Optimization and design for the curing process of solid azide propellant: Influence of typical components on the curing reactions of PBT binders with TDI

The effects of several typical ingredients on the thermal curing reactions of poly (3,3 '-bis-azidomethyl oxetane)-tetrahydrofuran (PBT) adhesive with curing agent toluene diisocyanate (TDI) were carried out using density functional theory. The results show that the most energetic favorable curing route is the reaction with the cured product with two urethane segments and its rate-determining step is the nucleophilic addition reaction of TDI with triethanolamine (TEA). Compared to the reaction of TDI with PBT, the reaction with trimethylolpropane (TMP) shows relatively lower activation free energy mainly due to the weakening of N-H bond in corresponding TS structures. After the pre-modification of TDI by diethylene glycol (DEG), the reactivity of the remaining isocyanate group is significantly decreased, which is supported by the smallest absolute value of torsion angle when the electron density in N-H and C-O bond areas maintains locally concentrated. In contrast, the final curing reactions with PBT are remarkably facilitated as the participation of the intermediate structures formed through TEA. The stronger hindrance to the last step curing reaction of biuret molecule obtained through the continuous reaction stemming from H2O basically attributes to the reduction in the electrophilicity of relevant -NCO groups.