Metal-ion coordinated phosphazene flame retardants enabling enhanced fire safety and mechanical performance of epoxy composites

The inherent flammability and brittleness of EP limit its application in the industrial field. Therefore, it is of great importance to simultaneously enhance the flame retardancy and toughness of EP. In this work, metal ion-coordinated P/N synergistic flame retardants, named as PIFe and PIZn, were designed and synthesized to concurrently improve the flame retardancy and mechanical properties of EP composites. The results showed that the LOI value of EP/PIFe-5 and EP/PIZn-5 increased to 30.5% and 33.2%, reaching a UL-94V-0 rating. Compared with Pure EP, the pHRR and TSP of EP/PIFe-5 decreased by 41.4% and 26.3%, and those of EP/PIZn-5 decreased by 63.8% and 44.9%, respectively, demonstrating a significant improvement in flame retardancy and smoke suppression. Metal ions catalyzed the carbonization of EP, participated in the formation of the char layer at high temperature. Furthermore, the covalent and non-covalent interactions between the flame retardants and EP matrix significantly improved the mechanical properties of the EP composites. The tensile strength and impact strength of EP/PIZn-3 increased by 35.1% and 140.0%, respectively. Compared with EP/PIZn-3, the mechanical properties of EP/PIFe-5 and EP/PIZn-5 have declined to a certain extent, but they are still superior to those of pure EP. Overall, this work presents a method for synthesizing flame retardants based on phosphazene-metal coordination structure, which enables to prepare EP composites with excellent comprehensive performances.