Tailoring the reactivity of printable Al/PVDF filament

Within the energetic materials and additive manufacturing (AM) communities, a number of aluminum/fluoropolymer (Al/FP) combinations have been identified for their suitability in various additive manufacturing techniques. For practical applications, such as in the case of a reactive wire or core in solid propellant, a range of selectable reactivity within a given Al/FP selection is needed. The purpose of this study was to alter the reactivity of aluminum/polyvinylidene fluoride (Al/PVDF) to produce a range of consistent burning rates, enabling the design of a printable reactive filament suitable for use as a reactive propellant core, or in other related applications. Three potential methods of tailoring the burning rate of Al/PVDF filaments were investigated: (1) selecting different aluminum fuel particles, (2) adjusting the stoichiometry of the material, and (3) changing the fuel particle size ratio from pure micro- to pure nano-aluminum. Reactive filaments consisting of PVDF and either mechanically activated aluminum-polytetrafluoroethylene (MA Al-PTFE), nanoscale aluminum (nAl), or mixtures of nano- and micro-aluminum (nAl:µAl) were tested to assess reaction speeds as well as intra- and inter-batch variability. Differential scanning calorimetry, thermogravimetric analysis, drop weight impact testing, friction testing, and porosity analysis were conducted on select materials. Filaments of 20 wt% nAl/PVDF and 32.2 wt% MA Al-PTFE/PVDF were printed using a material extrusion method into strands with dimensions, porosities, and burning rates comparable to their filament feedstock. This study determined that the selection of fuel particles and stoichiometry could reliably produce moderate burning rates between 17 and 40 mm/s. The burning rates of the mixed formulations were inconsistent in the mid-range (20–30 mm/s) with significant deviation indicating a threshold phenomenon potentially related to a shift from a slower to faster reaction mode.