Exploring mechanisms for agglomerate reduction in composite solid propellants with polyethylene inclusion modified aluminum

In composite solid propellants, shortening particle residence time at the burning surface and inducing particle microexplosions could decrease aluminum agglomeration, thus reducing two-phase flow losses in a rocket motor. We explore this by using aluminum particles modified with low-density polyethylene (LDPE) inclusion to drive intraparticle outgassing, which could break apart composite particles, yielding smaller and faster burning fragments during composite solid propellant combustion. We find that use of these particles in a propellant results in more prompt particle ignition, and surface residence time is decreased. For composite propellant burning at 6.9 MPa, mean coarse agglomerate diameter is decreased from 75.8 μm (spherical aluminum) to 29.0 μm (Al/LDPE 90/10 wt.% particles). Thermal analysis with DSC/TGA shows that 10 wt.% LDPE inclusion in aluminum (1.5% of propellant weight) results in enhanced oxidation characteristics that are more similar to nanoaluminum than neat spherical aluminum. Thermochemical equilibrium calculations indicate LDPE inclusion decreases specific impulse by 1.0% from 262.7 to 260.0 s, but it is expected that in a motor, LDPE inclusion could produce a net increase in specific impulse due to a substantially reduced agglomerate size. This work shows that reduced agglomeration is possible using gas generating inclusion materials that are only weakly reactive with aluminum.