Polymer/Melamine Aerogel Composites for Vibro-Acoustic Mitigation

Traditional aerogels are produced by sol-gel chemistry where a dilute polymer solution is taken to gelation. Polymer Aerogel 3D printing requires a high viscosity sol for stackable extrusion; however, this limits the time frame to print the materials prior to gelation. In response to this issue, NASA researchers have developed a novel dual solvent process to be used in additive manufacturing (3D printing). A dual-solvent formulation is employed during polymer aerogel precursor preparation to enable 3D printing of self-supporting structures. The system combines a high–boiling point aprotic solvent, which supports polymerization and network formation during aerogel synthesis, with a secondary low–boiling point solvent that partially evaporates during extrusion and printing. Preferential evaporation of the low–boiling component increases the local solids concentration and material viscosity at the nozzle and immediately after deposition, enabling filament stackability and shape retention without premature gelation. This approach decouples printability from bulk gel chemistry, allowing precise control of rheology during printing while preserving the desired aerogel microstructure and porosity after drying. A Two-Pot System: • Pot 1 contains a cross-linked polyamic acid solution and acetic anhydride or water scavenger, dissolved into a mix of high and low boiling point solvents (e.g., Dimethyl Sulfoxide (DMSO), n-methylpyrrolidone (NMP), or Dimethylformadie (DMF), with acetone or tetrahydrofuran (THF), ethanol, or methanol. • Pot 2 contains a base catalyst (e.g., trimethylamine or pyridine) and optionally a thickening agent (e.g., polyvinyl alcohol or polyvinyl acetate) to match viscosities. Dual-Solvent Chemistry: The low boiling point solvent evaporates rapidly upon extrusion, increasing the polymer concentration and viscosity, allowing the aerogel to retain its shape and gel quickly. • Additive Manufacturing Process: The two solutions are mixed at the extrusion tip of a syringe/nozzle-based 3D printer. This enables low-viscosity flow pre-extrusion and rapid solidification post-extrusion—solving a key challenge in 3D printing 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.

Researchers at NASA's Glenn Research Center have developed an approach to significantly improve 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, Glenn's approach accommodates a variety of different polymer cross-linkers, including isocyanates, acrylates, epoxies, polyimides, and polystyreneenabling 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.

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.

The Aerofoam composites have superior thermal and acoustic insulation properties when compared to conventional polyimide foams. In addition, they provide greater structural integrity than the fragile aerogel materials can provide independently. In general, polymer foams can provide excellent thermal insulation, and polyimide foams have the additional advantage of excellent high-temperature behavior and flame resistance compared to other polymer systems (they do not burn or release noxious chemicals). Incorporating aerogel material into the polyimide foam as described by this technology creates a composite that has been demonstrated to provide additional performance gains, including 25% lower thermal conductivity with no compromise of the structural integrity and high-temperature behavior of the base polyimide foam. The structural properties of Aerofoam are variable based on its formulation, and it can be used in numerous rigid and flexible foams of varying densities. Aerofoam has a number of potential commercial applications, including construction, consumer appliances, transportation, electronics, healthcare, and industrial equipment. In addition, these high-performance materials may prove useful in applications that require insulation that can withstand harsh environments, including process piping, tanks for transporting and storing hot or cold fluids, ship and boat building, and aerospace applications.