Organic phosphorus–nitrogen-treated carbon nitride combined with Ni(OH)2 metal-based flame retardant to improve flame protection of waterborne epoxy coatings

Graphitic-phase carbon nitride (CN) is a potential flame retardant due to its unique two-dimensional structure and thermal properties. Here, hexachlorocyclic triphosphate was grafted onto CN to form organic–inorganic crosslinked nanosheets (CPH) through the abundant reactive groups of polydopamine. Then the nickel hydroxide nanosheets were loaded onto the surface of CPH to obtain CPH@Ni(OH)₂ hybrids. The combination of Ni(OH)₂ and CPH can achieve multiple flame-retardant effects with excellent catalytic char formation and gas-phase flame-retardant properties. The results show that CPH@Ni(OH)₂/EP has the highest residual char (30.2%) in the thermogravimetric experiments, which lays the foundation for its high adiabatic properties. Compared with other samples, the CPH@Ni(OH)₂/EP sample exhibits the largest expansion height (24.1 mm), the largest expansion rate (18.83), and the smallest smoke density (38.6%), which indicates a significant increase in the expansion height and smoke suppression properties. The SEM reveals a denser and more complete residual char layer for the CPH@Ni(OH)₂/EP samples. Some metal oxides were embedded in the residual char, which was essential for improving the heat resistance and strength of the char layer. Evidently, CPH@Ni(OH)₂ can effectively improve the flame-retardant properties of epoxy coatings.