The measured times to thermal explosion at various initial temperatures in a new One-Dimensional Time-to-Explosion (ODTX) apparatus are compared to the Chemical TOPAZ predictions. Two series of pristine HMX spheres formulated with coarse and fine particles are tested for the first time in an ODTX apparatus. The pure HMX data clearly show that the presence of an endothermic binder in a PBX increases the times to thermal explosion, while the presence of an exothermic binder decreases the times to explosion. The magnitudes of these changes in explosion time depend upon the chemical stabilities and heats of reaction of these binders. A four-step decomposition model is developed for HMX, which includes the β to δ solid-phase transition as the first endothermic reaction. This model accurately reproduces the pure HMX curves. Decomposition models for the various binder components are then used with the HMX model to accurately reproduce the ODTX time-to-explosion curves. Comparisons are also made to times to thermal explosion obtained in various experiments involving aged PBXs, ramped temperature rate increases, unconfined explosives, and a larger size, cylindrical geometry called the scaled thermal explosion experiment.