Phosphorus-nitrogen co-modification of floral hydrotalcite synergized with zinc hydroxystannate for enhancing fire resistance of waterborne epoxy coatings

A novel flower-like flame retardant was employed to improve the flame retardancy of epoxy resins. Specifically, polydopamine (PDA) was first loaded on the surface of floral Ni-Al layered dimetallic hydroxide (Ni-Al/LDH) to endow it with excellent hydrophilicity and abundant reactive groups. Then, hexachlorocyclotriphosphorus (HCCP) was grafted onto the surface of LDH/PDA to ensure the introduction of a significant amount of P elements, which is essential for enhancing the flame retardancy of the composite. Finally, zinc hydroxystannate (ZHS) was anchored onto the surface of LDH/PDA/HCCP (LDH/PCP) by co-precipitation. The advantage lies in combining the char layer enhancement effect of Ni-Al/LDH, the gas-phase flame retardant and catalytic carbon-forming effect of ZHS to obtain highly efficient nanocomposite fire coatings. The results revealed that the LDH/PCP@ZHS/EP composite coating had the highest residual carbon rate (31.7 %) in the heat loss experiments, which provided the basis for its high thermal insulation performance. Tests on the thermal insulation performance of the coatings found that the backside temperature of LDH/PCP@ZHS/EP (162.6°C) was lower than that of other coatings, which is closely related to the high residual carbon and residual metal oxides. Meanwhile, the LDH/PCP@ZHS/EP specimen has the largest expansion height (34.6 mm), the largest expansion ratio (23.13), and the smallest smoke density (35.7 %), indicating that its expansion height and smoke suppression performance are significantly improved. The microstructure of the expanded char layer showed that the residual carbon was more complete and dense after the combustion of LDH/PCP@ZHS/EP. This also reflects the excellent thermal insulation and flame retardant properties of LDH/PCP@ZHS/EP composite coating.