Phosphorus-nitrogen doped UiO-66/cobalt-nickel borate nanoflowers combined MXene improves flame protection of waterborne epoxy coatings

This study in situ encapsulated hexachlorotriphosphonyl cyanide (HCCP) within the pore structure of UiO-66 to prepare zirconium-based metal-organic framework material (UHP) with high nitrogen and phosphorus content. Subsequently, using gradient assembly technology, a transition metal-based borate flame retardant layer (Co-Ni/B) and a polyethyleneimine (PEI) layer were sequentially constructed on the UHP surface. Afterwards, the UHP/Co/Ni-B composite was combined with MXene via electrostatic interactions, ultimately yielding a novel multilevel flame retardant material (MX@UHPB/PEI). Experimental results demonstrated that the MX@UHPB/PEI-containing sample exhibited the lowest backside temperature (163.4 ℃) and highest expansion characteristics (27.2   mm, 20.76), confirming its effective ability to block heat propagation. Furthermore, the MX@UHPB/PEI/EP sample exhibited the highest residual char content (31.1%) and the most intact, dense char layer. This is attributed to transition metals such as titanium, zirconium, cobalt, and nickel within the material catalyzing the conversion of more pyrolyzed small molecules into stable char under high-temperature conditions. Notably, the formation of this catalytic char layer suppressed further decomposition of intermediates during combustion, enabling the MX@UHPB/PEI/EP sample to achieve the lowest smoke density rating (36.4%). The development of this multifunctional composite flame retardant offers a novel approach to enhancing the protective performance of water-based epoxy intumescent fireproof coatings.