Flammability regimes of ionic liquids: Modelling thermal decomposition and flash point

Ionic liquids are commonly considered safe due to their negligible vapor pressure, making them attractive for applications such as solvents and battery electrolytes. Recent studies have shown that their flammability risk arises not from volatility, but from the thermal decomposition process. Conventional flash point tests and models, based on equilibrium vaporization, fail to accurately capture the behaviour of ILs under thermal stress. In this study, a mathematical model is developed to predict the flash point of ILs by accounting for all key phenomena involved: external and internal heat transfer, decomposition kinetics, and volatile combustion. By varying the operating conditions, the model identifies different thermal regimes, each characterized by distinct dominant mechanisms. Flash point predictions are compared against experimental data for four representative ILs: [BMIm][BF4], [HMIm][Cl], [BMIm][Br] and [BMIm][Cl]. Results show that regimes controlled by internal heat transfer and reaction kinetics, both strongly affected by the low thermal conductivity of ILs, can lead to lower flash points when stirring is not applied. The effects of different heating times and characteristic lengths are also explored. The model is used to assess the reliability of conventional testing procedures, such as the ASTM D93 method, highlighting the limitations of applying traditional flammability tests to thermally unstable ignitable liquids and providing regime-specific guidelines for flash point assessment. The proposed regime-based mathematical model helps interpret IL flash-point behavior under different thermal and mixing configurations, explicitly distinguishing stirred conditions used in standard tests from unstirred conditions that are representative of real applications where mixing is absent; the model is validated against literature flash-point data for four ionic liquids.