ADN Recrystallization and Microencapsulation with HTPB by Simple Coacervation

Abstract

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.

Microencapsulation of ADN with HTPB‐based membrane in the presence of the bonding agents Tepan or Tepanol**

“…The microencapsulated ADN were subjected to the dissolution test that consists of immersing the particles straight into distilled water; water passes through the structural defects in the capsule and starts dissolving the ADN inside, until it is completely dissolved, leaving the empty capsule floating on the water surface. For details, please refer to our previous work [12]. The dissolution time and rate are the parameters associated with capsule permeability; the first is the span between the moment of the immersion of the particle and the floating, and the second is the ratio of ADN particle mass to dissolution time.…”

“…A Büchi chiller, model F‐114, was employed as the cooling circulator, whose temperature remained below 10 °C; after solidifying, the round ADN particles were filtered, washed with dichloromethane, and remained inside a desiccator to remain dry; obtaining round ADN particles with radius under 600 μm. The process parameters applied in this method were given in a previous work [12]. The apparatus is schematically depicted in Figure 3.…”

“…It can be chemically incompatible with some other components of propellants, and it is highly hygroscopic [7][8][9]. Some proposals to tackle these issues include the co-crystallization of ADN with other components in an attempt to render it more resistant to moisture [10,11]; or in providing physical protection through the microencapsulation of ADN particles [12,13]. The latter is carried out in this work by the simple coacervation [14] method, in which pre-polymer molecules cluster around ADN in a proper dispersant, whose temperature and chemical affinity are adjustable; the clustered molecules are then hardened by a curing agent to achieve a homogeneous membrane, with low permeability and effective individual protection in the form of microcapsules.…”

Anti‐hygroscopicity technologies for ammonium dinitramide: A review**

“…Silva et al used polyurethane made from end-hydroxy polybutadiene as a coating material to encapsulate spherical ADN by microencapsulation. The results demonstrated the good compatibility between this polyurethane coating material and ADN, which led to good coating performance [88]. Santosh et al coated ADN particles with ethyl cellulose or polymethylmethacrylate (PMMA).…”

Preparation of low hygroscopicity ADN-based energetic composite microspheres and their performance study

“…The hygroscopicity was 10%, which still could not meet the requirements of propellant loading. Oliveira Silva et al 23 used polyurethane made of hydroxyl-terminated polybutadiene as a coating material, and moisture absorption tests showed that there was an important relationship between the formation of capsules and the number of layers as well as the processing time. The chemical compatibility between the capsules and ADN was determined at 60 ○ C, making it possible to use them as a protective coating.…”