Synthesis of a phosphorus‑silicone modifier imparting excellent flame retardancy and improved mechanical properties to a rapid cure epoxy

A liquid bi-functional phosphaphenanthrene siloxane molecule that imparts excellent flame-retardancy and improved mechanical properties to an epoxy amine network was identified and synthesized. The modifier was added to a reactive epoxy amine system to create a low viscosity, miscible, flame retardant resin potentially able to be manufactured via high-pressure resin transfer molding (HP-RTM). The additive reduced the viscosity of the epoxy resin system, remained miscible in the epoxy resin after cure producing a transparent network while simultaneously enhancing self-extinguishing flame retardancy, flexural properties and fracture toughness. At a concentration of 25%, the viscosity of the modified resin system prior to cure was very low (less than 100 mPa.s at 80 °C), cured rapidly (less than 8 min at 150 °C), exhibited a T_g over 110 °C, displayed improved flexural strength (13%), modulus (23%) and fracture toughness (23%), increased thermal stability and displayed self-extinguishing behaviour (UL 94 V-0). Examination after combustion revealed a transformation to a porous but stable char layer likely a result of the synergistic effects of the silicon and phosphorus. The results presented here using represent a promising development for next generation modified epoxy resin systems targeted towards rapid manufacturing of composite materials using HP-RTM.