(32d) How Carboranes Provide Tremendous Improvement in Thermal Stability of Thermosets for Aerospace and Defense Applications?

Cyanate ester (CE) is an important class of materials among high-temperature performance thermosets. It is used in aerospace launch vehicles, heat sinks, booms and trusses of satellites etc. due to its high glass transition temperatures (>220°C), excellent thermal stability, and low flammability. Current approaches to improve thermal stability of CE include incorporation of siloxanes or phosphorus based flame retardants (PFRs). Siloxanes demonstrate only a marginal improvement in thermal stability by acting as a barrier for heat and oxygen flow. On the other hand, PFRs reduce flammability of the matrix but they also result in lower char yields and glass transition temperature of CE. In this work, we have explored boron-based “carborane” additives to improve thermal properties of CE. We have demonstrated how the synergic interactions of boron with carbon and nitrogen from matrix can be applied for developing advanced materials. Carborane fillers were solvent blended at various mass loadings in the resin and cured to study their effect on thermal properties. Carborane-filled cyanate ester (CE) nanocomposites have an exceptionally high oxidative thermal stability as compared to the pristine resin. Our thermogravimetric analysis (TGA) experiments show that the ultimate char yield of the resin can be increased from 0% to as high as 76% with 30 wt% carborane loading at 1000°C in air. We interrogated the degradation mechanism of these carborane-CE composites by characterizing the reactions in the condensed phase (char) and the evolved gaseous products. The effect of degradation on char was studied through FTIR, Elemental Analysis, and solid-state 11B and 13C NMR. TGA evolved gases were evaluated using mass-spectrometer and FTIR. We have discovered that the covalent linkages between CE and carboranes are labile, producing a moderate temperature (ca. 450°C) initial degradation in TGA. However, at higher temperatures (> 650°C) the carborane structure disintegrates and is oxidized to boronic acid and boroxine structures. The boron oxides in the char promote crosslinking and result in tremendous thermal stability. We have outlined the variation in degradation mechanism for carborane containing CE systems in the inert and oxidative environment. Boron based composites are not well understood for high temperature applications and previous work has majorly focused on halogen, silicon or phosphorus based additives. This study will help elucidate the action of boron as thermal protection agent and better design of composites with other matrices.