Combustion Behavior of Solid Fuels Based on PTFE/Boron Mixtures

An experimental study was conducted to understand the combustion behavior of polytetrafluoroethylene (PTFE)/boron-based solid fuels for future hybrid rocket motor applications. Fuels were loaded with 10-40% boron powder (w/w). Two different types of PTFE were examined in this study, while a single type of boron powder was considered. No significant differences in the decomposition mechanisms for PTFE and a candidate solid fuel mixture were observed by differential scanning calorimetry (DSC) and temperature-jump (T-jump)/Fourier transform infrared (FTIR) experiments. Diffusion flame studies between solid fuels and gaseous oxygen were carried out to measure regression rates and to develop a fundamental understanding of the combustion behavior. The fuels with the lowest boron content readily extinguished upon removal of the supplemental oxygen flow. The fuels with the highest loadings of boron self-propagated after ignition. X-ray diffraction on postcombustion residue of the self-propagating material revealed graphite and boron carbide as the remaining products, while particles captured leaving the surface of the fuel under normal burning conditions were found to be mostly boric acid. Boron oxidation and magnesium fluorination were observed in the flame zone of the diffusion flame by UV-Vis emission spectroscopy (magnesium is the major impurity in the elemental boron powder used). The results of this study suggest that solid fuels comprising PTFE and boron show promise for improving the energy density of hybrid rockets.