Interfacial engineering of ammonium polyphosphate via dendritic mesoporous silica for effective and durable intumescent flame retardancy in polypropylene

Polypropylene (PP) exhibits poor flame retardancy and severe melt dripping behavior, which greatly limits its application in fields with stringent fire safety requirements. To address these issues, a novel inorganic core–shell flame retardant (APP@m-SiO₂) was developed in this study by coating ammonium polyphosphate (APP) with dendritic spherical mesoporous silica (m-SiO₂). The hydrolytic resistance, flame-retardant performance, and underlying mechanisms of the resulting composites were systematically investigated. The flame-retardant results demonstrate that, compared with neat PP, the prepared PP/IFR composites show significantly enhanced fire performance. Specifically, the PP-4 composite achieves a limiting oxygen index (LOI) of 34.8% and attains a UL 94 V-0 rating, while its peak heat release rate (pHRR) is reduced by 92.7% relative to pure PP. Moreover, after boiling in water for 168 h, the LOI value of PP-4 can still be maintained above 28%, indicating excellent water resistance. In addition, PP-4 exhibits negligible change in tensile strength, and its elongation at break is improved compared with that of PP-3. This work proposes a novel core–shell flame retardant strategy for fabricating polypropylene composites with both outstanding flame retardancy and superior hydrophobicity.