Flame Retardancy and Reinforcement–Toughening Mechanism of Epoxy Resin Modified with a Phosphorus–Nitrogen Synergistic Imidazolium Ionic Liquid

Abstract Epoxy resin (EP) is widely used in engineering applications, but it has inherent flammability issues, and balancing its flame retardancy with mechanical properties also poses challenges. In this study, a phosphorus and nitrogen-containing amide-functionalized imidazoline ionic liquid − 1-(carbamoylmethyl)-2-methylimidazolium dihydrogen phosphate ([CMMIm][H 2 PO 4 ]) was synthesized, and it was introduced as a multifunctional flame retardant modifier into EP. Through LOI, TGA, cone calorimeter method, Raman spectroscopy, scanning electron microscope (SEM), and mechanical tests, the effects of [CMMIm][H 2 PO 4 ] on flame retardancy, thermal degradation, carbonization formation and mechanical properties were systematically investigated. The addition of [CMMIm][H 2 PO 4 ] significantly improved the fire resistance of EP. The LOI value of EP-4 increased from 20.41% to 30.68%, while the PHRR value decreased from 923.43 kW·m − 2 to 563.61 kW·m − 2 , a reduction of approximately 39%. At 800°C, the carbonized residue content increased from 11.64% to 16.90%, and the ID/IG ratio decreased from 0.85 to 0.78, indicating a more stable carbonization structure. Meanwhile, EP-4 exhibited tensile strength of 73.91 MPa, bending modulus of 3522.38 MPa and impact strength of 16.75 kJ·m − 2 . These results indicate that [CMMIm][H 2 PO 4 ] is an effective strategy that can enhance the flame retardancy of epoxy resin while improving its mechanical properties.