Influence of structure of polymeric fuels on the combustion behaviour of composite solid propellants

Abstract

This is a first report in which the influence of the structure of polymeric binders, such as molecular weight (M), end-group functionality and cis-trans proportion, on the combustion behaviour of composite solid propellants (CSP) has been examined. In admixture with ammonium perchlorate (AP), dicarboxylic acids were used to simulate the commonly used telechelic polymers such as carboxy-terminated polybutadiene (CTPB) fuels. The burning rate (r) varies exponentially with the M of the acids and embraces the values for CTPB. The end-group effect diminishes at higher M due to dilution of the acid groups. The dependence of r on M is expressed as a newly defined molecular weight sensitivity of burning rate (σ_m). The σ_m values can be divided into three regimes: I (118 < M < 150), II (150 < M < 200) and III (M > 200) for polymeric fuels. The σ_m values in regimes I and II are explained by the selective tendency for anhydride formation; in regime III, fragmentation of the polymeric fuel occurs. In regimes I and II, volatilization of the acids occurs at lower temperatures than for their neat form and in regime II this volatilization is limited by the phase transition of AP. The volatilization rate decreases with increase in M, but the ratio of the condensed-phase to gas-phase heat release increases with M.