We have used a modified version of the Sandia Instrumented Thermal Ignition (SITI) experiment to develop a pressure-dependent, five-step ignition model for a plastic bonded explosive (PBX 9501) consisting of 95 wt. % octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazoncine (HMX), 2.5 wt. % Estane 5703 (a polyurethane thermoplastic), and 2.5 wt. % of a nitroplasticizer (NP): BDNPA/F, a 50/50 wt. % eutectic mixture bis(2,2-dinitropropyl)-acetal (BDNPA) and bis(2,2-dinitropropyl)-formal (BDNPF). The five steps include desorption of water, decomposition of the NP to form NO 2 , reaction of the NO 2 with Estane and HMX, and decomposition of HMX. The model was fit using our experiments and successfully validated with experiments from five other laboratories with scales ranging from about 2 g to more than 2.5 kg of PBX. Our experimental variables included density, confinement, free gas volume, and temperature. We measured internal temperatures, confinement pressure, and ignition time. In some of our experiments, we used a borescope to visually observe the decomposing PBX. Our observations included the endothermic - phase change of the HMX, a small exothermic temperature excursion in low-density unconfined experiments, and runaway ignition. We hypothesize that the temperature excursion in these low density experiments was associated with the NP decomposing exothermically within the PBX sample. This reactant-limited temperature excursion was not observed with our thermocouples in the high-density experiments. For these experiments, we believe the binder diffused to the edges of our high density samples and decomposed next to the highly conductive wall as confirmed by our borescope images.
A novel data processing technique has been developed to obtain thermal diffusivity, conductivity, and reaction heat release for energetic materials from Sandia Instrumented Thermal Ignition (SITI) experiments heated with a linear ramp temperature boundary condition. The method is based on the equivalence of the temperature reponses of: (a) ramped temperature boundary condition with no internal heat generation and (b) uniform heat generation (that is, with a negative value) with constant temperature boundary conditions; which is true regardless of the spatial domain. For the specific case analyzed herein (the SITI apparatus), the midplane temperature profile is well represented by a quadratic expression in the radial coordinate for both ramped boundary temperature and uniform heat generation responses. Internal temperature data from temperature ramped SITI experiments with various pyrotechnics, propellants, and explosives were analyzed. Quadratic fits to the temperature profile data were made and the associated fitting coefficients were converted to yield thermal diffusivity directly. Thermal conductivity was then determined from thermal diffusivity, given knowledge of the material's specific heat capacity and density. Finally, because of the equivalence of the cases (a) and (b) above, their individual contributions to a combined temperature profile can be easily separated, thereby yielding internal heat generation as well. This technique allows for measurements of properties for pressed and powdered materials over a range of densities and temperatures. The technique is demonstrated using pyrotechnic materials (KClO 4 and Ti / KClO 4 ), a composite solid propellant (herein referred to as -Propellant A‖, a class 1.3 AP-HTPB-aluminum propellant) and an explosive (PBX 9502).
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