Phosphorus-derived imidazolium salts with varied oxidation states: Tailoring latency, mechanical properties, and flame retardancy in single-component epoxy resins

Single-component epoxy resin (EP) is the premix of EP and latent curing agent, which is highly demanded in industries. However, current single-component EPs struggle to balance storage stability with mechanical properties and flame retardancy. To address this issue, three phosphorus-derived imidazolium salts (MPOx, x = 2, 3, 4) were synthesized using 2-ethyl-4-methylimidazole (EMI) and phosphorus-containing acids with different oxidation states (diphenylphosphinic acid, phenyl hydrogen phenylphosphonate, and diphenyl phosphate). The oxidation state of phosphorus significantly influenced thermal latency of MPOx, with higher oxidation states leading to improved latency. EP/MPO4 achieved the longest shelf life of 42 d at 25 °C. EP/MPO3 and EP/MPO4 exhibited enhanced tensile strength, modulus, and impact resistance compared to EP/EMI, but EP/MPO2 showed poor mechanical properties due to phase separation. All EP/MPOx achieved limiting oxygen index (LOI) exceeding 30 %, with EP/MPO3 showing the highest LOI of 34.0 % and significant reductions in heat release and smoke production. Flame-retardant mechanistic studies revealed a shift from gaseous-phase flame inhibition to condensed-phase promoting carbonization with increasing phosphorus oxidation state. Obviously, MPOx provides a tailored balance of latency, mechanical strength, and flame retardancy, making it a promising solution for advanced single-component EPs in aerospace, electronics, and optical applications.