Synthesis of Carbon-Zinc Oxide Microspheres Decorated with Ammonium Polyphosphate (APP) for Synergistic Flame Retardancy in Polypropylene Composites

A key strategy for improving polypropylene (PP) fire safety involves developing composites with enhanced flame-retardant properties. In this study, novel flame-retardant systems were developed through the sustainable synthesis of carbon microspheres (CMSs), carbon-zinc oxide microspheres (CZnMSs), and zinc oxide microspheres (ZnMSs). These microspheres were subsequently combined with ammonium polyphosphate (APP) to form synergistic flame-retardant grenades (FRGs). The FRGs were characterized using XRD, FTIR, UV-Vis, TGA, and SEM, and then incorporated into a PP matrix via melt mixing to produce PP-FRG composites. The composites were systematically evaluated for chemical interactions (FTIR), thermal stability and crystallinity (TGA/DSC), morphology (SEM), flammability (UL-94 and cone calorimetry), and mechanical performance (flexural testing). The results demonstrated that the incorporation of FRG in low concentrations (10 wt.%) led to a synergistic effect, improving both fire resistance and mechanical performance of PP-FRG composites compared to neat PP. Among all formulations, the PP-CZnMS/APP composite exhibited the most balanced behavior, combining effective flame inhibition, enhanced char formation, and improved structural integrity.