Flower-like Ni-Al/LDH synergistic polyaniline anchored to the carbon sphere surface to improve the fire performance of waterborne epoxy coatings

In this study, phytic acid -doped polyaniline (PPA) was loaded on the surface of carbon spheres, resulting in a classical C-N-P flame retardant system, that has a beneficial function for the formation of residual carbon. Subsequently, using this phosphorus-nitride carbon sphere (CS/PPA) as a carrier, flower-like Ni-Al-layered double hydroxide (Ni-Al/LDH) were uniformly grown on its surface, thus obtaining a novel nano-fire retardant with a graded structure to heighten the fire-resistant properties of aqueous epoxy resin. Importantly, the intervention of Ni-Al/LDH nanosheets not only imparts a gas-phase fire retardant function to the coating, but also the presence of Ni and Al bimetals can effectively catalyze the formation of more residual carbon in the combustion process. The highest residual carbon (30.7 %) in thermal weight loss experiments of the CS/PPA@Ni-Al/LDH/EP composite coating confirmed this view. Tests on the thermal insulation behavior of coatings illustrated that the backside temperature (168.8 °C) for CS/PPA@Ni-Al/LDH/EP was significantly smaller than that of other coatings, and that this optimum thermal shielding is inextricably linked to the high residual carbon and the residual metal oxides. Furthermore, the CS/PPA@Ni-Al/LDH/EP specimen shown the greatest expansion height (25.1 mm), expansion ratio (19.61) as well as the smallest smoke density rating (39.1 %), proving that its expansion property and smoke suppression were effectively improved. Moreover, the microstructure for the expanded carbon layer demonstrated that the CS/PPA@Ni-Al/LDH filled EP possessed a more complete and dense residual carbon after burning. Moreover, the high degree of graphitization of the residual carbon for CS/PPA@Ni-Al/LDH/EP determined its high thermal stability. The lowest cracked gas phase products and CO release of the CS/PPA@Ni-Al/LDH/EP composite coating during TG-IR testing confirmed its lowest fire toxicity. The development of such a composite flame retardant with a graded structure could help to advance the progress of waterborne intumescent fire coatings.