Preparation of Micronized β-HMX Using Supercritical Carbon Dioxide as Antisolvent

Abstract

This study investigated the micronization of HMX (cyclotetramethylenetetranitramine) using two types of recrystallization methods with supercritical carbon dioxide as an antisolvent, namely, the aerosol solvent extraction system (ASES) and gas antisolvent (GAS) processes. The effects of experimental variables on the recrystallization of β-HMX particles in the GAS process were also studied. In particular, we varied the temperature from 303 to 323 K, used CO2 feeding rates of 20 and 50 mL·min–1, and varied the agitation speed from 200 to 900 rpm in the GAS process. The precipitated HMX particles were characterized by field-emission scanning electron microscopy (FE-SEM), particle size analysis (PSA), and Fourier transform infrared (FT-IR) spectroscopy. Depending on the organic solvent used and the type of recrystallization method, the precipitated HMX particles showed a variety of morphologies, particle size distributions, and crystal phases. The HMX precipitated by the ASES process was transformed into micronized γ- or δ-HMX with a volume-mean particle size (D50) of 6.3–32.1 μm. On the other hand, in the case of the GAS process, micronized γ- or β-HMX was formed, with a volume-mean particle size (D50) of 5.3–37.45 μm. In particular, HMX dissolved in acetone was successfully precipitated to micronized β-HMX with a narrow particle size distribution through the GAS process.