Initiation of insensitive high explosives is affected by porosity in the 100 nm to micron size range. It is also recognized that as-pressed plastic bonded explosives (PBX) are heterogeneous in composition and density at much coarser length scale (10 microns-100 microns). However, variations in density and composition of these explosives have been poorly characterized. Here, we characterize the natural variations in composition and density of TATB-based PBX LX-17 with synchrotron radiation tomography and ultra small angle x-ray scattering. Large scale variations in composition occur as a result of binder enrichment at the prill particle boundaries. The pore fraction is twice as high in the prill particle as in the boundary. The pore distribution is bimodal, with small pores of 50-100 nm in radius and a broader distribution of pores in the 0.5-1.5 micron size range. The higher pore density within the prill particle is attributed to contact asperities between the crystallites that might inhibit complete consolidation and binder infiltration.

A series of plastic-bonded explosives (PBX) has been formulated with more binder than is normally contained in high-energy formulations. Adding a relatively small amount of binder to a material such as PBX 9501 (95/2.5/1.25/1.25 wt % HMX/Estane/BDNPA/BDNPF (the BDNPA and BDNPF form a eutectic that is frequently called simply the eutectic)) was found to decrease the shock sensitivity while not decreasing the energy of the explosive. The best compromise for a PBX 9501-type material contains about 92 wt % HMX. Adding additional binder does not continue to decrease the gap sensitivity of the formulation; however, the energy of the PBX decreases as expected. The higher-binder formulations are of potential use because of the possibility of formulating a PBX with energy similar to TATB formulations, such as PBX 9502 (95/5 wt % TATB/Kel-F 800), and with a higher strain to failure. 2 refs., 4 figs., 1 tab.

Two applications of thermal expansion measurements on plastic-bonded explosive (PBX) composites are described. In the first dilatometer application, thermal expansion properties of HMX-based PBX 9501 are measured over a broad thermal range that includes glass and domain-restructuring transitions in the polymeric binder. Results are consistent with other thermal measurements and analyses performed on the composite, as well as on the binder itself. The second application used the dilatometer to distinguish the reversible and irreversible components of thermal expansion in PBX 9502, a TATB-based explosive. Irreversible expansion of the composite is believed to derive from the highly-anisotropic coefficient of thermal expansion (CTE) values measured on single T A TB crystals, although the mechanism is not well understood. Effects of specimen density, thermal ramp rate, and thermal range variation (warm first or cold first) were explored, and the results are presented and discussed. Dilatometer measurements are ongoing towards gaining insight into the mechanism(s) responsible for PBX 9502 irreversible thermal expansion.