Synthesis of a charring agent containing triazine and benzene groups and its intumescent flame retardant performance for polypropylene

•A liner charring agent (ETDPCs) containing benzene and triazene is synthesized.•ETDPCs exhibited excellent thermostability and char-forming ability.•LOI value of 30.4% for PP is reached with optimal ratio of FR and ETDPCs (3:1).•Synergistic effect of FR and ETDPCs suppresses the heat transfer and smoke release.•The continuous and dense graphitic char layer endows it with a high barrier effect.