Electrolytic manganese residue to metal phosphides via ball milling: Enhancing flame retardancy, mechanical properties, and electrical conductivity of epoxy resin

Landfill or open-air stacking of waste electrolytic manganese residue (EMR) poses environmental pollution risks due to the presence of harmful substances. EMR contains SiO2, Al2O3, and Fe2O3, which offer resource utilization and flame retardancy potential. A ball milling-based surface treatment strategy was proposed to modify EMR by incorporating red phosphorus (RP) and graphitic carbon nitride (g-C3N4), resulting in flame retardant EMR-PCN containing metal phosphides. Adding EMR-PCN to epoxy resin (EP) enhanced the flame retardancy of EP, addressing its inherent fire hazard. The EP composite incorporating 5 wt% EMR-PCN (EP/5EMR-PCN) demonstrated a limiting oxygen index of 33.0% and achieves V-0 level in vertical combustion test. Compared to EP, EP/5EMR-PCN showed significant reductions in peak heat release rate (65.3%), total heat release (62.2%), total smoke production (40.7%), and average CO yield (53.8%). Notably, EP/5EMR-PCN achieves a Flame Retardancy Index of 8.37, outperforming most g-C3N4-based flame retardants. Besides, increased hydrophobicity optimized the storage of EMR-PCN. Improved compatibility of EMR-PCN with EP enhanced its mechanical properties. Additionally, volume and surface resistance reductions in EP/5EMR-PCN mitigated static cumulative effects, expanding its application potential.