Aging Behavior of ADN Solid Rocket Propellants and Their Glass-Rubber Transition Characteristics

Abstract

Solid rocket propellants (SRPs) using ammonium dinitramide (ADN) as oxidizer are of interest because they belong to the classes of “green propellants” and signature-reduced propellants. The present study investigated several ADN-based rocket propellant formulations containing different prepolymers (glycidyl azide polymer (GAP); Desmophen® D2200), curing agents (bis-propargyl succinate (BPS); Desmodur® N3400), and filler types (aluminum (Al); octogen (HMX)). Ammonium perchlorate (AP)-based formulations have also been manufactured to make comparisons. SRP formulations have been investigated using dynamic mechanical analyses (DMA), mass loss, and tensile strength measurements. The accelerated aging program was between 60 and 85 °C with aging times adjusted to a thermal equivalent load of 15 years at 25 °C. The dynamic mechanical behavior of the ADN formulations differs from the hydroxyl-terminated polybutadiene (HTPB)-based materials: results show only one obvious peak in the loss factor curve instead of two. Fuel fillers, oxidizers, and curing agents have influences on the glass-rubber transition temperature (Tg) and the peak broadness. The loss factor peak of the GAP formulations is broader than the one of the Desmophen® formulations. Lowering of Tg by using AP instead of ADN was found. DMA investigations revealed distinct changes in the shape of the loss factor curves. Their detailed analyses with exponentially modified Gaussian (EMG) functions showed that the loss factor curves have two parts with different molecular mobilities during the transition of the material from energy-elastic (glassy) to the entropy-elastic (rubbery) state. Aging acts strongly on the part with restricted mobility.