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 in situ 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.