Selective polyurethane degradation enables non-destructive recovery of high-value HMX from decommissioned composite propellants

A nitric acid-based route was developed for the safe and efficient recovery of HMX from decommissioned composite propellants via selective degradation of the polyurethane (PU) binder. At 60 °C, the combined acidity and oxidizing capacity of nitric acid promoted PU chain scission and dissolution, leading to pore formation and interfacial debonding that facilitated HMX release. The temperature increase from nitric acid-aluminum reactions remained controllable. The effects of propellant size, acid concentration, liquid-to-solid ratio, reaction time, and temperature were evaluated using HMX yield, purity, and Al 3+ concentration in the filtrate as key metrics. Under optimized conditions, the HMX yield and purity reached 98.2% and 98.89%, respectively. FTIR, micro-FTIR, 1 H NMR, and gel permeation chromatography (GPC) analyses of pure PU degradation products identified the major degradation pathways of hydroxyl‑terminated PU in nitric acid, and molecular simulations were used to calculate reaction energies. SEM–EDS mapping at different depths confirmed interfacial selectivity and surface-to-interior propagation of the reaction front. Kinetic analysis showed that aluminum dissolution followed an interfacial reaction-controlled regime, with an apparent activation energy of 52.52 kJ mol -1 , whereas HMX release followed a first-order model with an apparent activation energy of 69.33 kJ mol -1 and was governed by binder-degradation-induced pore opening and mass transfer. The recovered HMX retained the β-HMX crystal form and its original ultrafine particle-size distribution. Overall, this route eliminates the need for fine propellant pulverization, simplifying the process and improving operational safety.