Nonlinear relaxation behavior and competing aging mechanisms in GAP-based propellants under thermal aging

Abstract The glycidyl azide polymer(GAP)-based propellant, as a category of composite propellant with higher energy efficiency, exhibits a distinct nonlinear change in viscoelastic behaviours during the aging, which differs from the single change trend behaviours of other traditional propellants. This study investigates the relaxation behaviours of solid propellants under thermal aging conditions at 60°C. Nuclear Magnetic Resonance and high-performance liquid chromatography tests are used to reveal the aging mechanisms for the nonlinear variation in the relaxation curves of GAP-based propellants. The aging process of GAP-based propellants can be divided into three stages. In the first stage, the propellants are primarily influenced by post-curing reactions. Subsequently, the aging process is affected by both the scission of the crosslinking network and the degradation reactions of the plasticizer. These two mechanisms operate a competitive effect on relaxation, due to their alterations of the propellant's free volume microscopically. In addition, this work presents a predictive model for the relaxation modulus based on the established aging mechanisms, indicating the nonlinear fluctuations of the aging shift factor. The model effectively predicts the variations in the relaxation modulus during the thermal aging process of GAP-based propellants.