Thermal stability and flammability of several quaternary ammonium ionic liquids

Abstract

With the development of potential large-scale industrial application of quaternary ammonium ionic liquids in the fields of gas separation, high-temperature lubricants and even rocket propellants, deep investigations are required to reveal the possible hazards of thermal decomposition, the flammability and the hazardous gaseous products. In this study, the thermal stability of five typical quaternary ammonium ionic liquids, [N2222][NTF2], [N2222][PF6], [N2222]Br, [N4444]Br and [N4444][PF6], were studied using thermogravimetric analyser (TGA). The results show that the thermal stability of ionic liquids follows an order of [N2222][NTF2] > [N2222][PF6] > [N4444][PF6] > [N2222]Br > [N4444]Br, indicating that the thermal stability is obviously affected by the anion, with an order of [NTF2]− > [PF6]− > Br−. Besides, increasing the alkyl chain length in the cation will reduce the thermal stability. In addition, Fire Testing Technology (FTT) cone calorimeter was used to investigate the flammability and found that the longer alkyl chains of quaternary ammonium ionic liquids make great contribution on larger the total heat release, the peak value of heat releasing rate and shorter ignition time. Moreover, with the same cation, [N2222]Br and [N4444]Br were shortened the ignition time and produced larger amount of heat than [N2222][PF6] and [N4444][PF6], respectively. It clearly shows that the risk of fire is also affected by the anion, with an order of Br− > [PF6]− > [NTF2]−. Finally, the decomposition products were measured by thermogravimetry coupled with Fourier-transform infrared spectroscopy (TG-FTIR), and it is deduced that the nucleophilic Br− from [Nmmmm]Br (m = 2,4) and F− from the [NTF2]− and [PF6]− during decomposition will attack the alkyl chain of the cation, which generated gaseous products such as fluoroalkanes, bromoalkanes, olefins, etc.