Effect of catalyst mixing procedure on subatmospheric combustion characteristics of composite propellants

Subatmospheric combustion characteristics of ammonium perchlorate (AP)/hydroxyl-terminated-polybutadiene (HTPB) propellants prepared using two different catalyst mixing procedures were studied. In the first one, identified as the wet-mixing process, copper chromite catalyst (CC) was mixed with the binder (HTPB + di-2-ethylhexyl adipate); and to this CC-binder mix, AP and the curing agent [toluene di-isocyanate (TDI)] were then added. In the second, identified as the dry-mixing process, CC was first mixed with AP, and to this dry-mixture, the binder and toluene di-isocyanate were then added. For such propellants of different mixing procedures, with various AP particle sizes, the measurements of burning rates up to 1 bar, low-pressure deflagration limit (LPDL), and the scanning electron microscope (SEM) study of extinguished samples at LPDL were carried out. Results of the study show a higher burning rate for dry-mix propellents, and this higher burning rate is AP particle-size and pressure dependent. LPDL is also AP particle-size dependent, and for catalyzed propellant it is lower than that of an uncatalyzed one. The two procedures of CC mixing do not demonstrate any significant difference in LPDLs. The micrographs of wet-mix and dry-mix propellants look similar; however, the strong effect of low pressure in increasing the surface and subsurface reactions is brought out by the SEM study.