The synergism between melamine and expandable graphite on improving the flame retardancy of polyamide 11

Expandable graphite (EG) has attracted more and more attention in fire science society due to its excellent char-forming ability, however, it cannot meet commercial flame-retardant requirements because of the low intensity of the char. This work reported our very recent efforts on improving the char quality of EG by introducing melamine (MEL) in order to enhance the fire resistance and thermal stability of polyamide 11 (PA 11) composite. The flammability characterized by limiting oxygen index, UL-94 vertical burning, and cone calorimeter (cone) tests shows that the presence of both EG and MEL can significantly improve the flame retardancy and thermal stability of PA 11 composites. The scanning electron microscopic analysis shows that EG and MEL are beneficial to form compact char layers that can isolate the matrix from heat and oxygen. It is proposed that the formation of hydrogen bonds between MEL and PA 11 are crucial for improving the flame retardancy of the composites.