Effect of graphite and its ball-milled mixture with organic fluoride on inhibiting aluminum agglomeration in HTPB/AP/Al propellants

It is well-established that incorporating fluorine enhances the combustion efficiency of aluminum in composite solid propellants, while the addition of graphite reduces both the burning rate and the burning rate-pressure exponent of these propellants. In this study, a series of HTPB/AP/Al composite propellants were prepared with graphite, organic fluoride, and their mixtures, respectively. The combustion performance of each formulation was systematically evaluated. Results from burning rate tests demonstrate that the incorporation of graphite, organic fluoride, or their mixture significantly decreases both the burning rate and the burning rate-pressure exponent. Analysis of the condensed combustion products reveals that both graphite and organic fluoride effectively mitigate aluminum agglomeration, as evidenced by reduced particle sizes and lower active aluminum content. High-speed imaging of the combustion process for propellants containing these additives revealed a distinctive “flocculent” structure. This phenomenon is attributed to weakened interfacial interactions between the carbon skeleton formed during binder combustion and aluminum particles. Both graphite and organic fluorides promote the formation of this flocculent structure, characterized by smaller, loosely aggregated aluminum particles that detach from the combustion surface prior to complete melting.