Phosphorus-containing imidazole-based ionic liquid curing agents with different functional groups: synergistically enhancing flame-retardant and mechanical properties while extending processing time

Commercial epoxy resins (EP) face key challenges such as flammability, brittleness, poor processability, and incompatibility, limiting their widespread use. This study presents the synthesis of phosphorus-based ionic liquid curing agents using n-Octyl phosphonic acid (OPA) and various imidazole derivatives through a supramolecular assembly strategy. The design of these curing agents aims to improve the flammability, brittleness, and processing performance of EP. The results show that ZMI, 1-ZMI, and B-ZMI, due to the inherent properties of ionic liquids, exhibit excellent compatibility with EP. Compared to EP/IM, EP/1-EIM, and EP/BIM, the mechanical and flame-retardant properties of EP/ZMI, EP/1-ZMI, and EP/B-ZMI are significantly enhanced, with all samples achieving UL-94 V-0 ratings. Additionally, the ionic bonds exhibit a “molecular switch” effect, extending the processing time of EP/ZMI, EP/1-ZMI, and EP/B-ZMI. Notably, EP/1-ZMI, where only the nitrogen at the 3-position of imidazole participates in the curing reaction, remained unchanged for 360 min at 60 °C, but rapidly gelled in just 4.5 min at 150 °C. This effect was not observed with other imidazole derivative curing agents with different substituents. In conclusion, all three curing agents effectively enhanced the mechanical and flame-retardant properties of EP, with 1-ZMI demonstrating superior processability. This approach offers a simple yet effective strategy for enhancing flame retardancy and toughness, with strong potential for industrial applications.