Phosphorus-nitrogen synergistic flame retardant (PNFR) towards epoxy resin with excellent flame retardancy and satisfactory mechanical strength: An insight into pyrolysis and flame retardant mechanism

A novel PNFR (BOPOA) was synthesized and employed to flame retardant epoxy resin (EP). 4,4′-Diaminodiphenylmethane (DDM) and BOPOA co-cured EP exhibited satisfactory mechanical properties, with the maximum tensile and flexural strength being 61 % and 31 % higher than those without BOPOA (BOPOA-0). Limiting oxygen index of DDM cured EP with 1.5wt% phosphorus introduced (BOPOA-1.5) increased by 38 % than BOPOA-0, and it passed UL94 V0 rating. Time to flameout was shortened by 45 %, and the peak and average heat release rate and total heat release was decreased by 25 %, 24 % and 24 %, respectively. BOPOA promoted graphitization of carbon residues and improved their stability. Evolved gas analysis proved that BOPOA not only changed pyrolysis pathways of EP and reduced combustible gas substantially, but also released phosphorus-containing free radicals and interrupted chain reactions of combustion. This work presents a facile pathway for fabricating high performance EP with excellent flame retardancy and satisfactory mechanical strength.