An Efficient Flame Retardant and Smoke Suppressant for Polyformaldehyde Based on Metal‐Ion‐Modified Ammonium Polyphosphate

A series of metal ion–modified ammonium polyphosphates (M‐APPs) were synthesized and employed as acid sources in combination with benzoxazine (Si‐BOZ) and melamine (ME) to construct intumescent flame‐retardant polyoxymethylene (POM) composites. The influence of different metal ions on the flame‐retardant efficiency, smoke‐related parameters, and mechanical properties of the composites was systematically investigated by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and cone calorimetry. The results demonstrate that metal ion modification markedly enhances the overall fire safety performance of POM. Among the investigated systems, the POM/Mg‐APP composite exhibited the most outstanding performance, achieving a UL‐94 V‐0 rating with a remarkably high limiting oxygen index (LOI) of 62.7%. Compared with the conventional POM/APP system, the incorporation of Mg‐APP led to significant reductions in total heat release (THR) and total smoke production (TSP) by 39.7% and 68.4%, respectively. The improved fire performance is attributed to the catalytic effect of magnesium ions on condensed‐phase char formation, resulting in a dense, continuous, and highly graphitized protective carbon layer. These results indicate that Mg‐APP is an efficient multifunctional flame retardant that simultaneously improves flame retardancy and suppresses smoke and toxic gas emission in POM composites.