Cage‐shaped octaphenyl silsesquioxane with micro‐nano dispersibility for strengthening intumescent flame retardancy in polypropylene composites

Abstract In this work, a silicon‐, phosphorus‐, nitrogen‐containing flame retardant system was constructed via the combination of the caged organic octaphenyl silsesquioxane (OPS) and an intumescent flame retardant (IFR) composed of piperazine pyrophosphate and melamine polyphosphate. Compared with the IFR mixture alone, the designed OPS/IFR system simultaneously endowed polypropylene (PP) with a higher charring flame‐retardant effect and better mechanical properties under the same loading amount. With only 0.5 wt% OPS and 19.5 wt% IFR in PP, both 3.2 mm‐ and 1.6 mm‐thick flame‐retardant composite reached UL 94 V‐0 level, while the sample with IFR only passed the 3.2 mm‐thick UL 94 V‐0 rating. Moreover, compared with IFR, OPS/IFR with a specific mass ratio also performed higher limited oxygen indexes, improved anti‐dripping behavior in the glow‐wire test, and suppressed the heat and smoke release to a larger extent of PP composite. The main mechanism was that a phosphorus/silicon synergistic charring effect occurred in PP composites during the combustion, thus forming more compact char layers with rich phosphorus/silicon compounds. The investigation of the multielement OPS/IFR system also disclosed a more effective flame retardant working way in PP through synergistic charring effect. Meanwhile, due to the mico‐nanoscale dispersibility of OPS in PP, the constructed composites kept higher mechanical performances compared with the sample loaded with IFR alone.