Polycyclic siloxanes: Base resins for novel high temperature resistant platinum curing transparent silicone adhesives

There is increasing demand for polymers having optical transparency along with high thermal stability for specific applications. Herein, the synthesis of high performance, mainly ethylene bridged, polycyclic silicones possessing multiple functionalities like optical clarity, high temperature stability and inherent flame retardancy is reported which is formulated further as high temperature resistant polymeric adhesive. These specialty silicones were synthesized from cyclic siloxanes through hydrosilylation reactions. The reaction conditions were optimized to synthesise the polycyclic silicone system with varying 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (D4V): 1,3,5,7-tetramethylcyclotetrasiloxane (D4H) ratio and the effect of cyclomer ratios on various physical, thermal, dynamic mechanical characteristics were explored. Among different cyclomer feed ratios, the ratio of 1:1 resulted in effective hydrosilylation with improved thermal stability. High temperature resistant adhesive formulations were made with a polycyclic silicone polymer as base resin, which showed improved strength at 350 °C than at RT, and retained 25% of its strength at 450 °C. All conventional silicone polymeric adhesives fail catastrophically at this temperature. The network formation in the polymer resulted in higher decomposition temperature and increased flame retardancy, which are essential requirements for high temperature resistant adhesives.