Construction of magnesium–molybdenum–phosphorus multi-component flame retardant and its performance in flexible PVC composites

To address the high flammability and toxic smoke emission of flexible PVC (fPVC), a magnesium-molybdenum-phosphorus multi-component flame retardant (MO@MH-PEPE) was constructed by surface-modifying self-synthesized molybdenum oxide-hybridized magnesium hydroxide (MO@MH) with phenolic epoxy phosphate ester (PEPE). Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) confirmed the chemical grafting of PEPE onto MO@MH via P-O-Mg bonds, enhancing interfacial compatibility. When incorporated into fPVC, the fPVC/MO@MH-PEPE composite exhibited superior flame retardancy and smoke suppression: limiting oxygen index (LOI) increased to 32.0%, UL-94 reached V-0 rating, peak heat release rate (pHRR) and total smoke production (TSP) decreased by 47.16% and 75.15% compared with the fPVC/MH composite, respectively. The char residue yield (50.00 wt%) and graphitization degree significantly improved, attributed to Mo6+/Mo4+ redox catalysis and phosphoric acid charring. Thermogravimetry analysis-FTIR (TGA-FTIR) revealed gas-phase flame inhibition via H2O dilution. Furthermore, PEPE modification optimized mechanical properties, increasing tensile and impact strength by 28.35% and 6.50% over fPVC/MO@MH, supported by SEM-proven interfacial adhesion. This work demonstrates a synergistic Mg-Mo-P system for high-performance fPVC composites.