Polymer Cross-Linked Aerogels (X-Aerogels)

GRC's aluminosilicate aerogel composites are fabricated using a sol-gel technique. A sol is formed by hydrolyzing an alumina dispersion in acid solution; the alumina may be combined with a silicon precursor to create a sol. Fabrics, papers, and felts are used as reinforcing fibers to form an aerogel composite. The aerogel adheres to the reinforcement without use of sizing or organic binders. (In the case of sized fabrics, the sizing is first removed by heat cleaning.) Composites can be fabricated in a batch process, impregnating individual layers of paper, felt or fabric with the precursor sol, or in a roll-to-roll process. The sol is allowed to gel, and then aged for several days prior to supercritical drying using liquid CO2. Heat treatment of the super critically dried composites can be used to tailor the alumina or Aluminosilicate crystal structure and pore size. In contrast to commercially available insulations, GRC's innovation provides extremely low thermal conductivity (60 mW/m-K at 900°C in argon) at high temperatures, thus enabling use at higher temperatures and improving applicability. In addition, GRC's unique process provides very good adhesion of the aerogel to its reinforcing fibers in alumina papers and zirconia felts, eliminating the spalling seen in other aerogel composites. Finally, GRC's innovation demonstrates low density and extreme resilience to high temperatures and harsh conditions. Seven layers of composite material of 1.25 mm/layer produced a temperature drop of 700°C when tested in the 8-foot high-temperature wind tunnel (8 HTT) at NASA's Langley Research Center. The technology also has withstood heat tests of up to 1200°C. In combination with other insulators, it has withstood fluxes of up to 65 W/cm2, producing a temperature drop of 625°C across 8 mm.

Innovators at NASA Glenn have devised a new method for harnessing the high transmission and clarity associated with optical glasses in a robust polyimide aerogel. This process uses sol-gel synthesis technology with aromatic dianhydrides and diamines as the precursors, and a trifunctional triacid chloride that arranges itself into a three-dimensional (3D) matrix with a low refractive index. The liquid portion of the gel is then removed by supercritical fluid extraction in order to produce the polyimide aerogel and maintain the desired 3D structure without pore collapse. The result is a cross-linked polyimide aerogel that allows for light wave transmittance while retaining low thermal conductivity. This unique material can be made into thin blocks, or highly flexible films as thin as 0.5 mm. While some embodiments have a yellow color, other embodiments may be nearly colorless. When compared to high-opacity polyimide aerogels, they have much greater surface area (up to 880 m2/g) and a very homogenous pore size (10 to 20 nm) with only a minor penalty in density (0.15 g/cc vs 0.10 g/cc). These strong, optically transparent aerogels incorporate a number of unique properties with applicability to a host of potential new applications, making this innovation a game-changer in the global aerogel market. Glenn welcomes co-development opportunities.

Aerogels are highly porous, low-density solids with extremely small pore sizes, fabricated by forming a gel from a solution in the wet-gel state that is then converted to the dry-solid state without compaction of the porous architecture. Aerogels make excellent electrical, thermal, and acoustic insulators. However, most inorganic silica aerogels are fragile and shed dust. The NASA Glenn team is the first to synthesize three-dimensional polymer aerogel networks of polyimides cross-linked with multifunctional amine monomers. Compared to silica aerogels, these aerogels retain small pore sizes and low thermal conductivities, but are distinguished by their flexibility. Polyimide aerogels are not brittle, fragile, or dusty like silica aerogels. Plus, polyimide aerogels possess the beneficial characteristics and strength of polyimide materials. The results are cross-linked polyimide aerogels with little shrinkage, low densities, high compression and tensile strengths, and good moisture resistance. They can be fabricated or machined into net shape parts, which are strong and stiff, or cast as thin flexible films with good tensile properties. Extremely customizable, polyimide aerogels can be formed into any configuration (e.g., wrapped around a pipe, sewn into protective clothing, or molded into a panel to act as a heat shield in a car). In short, Glenn's innovation improves the performance, adaptability, and affordability of aerogels in a broad number of applications.

Polyamides are polymers that are similar to polyimides (another polymer that has been developed for use in aerogels). However, because the amide link is a single chain while the imide link is a ring structure, polyamide aerogels can be made less stiff than polyimides, even though a similar fabrication process is used. The precursor materials can be made from any combination of diamine and diacid chloride. Furthermore, NASA Glenn researchers have found methods for using combinations of diamines and disecondary amines to produce polyamide aerogels with tunable glass transition temperatures, for greater control of features such as flexibility or water-resistance. In the first step of the fabrication process, an oligomeric solution is produced that is stable and can be prepared and stored indefinitely as stock solutions prior to cross-linking. This unique feature allows for the preparation and transport of tailor-made polyamide solutions, which can later be turned into gels via the addition of a small amount of cross-linker. When the cross-linking agent is added, the solution can be cast in a variety of forms such as thin films and monoliths. To remove the solvent, one or more solvent exchanges can be performed, and then the gel is subjected to supercritical drying to form a polyamide aerogel. NASA Glenn's polyamide aerogels can be fully integrated with the fabrication techniques and products of polyimide aerogel fabrication, so hybrid materials which have the properties of both classes are easily prepared. As the first aerogels to be composed of cross-linked polyamides, these materials combine flexibility and transparency in a way that sets them apart from all other polymeric aerogels.

The approach in developing the LCX system was to provide a combination of advantages in thermal performance, structural capability, and operations. The system is particularly suited for the complex piping, tanks, and apparatus subjected to the ambient environment common in the aerospace industry. The low-cost approach also lends the same technology to industrial applications such as building construction and chilled-water piping. The system can increase reliability and reduce life cycle costs by mitigating moisture intrusion and preventing the resulting corrosion that plagues subambient-temperature insulation systems operating in the ambient (humidity and rain) environment. Accumulated internal water is allowed to drain and release naturally over the systems normal thermal cycles. The thermal insulation system has a long life expectancy because all layer materials are hydrophobic or otherwise waterproof. LCX systems do not need to be perfectly sealed to handle rain, moisture accumulation, or condensation. Mechanically, the LCX system not only withstands impact, vibration, and the stresses of thermal expansion and contraction, but can help support pipes and other structures, all while maintaining its thermal insulation effectiveness. Conventional insulation systems are notoriously difficult to manage around pipe supports because of the cracking and damage that can occur. Used alone or inside another structure or panel, the LCX layering approach can be tailored to provide additional acoustic or vibration damping as a dual function with the thermal insulating benefits. Because LCX systems do not require complete sealing from the weather, it costs less to install. The materials are generally removable, reusable, and recyclable, a feature not possible with other insulation systems. This feature allows removable insulation covers for valves, flanges, and other components (invaluable benefits for servicing or inspection) to be part of original designs. Thermal performance of the LCX system has been shown to equal or exceed that of the best polyurethane foam systems, which can degrade significantly during the first two years of operation. With its inherent springiness, the system allows for simpler installation and, more importantly, better thermal insulation because of its consistency and full contact with the cold surface. Improved contact with the cold surface and better closure of gaps and seams are the keys to superior thermal performance in real systems. Eliminating the requirement for glues, sealants, mastics, expansion joints, and vapor barriers provides dramatic savings in material and labor costs of the installed system.