Investigation of Crystallisation Conditions to Produce CL-20/HMX Cocrystal for Polymer-bonded Explosives

Since its discovery in 2012, multiple techniques to generate the CL-20/HMX cocrystal have been published. However, as yet no assessment or trial has been reported of crystallisation methods capable of producing the cocrystal in a size region and production scale suitable for its use in polymer bonded explosives (PBX). This paper provides insight into the selection of suitable crystallisation methods, solvent selection and process optimisation with a focus on the efficient production of high-quality cocrystals for use in PBX. Through extensive solvent screening, acetonitrile was identified as the best solvent for solution-based crystallisation, due to its capability to produce compact parallelepipedic crystals and its comparably wide cocrystal phase region. Crystallisation conducted at 60 °C was found to increase the conversion rate and the material efficiency compared to room temperature. By application of an advanced seeding procedure, high-quality cocrystals in the size region of 180-250 mm were produced in laboratory-scale antisolvent and cooling crystallisations. By pilot-plant-scale batch reaction cocrystallization, cocrystals with a volume-weighted mean diameter of 33 mm were produced in quantities of 250 g per day