Inter molecular azide–diisocyanate coupling: new insights for energetic solid propellants

Abstract

Hydroxyl terminated azide binders can undergo a spurious reaction with diisocyanates to form tetrazoline-5-one via an inter molecular 1,3-dipolar cycloaddition reaction apart from urethane/allophanate groups which has been overlooked. This has serious implications on solid propellants. The computed activation barrier using density functional theory (DFT) for urethane formation reaction is 28.4 kJ mol⁻¹ and that for tetrazoline-5-one formation reaction is 108.0 kJ mol⁻¹. DFT studies reveal that the rate limiting step of the reaction is 1,3-dipolar cycloaddition between azide and isocyanate. A dual cure was observed in the temperature ranges 42–77 °C and 78–146 °C by differential scanning calorimetry (DSC) and rheological studies, confirming multiple reactions. Tetrazoline-5-one formation was confirmed by Fourier transform infrared spectroscopy (FTIR) and solid state nuclear magnetic resonance spectroscopy (NMR).