The aim of the present work was to address the hazardous vulnerability of energetic materials to accidental initiation. An improved form of the explosive RDX with a significantly reduced sensitivity to stimuli including shock and impact was sought. The direction of this research was to investigate the effect of RDX crystal size reduction down to nano-scale dimensions on the sensitivity characteristics. Although size reduction of energetic crystals has been demonstrated often to result in a sensitivity reduction, the effect at the nano-scale particle size has not been investigated.

To generate nanocrystalline RDX a recrystallization process was developed based on rapid expansion of supercritical solutions (RESS). Compressed carbon dioxide was utilized as the solvent for RDX. Effect of key process parameters including the pre­expansion temperature and pressure, post-expansion pressure, and nozzle dimensions was investigated experimentally and via mathematical modeling.

The RESS process yielded pure RDX with a mean crystal size ranging from around 100 to 1000 nm with a relatively narrow size distribution and near spherical particle shape. Continuous processing with efficient particle collection and solvent recycling was accomplished enabling bulk production.

The sensitivity of RDX recrystallized by RESS was tested to stimuli including electrostatic discharge, impact, and shock was tested. Both pure samples as well as wax-based formulations were tested. Two types of recrystallized RDX were tested, Type A and Type B, with a corresponding specific surface area of around 20 and 6 m²/g. Both samples exhibited a significantly lower sensitivity to shock and impact compared to the reference 4.8 micron RDX. The coarser Type B nano RDX was found to be the least sensitive with all the tests. This indicates the existence of a minimum in sensitivity with crystal size.