Roles of multi-hierarchical char in flame retardancy for epoxy composites induced by modified thermal conductive fillers and flame-retardant assembly

Flame retardancy and thermal conductivity are contradictory usually. In this paper, furan and vinyl groups were grafted on the surface of aluminum oxide (Al 2 O 3 ) and aluminum nitride (AlN), respectively; carbon nanotubes (CNTs) were assembled on the surface of aluminum diethylphosphonate (ADP) to construct ADP@CNTs; they were used to regulate the flame retardancy and thermal conductivity of epoxy (EP). With the incorporation of 50 wt% modified Al 2 O 3 /AlN and 6.5 wt% ADP@CNTs, the thermal conductivity of EP composites is improved to 0.706 W/m·K, which is 188 % more than that of EP, meanwhile the UL-94 V-0 rating is achieved. The furan ring of Al 2 O 3 and vinyl of AlN can perform the Diels-Alder reaction to form a thermally conductive network, thus the heat can be carried away and the ignition time can be prolonged at the early stage of combustion; while a multi-hierarchical char layer is formed based on these inorganic fillers and ADP after ignition, which breaks the thermal conductive pathway and prevents the heat from transferring resulting in good flame-retardant properties of EP composites. It provides a novel perspective and solution for satisfied flame retardancy of EP composites with high thermal conductivity.