Multi‐component crystallisation studies of environmentally benign propellants

Abstract

This research aims to explore novel, multi-component crystals based on dinitramide salts as potential applications for environmentally benign propellants.

Crystal engineering was used in this study to create new oxidisers as a means of enhancing the performance of existing energetic materials by changing the physical and chemical properties of crystalline solids.

In the co-crystallisation studies, multi-component crystals based on ADN were successfully synthesised incorporating guanidinium dinitramide (GDN) and guanidinium nitrate (GN) via different crystallisation techniques including evaporation, diffusion and mechanical grinding. The crystal structures of the multi- component crystal were obtained by single-crystal X-ray diffraction and show that the dinitramide is extensively hydrogen bonded. For ammonium dinitramide: guanidinium dinitramide (ADN:GDN), the crystal density lies approximately midway between the two co-formers and has a positive oxygen balance of 5.51%. The co- crystal of ammonium dinitramide: guanidinium nitrate (ADN:GN) has a density that surprisingly lies far from midway between the two co-formers and is closer to that of ADN. The oxygen balance of ADN: GN is 0.00%.

The physicochemical and energetic properties, including density, thermal expansion, thermal stability, hygroscopicity, impact sensitivity and calculated energetic performance (e.g., detonation velocities and detonation pressures) of both the multi-component crystals have been investigated. The syntheses of ADN:GDN and of ADN:GN can be scaled-up using grinding techniques and resonant acoustic mixing (RAM). ADN: GDN has a higher melting point (111 °C) higher than that of ADN whereas ADN:GN has lower melting point at 89 °C. The detonation velocity was calculated by using EXPLO-5 program based on densities, chemical composition, and heats of formation. The detonation velocities and pressures of both co-crystals are is higher than that of ADN. Humidity testing of ADN: GN and ADN:GDN was performed using a humidity chamber at 55% RH and 25 °C. Both these materials are less hygroscopic than ADN. Impact sensitivity testing using a BAM Fall Hammer gave an impact sensitivity for ADN: GDN of 17 J compared to ADN (3-4 J).

The compatibilities of the multi-component crystals with various polymer binders were studied. From these studies, ADN:GDN was compatible with hydroxyl- terminated polybutadiene (HTPB) and polycaprolactone (PCL). ADN:GN had fair compatibility with HTPB, but was compatible with PCL. Both compounds were incompatible with polyethylene glycol (PEG).