Promoting aluminum nanoparticle ejection from HTPB via exothermic polymer-bound mesoparticles

Abstract

The incorporation and subsequent ejection of aluminum nanoparticles (nAl) from hydroxyl‑terminated polybutadiene (HTPB) remains a significant challenge due to the high specific surface area of nAl and strong interfacial interactions with the polymer matrix. We demonstrate that one approach to facilitate particle removal is the assembly of nAl into mesoparticles (MPs). MPs are micron sized composite particles bound by a polymer binder. In this work, nAl mesoparticles were fabricated using nitrocellulose (NC), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), and polypropylene carbonate (PPC) binders and evaluated for their ability to eject nAl from regressing HTPB. MPs formulated with NC and PAN successfully ejected nAl and supported consistent fuel regression. While, in contrast, PVDF and PPC based MPs did not. The observed behavior is attributed to the exothermic decomposition of NC and PAN, which provides localized heat release at the HTPB-MP boundary that enhances the pyrolization of surrounding HTPB, generating gas and thereby promoting particle ejection. Novelty and significance statement: This work introduces a unique strategy for enabling aluminum nanoparticle (nAl) utilization in air-breathing polymer fuels. This is done by synthesizing micron scale mesoparticles made up of nAl and a polymer binder and incorporating them into HTPB. The use of mesoparticles provides two fundamental advantages. The first, is given the choice of binder, properties like gas generation, local oxidation of nAl from decomposition products and decomposition enthalpies can be quantified and used to explain an ejection mechanism. The second, is due to the low surface areas of mesoparticles, high loadings of nAl can be incorporated into the polymer fuel grains. These findings provide a mechanistic insight into nanoparticle ejection, as well as a practical significance for increasing the energy density of air-breathing propulsion systems.