Computational Study on the Lifting of Aluminum Particles from a Hydroxyl-Terminated Polybutadiene Burning Surface

The addition of aluminum particulates to polymer fuels is desired as a means to increase energy density. While nanosized aluminum has some attractive features with respect to its micrometer counterpart in terms of the burn rate, when incorporated into a fuel such as hydroxyl-terminated polybutadiene (HTPB), its release is often retarded, leading to crust formation on the fuel grain surface. Here, we undertake a molecular dynamics study to understand the size dependence of the polymer–particle interaction and how this impacts the size dependence of particle ejection. Comparing the interaction energy with the kinetic energy imparted to particles from the lifting force during polymer pyrolysis, we find that indeed, nanosized aluminum, due to its increased particle–polymer interaction binding energy, does not eject from the surface, while micrometer aluminum will. This is consistent with the experimental observation in a stagnation-flow burner. Further theoretical analysis indicated that replacing Al nanoparticles by nanosized Al/nitrocellulose (NC) mesoparticles may enhance the lifting of particles since the gas expansion from NC decomposition can generate sufficient kinetic energy to overcome the binding energy with the polymer.