Synthesis, structure–property and flame retardancy relationships of polyphosphonamide and its application on epoxy resins

A series of polyphosphonamides (PPDA) with heteroatoms and phosphonamide structures are proposed as highly effective flame retardants in epoxy resins (EP). The PPDAs were synthesized by solution polycondensation and well characterized. The impact of the heteroatoms on the thermal properties, pyrolysis route and flammability of PPDAs and their EP composites was investigated in detail. All the PPDAs show good thermal stability, high glass transition temperatures (T-g) and low flammability, associated to their backbone structures. With the addition of PPDAs, the flame retardancy of the composites was gradually enhanced. It is noteworthy that the heat release rate and thermal properties highly depend on the chemical environment of the phosphorus atom in PPDAs. The flame retardant mechanisms are suggested to enhance the charring of EP, protect the matrix in the condensed phase, and have a flame inhibition effect in the gas phase. The potential for increasing flame retardancy and maintaining high Tg and high fracture toughness is highlighted in this study.