Tetrazole-grafted-hydroxyl terminated polybutadiene: A novel energetic binder for solid rocket propulsion

Abstract

The present study reports on the synthesis, characterisation, and energetic aspects of poly-nitrogen containing azole-grafted hydroxyl terminated polybutadiene (HTPB) based energetic binders (EBs). A comprehensive computational analysis at the B3LYP/6-311++G(d,p) level of theory reveals that azole-grafted HTPB variants exhibit significantly higher HoF (1061.6 to 1659.8 kJ/kg) and densities (1.26 to 1.61 g/cc) as compared to traditional non-energetic binders such as HTPB (specific heat of formation (HoF): 781.8 kJ/kg, density: 0.92 g/cc),. Their ballistic properties, including specific impulse (Isp: 266.2 to 283.8 s) and density specific impulse (ρIsp: 484.5 to 547.4 g/cc.s) are also superior as compared to HTPB (Isp: 283.8 s, ρIsp: 547.4 g/cc.s). Moreover, the synthetic methodology for grafting azole compounds onto HTPB, utilizing cost-effective, metal-free, and environmentally sustainable procedures with straightforward iodine and base mediation, underscores its potential for large-scale industrial applications. In addition to its advantages in synthesis and energetic properties, tetrazole-grafted HTPB exhibits favourable physicochemical properties: it has a low molecular weight ([Mn(VPO)] = 3023), high specific gravity (1.25 g/cc at 30 °C), moderate flowability (η = 623.6 Pa.s at 30 °C and 62.7 at 60 °C), a low glass transition temperature (−57 °C), mechanical insensitivity (low impact sensitivity and no ignition up to 400 °C), high thermal stability (onset temperature, Td,onset = 190 °C, TDSC = 246 °C), and a desirable hydroxyl value (47 mg/KOH). These properties make tetrazole-grafted HTPB an appealing choice for advancing novel propellant composites. Overall, these findings not only pave the way for advancements in HTPB-based propellants but also inspire further exploration in real-world applications, highlighting the transformative potential of azole-grafted HTPB in propellant technology.