Novel phosphorus-containing halogen-free ionic liquid toward fire safety epoxy resin with well-balanced comprehensive performance

Most flame retardants used in epoxy resin (EP) inevitably affect its curing process, mechanical properties as well as transparency. To solve this problem, a novel phosphorus-containing halogen-free ionic liquid ([Dmim]Tos), composed of imidazole cation modified with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and tosylate anion, has been designed and used as a flame retardant for EP. DSC non-isothermal curing scans show that [Dmim]Tos has accelerating effect on the curing of EP. The addition of [Dmim]Tos enhances not only the crosslinking density but also the modulus of EP. The unnotched izod impact strength and the glass transition temperatures (Tgs) of EP/[Dmim]Tos are a little lower than pure EP because of the plasticization of unreacted [Dmim]Tos. Due to the good affinity with epoxy resin, low [Dmim]Tos addition almost has no effect on the transparency of EP. Besides, incorporation of 4 wt% [Dmim]Tos into EP makes it pass UL-94 V-0 level and increases the LOI value to 32.5%. Cone calorimeter test indicates the peak heat release rate (p-HRR) of EP/4.0[Dmim]Tos reduces by 37% compared with EP. The flame-retardant mechanism of EP/[Dmim]Tos has been investigated in detail. It is found that the flame-retardant action of [Dmim]Tos on EP is in the gas phase for quenching effect of phosphorus-based radicals and the condensed phase for dehydration of phosphorous fragments.