Facile Synthesis of Alkali Metal Polyhedral Oligomeric Silsesquioxane Salt and Its Application in Flame-Retardant Epoxy Resins

A series of metal polyhedral oligomeric silsesquioxane salts (POSS-M, M = Li or Na or K) were facilely synthesized using the click reaction and neutralization reaction. The chemical structure of POSS-M was confirmed by FT-IR, ¹H, ¹³C, ²⁹Si NMR, and XRD patterns. The well-characterized POSS-M was introduced into epoxy resins. The epoxy composites containing POSS-Na achieved the UL-94 V1 rating, and the highest LOI value also came from EP/POSS-Na, which was 27.4%. A dramatical decrease in pHRR was observed in all three composites. With 3 wt % POSS-M, the pHRR of epoxy composites dropped by up to 64.2%. Notably, compact char layers with sodium silicate on the surface were clearly observed for the first time, extending the silicon-containing flame retardant mechanism.