Untangling the Efficient Boron-Initialized Hydroxyl-Terminated Polybutadiene Combustion for High Energetic Solid Propulsion Systems

Highly energetic boron (B) particles embedded in hydroxyl-terminated polybutadiene (HTPB) thermosetting polymers represent stable solid-state fuel. Laser-heating of levitated B/HTPB and pure HTPB particles in a controlled atmosphere revealed spontaneous ignition of B/HTPB in air, allowing for examination of the exclusive roles of boron. These ignition events are probed <i>in situ</i> via simultaneous spectroscopic diagnostics: Raman and infrared spectroscopy, temporally resolved high-speed optical and infrared cameras, and ultraviolet-visible (UV-vis) spectroscopy. The emission spectra unravel two stages of the B/HTPB ignition─the exoergic ignition of boron followed by HTPB combustion. It was found that HTPB readily absorbs the energy from the irradiating carbon dioxide (CO₂) laser but efficiently transfers that thermal energy to the densely arranged boron particles due to the lower heat capacity of the latter. This transferred energy causes a surge in temperature for the boron particles, leading to ignition (in an oxygen environment) in B/HTPB, unlike the case with HTPB alone. The accumulated energy from the second stage of boron ignition triggers the decomposition of HTPB in conjunction with hydrogen abstraction to produce radical precursors via boron oxides (BO and BO₂)─the key emitting intermediates detected. Along with conventional combustion products such as carbon dioxide (CO₂) and water (H₂O), the formation of partially oxidized gaseous products such as methanol (CH₃OH) and methyl vinyl ether have also been detected as a tracer of diverse oxidation events, suggesting a complex oxidation chemistry within HTPB and overall depict crucial insights for its use as a solid rocket fuel.