Effect of 1-butyl-3-methylimidazolium hexafluorophosphate as the humectant on the thermal decomposition of nitrocellulose

Nitrocellulose is a versatile and thermosensitive material. Its thermal instability has caused a considerable number of fire and explosion accidents during production, transport, storage and use. Humectants are commonly used to increase the safety of nitrocellulose. Alcoholic humectants only retard but do not inhibit the spontaneous combustion of nitrocellulose. 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF₆]) contains phosphorus, fluorine and nitrogen elements, which has excellent flame retardant properties. In addition, the ionic liquid containing fluorine elements can effectively absorb noxious gases. In this study, the thermal stability of [Bmim][PF₆] was evaluated by thermogravimetric analyzer (TG). Furthermore, the feasibility of [Bmim][PF₆] as a humectant and its superiority over ethanol were verified by ignition point tests and nitrogen dioxide absorption experiments. In addition, differential scanning calorimeter (DSC), adiabatic acceleration calorimeter (Phi-tec II) and kinetics evaluation were performed to characterize the effect of [Bmim][PF₆] and ethanol on the thermal stability of nitrocellulose. The results show that [Bmim][PF₆] has high thermal stability and can inhibit the decomposition of nitrocellulose by absorbing nitrogen dioxide. Its uptake of nitrogen dioxide was 0.5557 g g⁻¹ at room temperature. The results of calorimetric experiments and kinetic evaluation show that ethanol increases the decomposition temperature of nitrocellulose, but also increases the severity of its thermal runaway. [Bmim][PF₆] improved the safety of nitrocellulose by reducing the severity of nitrocellulose decomposition through NO2 absorption, which indicates that [Bmim][PF₆] is more compatible with the requirements of intrinsic safety as the humectant.