Bimetallic Effect of Hybrid Flame Retardant on Flame Retardant Performance in Polypropylene

Polypropylene composites are widely used for their excellent properties, but poor flame retardance is a major concern. The phosphorus content of the material in intumescent flame retardant (IFR) has a direct effect on retarding smoke and dehydrating to carbon during combustion. This study presents a hybrid flame retardant (MPA@ZrP-Zn) synthesized by noncovalent bonding between melamine and phytic acid molecules, containing a large number of phosphorus atoms in the system. Theoretical calculations confirm that the structure formed by zinc ions and melamine phytate (MPA) is more stable, and the overall electrophilicity index is at the level of a moderate Lewis acid. In addition, the Zr-Zn bimetallic effect significantly improves the drip resistance of IFR and reduces smoke emissions during combustion. UL-94 and LOI tests have shown that PP composites with MPA@ZrP-Zn (PPZn30) can achieve a V-0 rating and 33.5% of the LOI value. The THR and PHRR were 15.1% and 54.2% lower, respectively, for PPZn30 (compared to pure PP), and the I_D/I_G curves further indicate a higher degree of graphitization of the carbon layer in the combustion residue, which should be ascribed to the bimetallic effect. Therefore, this work puts forward a strategy to achieve high-performance PP composites with excellent flame retardant properties, which exhibit huge applications in electrical appliances and energy vehicles.