A polyphosphate amide synthesized by a solvent-free route to enhance the flame retardancy and toughness of epoxy resins

To enhance the flame retardancy and mechanical performance of epoxy resin, a P/N-containing oligomer, polyphosphate amide (PPA) was synthesized by 1,4-phenylenediamine with dimethyl methyl phosphonate without using any organic solvent. With only 2% PPA addition in EP, the sample passed the UL-94 V-0 grade, and its limited oxygen index reached 34.0%. In the cone calorimeter test, the peak heat release rate, total heat release, and total smoke release of the EP containing 2% PPA were reduced by 23.6%, 16.0%, and 22.7% compared with that of control EP, respectively. The toughness of the pure EP was 5.0 MJ m −3 , and its impact strength and shear strength were 4.1 kJ m −2 and 8.4 MPa, respectively. With 2% PPA, the toughness, impact strength, and shear strength of EP/PPA2 were increased to 10.3 MJ m −3 , 8.8 kJ m −2 , and 14.8 MPa, respectively. This work provided a green and feasible strategy to fabricate a flame-retardant oligomer that was capable of enhancing the mechanical performance and fire resistance of the EP simultaneously.