Construstion of a Phosphazene-Based Flame Retardant with Reactive Epoxy Group and its Application Achieves the Toughness Enhancement, Strength Enhancement and Fire Safety of Epoxy Resin

It is well known that improving the flame retardancy of epoxy resins (EPs) with high loadings of flame retardants often leads to severe damage to mechanical properties. Thus, a new flame retardant referred to as NEP was synthesized and incoprated to EP for achiving the conspicuous flame retardancy and mechanical properties. The successful synthesis of NEP was proved by 1 H NMR, 31 P NMR and fourier transform infrared spectroscopy (FTIR). Upon the 5.0 wt% NEP added, the impact strength, tensile strength and elongation at break of NEP/EP-5.0% were 37.6 kJ/m 2 , 55.3 MPa and 12.5. Compared with that of pure epoxy, they were increased by 375.9%, 13.8% and 37.4%, respectively. The reasons for strengthening are that NEP contains a large proportion of benzene rings, which increase the rigidity of the EP matrix. Meanwhile, NEP containing epoxy groups slows down the crosslinking rate of the epoxy system, thereby reducing the cross-linking density of flame retarded EPs and making them toughened. The phosphorus and nitrogen elements in the NEP led to the formation of denser char residue. The sample of NEP/EP-7.5% achieved a V-0 rating in the vertical UL-94 test and had a high limiting oxygen index (LOI) of 30.5. The sample with 10.0 wt% loading of NEP possessed a lower p -HRR, THR and TSP of 339 kW/m 2 , 35.7 MJ/m 2 and 16.6 m 2 , reducing by 61.9% , 52.9% and 11.7% compared to that of pure epoxy, respectively. Finally, the flame retarded EPs have good potential applications because of the glass transition temperature ( T g ) and thermal stability meeting the industrial needs.