Application of Internal Seeding and Temperature Cycling for Reduction of Liquid Inclusion in the Crystallization of RDX

In the cooling crystallization of cyclotrimethylene trinitramine (RDX) from γ-butyrolactone, an internal seeding method combined with temperature cycling, was applied to suppress liquid inclusion and improve particle size and size distribution for RDX at initial saturation temperatures of 60−80 °C. The process parameters including initial cooling rate, the range of the temperature cycle, and recooling rate during crystal growth period were determined based on direct measurements of size and amount of liquid inclusion of RDX crystals. Crystal size was measured by a dynamic light scattering particle size analyzer. Liquid inclusion was characterized in mass and area measurement based content by gas chromatography and matching refractive index method combined with image analysis. In the present work, RDX particles with average crystal sizes of 169.0−322.0 μm and inclusion area contents of 4.4−5.3% were obtained. Those values imply that the crystal quality of RDX is much improved compared with those of commercial RDXs such as Hanwha-RDX (conventional grade; average crystal size of 158.0 μm and inclusion area content of 9.3%) and I-RDX (reduced sensitivity grade; average crystal size of 218.0 μm and inclusion area content of 5.0%). For instance, the average crystal size of RDX obtained from a saturated solution at 70 °C by the present method is 290.3 μm, which was much larger than that of I-RDX, but the inclusion area content was about 4.6%, which was even lower than that of I-RDX.