High-Temperature-Performance Cyanate Ester Composites with Carboranes

Cyanate esters (CEs) are an important class of materials among high-temperature-performance thermosets. They are used in aerospace launch vehicles, heat sinks, booms, trusses of satellites, etc., due to their high glass transition temperatures (>220 °C), excellent thermal stability, and low flammability. Current approaches to improve the thermal stability of CEs include incorporation of siloxanes or phosphorus-based flame retardants. In this work, we have explored boron-based hydroxy (PD)- and epoxy (EP)-functionalized carborane additives to improve the thermal properties of CEs. Carborane fillers were solvent-blended at various mass loadings in the resin and cured to study their effect on thermal properties. PD and EP carboranes react with CEs to form iminocarbonates and oxazolidinone linkages, respectively. Cure kinetic studies at different wt % loadings explained that carboranes catalyze the curing reaction by reducing the curing activation energy by about 54 and 26% for 10 wt % loadings of PD and EP carboranes, respectively. In addition, carborane-filled CE nanocomposites demonstrate an exceptionally high thermal stability as compared to the pristine resin in air and inert environments. Our thermogravimetric analysis (TGA) experiments show that the ultimate char yield of the resin can be increased from 0% to as high as 76 and 82% with 30 wt % PD and EP carborane loadings, respectively, at 1000 °C in air. The initial degradation temperature T d,5 of the composites decreased with increasing carborane loadings in both air and argon. For instance, T d,5 values for CE were 465 and 471.6 °C in argon and air, while those for P20 were 437.4 and 452.1 °C, respectively. Modulated TGA studies gave evidence of the effect of carboranes on degradation mechanism and kinetics in air and inert environments. The effect of bonding between carboranes and CEs at various loadings on the thermal expansion of the matrix was also studied using a thermomechanical analyzer. PD carborane reduced the T g for P20 to about 225 °C, while CE had T g > 350 °C.