NASA's Glenn Research Center (GRC) has developed and produced ultra-lightweight polymer cross-linked aerogels (X-Aerogels). These mechanically robust, highly porous, low-density materials are three times denser than native aerogels, but more than 100 times stronger. Aerogels are ultra-lightweight glass foams with extremely small pores (on the order of 10 to 50 nanometers). These materials are extremely good thermal insulators, with R values ranging from 2 to 10 times higher than polymer foams. Unlike multilayer insulation, aerogels do not require a high vacuum to maintain their low thermal conductivity, and can function as good thermal insulators at ambient pressure. In addition, they are good electrical insulators and have low refractive indices, both approaching values close to air. Aerogels are also excellent vibration-damping materials. Traditional aerogels, however, suffer fragility and poor environmental durability.
This approach significantly improves the mechanical properties and durability of aerogels without adversely affecting their desirable properties. This approach involves coating conformally, and cross-linking the individual skeletal aerogel nanoparticles with engineering polymers such as isocyanates, epoxies, polyimides, and polystyrene. The mechanism of cross-linking has been carefully investigated, and is made possible by two reactions: a reaction between the cross-linker and the surface of the aerogel framework, and a reaction propagated by the cross-linker with itself.
By tailoring the aerogel surface chemistry, this approach accommodates a variety of different polymer cross-linkers, including isocyanates, acrylates, epoxies, polyimides, and polystyrene, enabling customization for specific mission requirements. For example, polystyrene cross-linked aerogels are extremely hydrophobic, while polyimide versions can be used at higher temperatures. Recent work has led to the development of strong aerogels with better elastic properties, maintaining their shape even after repeated compression cycling. By tailoring the internal structure of the silica gels in combination with a polymer conformal coating, the aerogels may be dried at the ambient condition without supercritical fluid extraction.