Effect of Cage and Ladder Configuration of Polyphenyl Silsesquioxane on Mechanical Performance and Flame Retardancy of Ethylene–vinyl Acetate Composites

Improving the flame retardancy of ethylene–vinyl acetate copolymer (EVA) while maintaining its mechanical properties is critical for EVA applications. Polyhedral oligomeric silsesquioxane (POSS) can be a molecular structure designed to improve the flame retardancy of polymers while minimizing damage to the mechanical properties of polymers. In this study, two types of POSS with the same chemical structure but different spatial structures, which are named cage octaphenyl POSS (OPS) and ladder polyphenyl silsesquioxane (PPSQ) are prepared. The miscibility and dispersibility of these two types of POSS in the EVA matrix and their effects on mechanical properties, dielectric properties, and fire behaviors are also investigated. Compared to OPS-based EVA composite, the PPSQ with ladder structure has good miscibility and dispersion state in EVA matrix, obtaining better toughness, dielectric properties, and flame retardancy under cone calorimeter test. However, OPS-based EVA composite obtains superior flame retardancy than that of PPSQ-based EVA composite under small fire test (limiting oxygen index and UL-94 vertical burning tests). The mechanism of these two separate flame-retardant performances under various scenarios is explored. A novel flame-retardant model is proposed, as well as a new understanding of the role of POSS in EVA combustion. This study provides powerful guidance for obtaining EVA composites with excellent flame retardant and mechanical properties.