Quantitative analysis of damage in an octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazonic-based composite explosive subjected to a linear thermal gradient

The microstructure within a slowly heated, consolidated explosive will be influenced by both physical changes and chemical reactions prior to thermal ignition. Thermal expansion, exothermic decomposition, endothermic phase change, and increased binder viscosity play significant roles in the cook-off to detonation. To further explore the details of this intricate cook-off process, we have conducted a series of experiments in which a carefully controlled temperature gradient has been applied along a cylinder of PBX 9501 [94.9/2.5/2.5/0.1-wt % octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX)/Estane 5703/a eutectic mixture of bis(2,2 dinitropropyl) acetal and bis(2,2-dinitropropyl) formal [abbreviated BDNPA-F]/Irganox] and maintained for a specified amount of time. After heating and subsequent cooling of the PBX 9501, the sample morphology has been probed with polarized light microscopy and small-angle x-ray scattering. Using these techniques we have quantitatively characterized the particle morphology, porosity, and chemical state of the explosive as a function of position, and therefore thermal treatment. Results of the analyses clearly show that thermal damage in PBX 9501 can be classified into two separate temperature regimes—an initial low-temperature regime (155–174 °C) dominated by the endothermic β-δ crystalline phase change, thermal expansion, and Ostwald ripening, and a high-temperature regime (175–210 °C) dominated by exothermic chemical decomposition. The results further show the complex interplay between the evolving sample morphology and the chemical reactions leading to a potential thermal self-ignition in the explosive.