Ammonium dinitramide (ADN) has been considered the potential substitute for ammonium perchlorate in solid green propellants. However, it has also some drawbacks due to its high hygroscopicity and chemical incompatibility with some of the components present in composite propellant formulations. On the other hand, ADN melts at temperatures below degradation, which makes emulsion crystallization an important method to prepare spherical ADN particles that benefit both propellant slurry processing and casting. Spherical ADN particles were prepared by emulsion crystallization to be used for the microencapsulation studies, and whose method was assessed to better understand which parameters may affect the ADN particles formation. Microencapsulation protects ADN particles as it addresses hygroscopicity besides the given protection from chemical incompatibility. Polyurethane made of hydroxyl-terminated polybutadiene was selected as a coating material because of its common use in propellants as a binder. Also, the good performance of methylene diphenyl diisocyanate as a curing agent was such that it was used in the whole study. The number of layers and the processing time played important roles in the formation of the capsule as seen in the impermeability tests. Besides, drying at 50°C for 168 h showed to be even more beneficial on average for the capsule protective property. Chemical compatibility between the capsule and ADN was confirmed at 60°C which allows it to be used as a protective coating. However, the obtained average coating thickness of ten microns has to be reduced yet with more efficient protective coating materials.

In composite solid propellants, the oxidizer in the form of particles is embedded in a polymeric matrix. In general, these oxidizers consist in inorganic salts that are hygroscopic, chemically incompatible or sensitive to friction or impact, so that microencapsulation can be applied as a mean to provide a protective coating layer. This work aims to assess the effectiveness of the spout-fluid bed method to perform microencapsulation of ammonium perchlorate particles with acrylic-based resin. The formed coating integrity was assessed by an optical stereomicroscope for samples with one, two and four layers of coating before and after dissolving the cores in water. The parameters utilized in this method provided a complete and individualized encapsulation with sufficient integrity. Therefore, the spout-fluid bed method proved to be effective, particularly with the application of multiple layers.