Effects of Drying on Energetic Materials Produced by Spray Flash Evaporation

Spray atomization, particularly spray flash evaporation (SFE), exhibited interesting properties such as a small droplet size and rapid droplet evaporation, which facilitate the recrystallization of energetic materials (EMs) with enhanced physicochemical properties. This study, for the first time, explores the effectiveness of the drying stage after atomization in the SFE technique in removing the solvent and altering the physicochemical properties (crystallinity, size, morphology, and performance) of cyclotrimethylenetrinitramine (RDX), a widely used explosive, when obtained at 160 °C and 40 bar. Two different and successive drying steps were applied to particles. They were first dried directly in the SFE process for a short period, about 1 min following the end of atomization, at a very low pressure of 11 mbar and under dynamic vacuum. Then, particles were exposed to a second ex situ drying under 24 h at 800 mbar and 40 °C. It was found that SFE reduced the particle size as well changing the particle morphology and conserving the crystalline phase. Particles exhibited less regular, nonspherical morphologies, bridge-like structures, and some displayed cavities, likely resulting from buckling associated with fast evaporation and crystallization processes. Particles also showed the presence of a residual solvent (acetone). Unexpectedly, even the second drying process failed to fully eliminate residual acetone, which probably remained trapped within the particles. Results uncover an Ostwald ripening where prolonged drying led to particle growth and cavity disappearance, likely accentuated by the presence of residual solvent, promoting RDX diffusion on the particle surface. Importantly, the morphology and particle size also dictate the sensitivity. Smaller RDX particles with significant surface defects, produced by SFE and drying during a short time, exhibit a lower impact and friction threshold. The second drying step, restoring near-spherical morphology and removing morphology defects while maintaining a lower particle size compared to that of the raw material, raised the impact threshold to 8.5 J while leaving friction and electrostatic discharge sensitivities mainly unchanged. These results pave the way to advance the spray atomization process in EM recrystallization, with a particular emphasis on the final drying stage. Notably, the presence of even trace amounts of residual solvent within the RDX particles can highly influence their physicochemical properties, underscoring the need for precise control of the drying after atomization to optimize particle quality and performance.