Extraordinary roles of waste aluminum sludge in enhancing flame retardancy, smoke suppression and electric insulation of intumescent flame-retardant coating for reliable cable tapes

Enhancing the flame retardancy of cable tape coatings while maintaining low electrical conductivity during fire incidents remains a significant challenge. In this study, we address this issue by incorporating waste aluminum sludge (AS) into an intumescent flame retardant (IFR)/vinyl acetate–ethylene (VAE) coatings, thus optimizing the flame retardancy, mechanical properties, and electrical insulation of fiberglass tapes. AS, composed of boehmite and bayerite phases with a sheet-like structure, interacts chemically with IFR/VAE during combustion, yielding thermally stable minerals (Al(PO₃)₃ and AlPO₄). Concurrently, its layered morphology promotes the formation of a compact and well-ordered carbonaceous char. The resulting hybrid mineral/carbon char acts as a robust physical barrier, effectively impeding heat and mass transfer, while disrupting the conductive carbon network. This dual mechanism leads to a remarkable enhancement in fire safety and electrical insulation performance, including: a 77.2 % reduction in peak heat release rate, a 29.0 % decrease in total heat release, a 58.5 % suppression in total smoke production, a 759.2 % increase in surface resistivity, a 45.7 % improvement in tensile strength, and a 2.4 % rise in limiting oxygen index. This work not only presents a sustainable strategy for upcycling industrial waste into high-performance flame retardants but also elucidates the mechanistic role of aluminum oxides/hydroxides in modifying the combustion behavior of IFR systems.