Hot spots sensitization and optical detonation measurements of emulsion explosives

Emulsion explosives are non-ideal explosives. The composition, rheological structure and its detonation properties were presented in previous ICT Conferences. Emulsion matrix is only detonable if it is sensitized (generally by hollow microspheres dispersed in the emulsion gelatin phase). Two kinds of spheres were used in experiments - expanded dry micro-spheres of 50 µm mean diameter (DE) and 5 mm expanded polystyrene spheres (EPS). The last big EPS spheres were successively dispersed in emulsion gelatin (emulsion matrix) and in emulsion explosive sensitized with DE microspheres. This sensitization mechanism can be explained by the deformation and collapse of the hollow microspheres, under shock detonation transmission, generating compressed hot gas and very high shear stress conditions in two lateral pockets. These two hot spot zones insure ignition of the outside emulsion matrix, generating, by the progress of reaction, cells and compressed layers, very well recorded in experimental tests, by the printed figures in copper witness plates. The experimental micro dynamic measurements were performed using optical techniques, using optical fibers and fast opto-electronic converters (presented previously in ICT Conferences). Two kinds of optical multimode fibers can be used in these measurements: PMMA and silica fibers. In a similar way, two kinds of opto-electronic receivers were used, as a function of optical range (maximum sensibility at 650 nm and 850 nm). Rise time is less than 50 ns. Silica fibers of 60 µm present more precise experimental measurements, due its reduced diameter and its more difficult ignition behavior with atmospheric air, when shocked from expansion of detonation products. Record signals show clearly the collapse of the EPS sphere. It shows the arrival of shock detonation front in sphere, the disappearing of light during collapse of sphere, and at the end, the very intense light generated by hot spots. Then two reaction progress regions seem to be developed. Their progression generate, sometimes, a central reaction zone, proved by the printed crater at the copper witness plate. This complex phenomena ensure the self-sustained detonation progression.