Synergistic POSS-based hybrids: Toward high-performance polypropylene composites with integrated flame retardancy and thermal stability

To address the inherent trade-off between flame retardancy and mechanical properties in polypropylene (PP), a novel polyhedral oligomeric silsesquioxane-based flame retardant (POSS-PPA) was designed and synthesized, and its synergistic effects with melamine polyphosphate (MPP) and piperazine pyrophosphate (PAPP) in PP composites were systematically investigated. Structural characterization confirmed the successful incorporation of phosphate groups onto a typical T8 caged POSS skeleton. Thermogravimetric analysis revealed that the char residue at 800 °C reached 63.9%, significantly higher than those of MPP (49.6%) and PAPP (31.0%). According to UL94 vertical burning tests and cone calorimetry measurements, the composite achieved a V-0 rating at a thickness of 1.6 mm. Notably, the composite containing four parts of the additive exhibited a peak heat release rate (PHRR) of 115.05 kW·m-2, representing an 87.9% reduction compared to neat PP, while the total smoke production (TSP) was reduced to 190.63 m2·m-2, corresponding to a 78% decrease. Moreover, its notched impact strength reached 5.46 kJ·m-2, marking a 22.4% enhancement relative to the composite without POSS-PPA. This study presents a viable strategy for developing halogen-free flame-retardant PP composites with well-balanced flame retardancy, smoke suppression, and mechanical robustness.