Thermal stability and flammability of several quaternary phosphonium ionic liquids

Potential thermal risks and combustion performances of quaternary phosphosnium ionic liquids should be concerned to prevent the fire and poisoning accidents during industrial production, storage and transportation. TG-DSC, TG-FTIR and FTT cone calorimeter were used to assess the thermal stability, hazardous gas production and combustibility. The results show that the thermal stability of five quaternary phosphonium ionic liquids, i.e., [P-4444][PF6], [P-4444]Br, [P-4444][NTF2], [P-12444]Br and [P-4444][BF4], follows an order of [P-12444]Br < [P-4444]Br < [P-4444][PF6] < [P-4444][BF4] < [P-4444][NTF2], indicating that the thermal stability of quaternary phosphonium ionic liquids is mainly affected by anions with an order of Br- < [PF6](-) < [BF4](-) < [NTF2](-). In addition, longer chains tethered with the cation are not favorable for thermal stability. TG-FTIR results show that a large amount of toxic and harmful gases were released with a large amount of heat during the decomposition process. According to the FTIR of the decomposition products with temperatures, it is hypothesized that the anion may initiate a nucleophilic attack on fluoroalkanes and bromoalkanes, and other gaseous products are produced by the alkyl chains on cations. Based on the cone calorimeter experimental results, the flammability follows an order of [P-12444]Br > [P-4444]Br > [P-4444][PF6] > [P-4444][BF4] > [P-4444][NTF2], indicating that with the same cation, the fire risk of the ionic liquids is also dominated by the anion structures. The monatomic anion Br- has the highest fire hazard, while [NTF2](-) has the lowest, implying that more halogen elements in anion make greater inhibiting effect on the combustion.