Stabilization and spheroidization of ammonium nitrate: Co-crystallization with crown ethers and spherical crystallization by solvent screening

• Undesired properties of NH4NO3 were minimized by co-crystal formation.
• Spherical crystallization was employed to enhance the flowability of co-crystals.
• Co-crystals were smokeless upon heating and exhibited a slower burning rate.

Although ammonium nitrate (AN) is a clean burning (chlorine free), non-corrosive, low-hazard, low-flame temperature, inexpensive, easily available energetic material and gives maximum gas horse power per unit weight, propellants incorporating AN suffer from three significant drawbacks: (1) polymorphic (phase) transition near ambient temperature, (2) caking and smoke generation upon burning, and (3) poor flowability for packing. The first two disadvantages could be minimized by forming non-hygroscopic, smokeless 1:1 co-crystals of ammonium nitrate–benzo-18-crown-6 (1:1 co-crystals of AN–B18C6) with a melting point of 125–129 °C, produced from the antisolvent precipitation method based on the systems of: ethanol–methyl-tert-butyl-ether (MTBE), methanol–ethyl acetate and methanol–MTBE. From the smoke test and burning characteristics, 1:1 co-crystals of AN–B18C6 is smokeless upon heating and exhibits a slower burning rate than the one of AN. Therefore, co-crystallization may become a solid-state modification method in the areas of propellants and explosives. The third drawback could further be eliminated by forming spherical agglomerates of 1:1 co-crystals of AN–B18C6 through the addition of water as a bridging liquid into the co-crystal slurry resulted at the end of the antisolvent precipitation method. Spherical agglomerates of 1:1 co-crystals of AN–B18C6 grown from Phase IV AN powders (0.32 mmol) and B18C6 crystals (0.32 mmol) in 0.64 mL methanol + 11.6 mL MTBE + 0.08 mL water gave relatively high yield of 87.3%, high population density of 20.6/cm3, high sphericity, of 0.86, high friability of 100 showing a greater strength, and low angle of repose of 25.2° indicating a good flowability.

Smoke tests of the purchased Form IV ammonium nitrate (AN) powders at (a) 160 °C, (b) 200 °C, and (c) 255 °C, and 1:1 co-crystals of 1:1 co-crystals of ammonium nitrate–benzo-18-crown-6 (AN–B18C6) at (d) 125 °C, (e) 140 °C, and (f) 170 °C.Figure optionsDownload as PowerPoint slide