AERoBOND: Large-scale Composite Manufacturing

The AERoBOND and AERoBOND+ technologies are composite resin materials design innovations that enable new methods for composites joining and manufacturing. The resins are formulated with carefully selected off-set stoichiometries to delay/control the cure such that initial curing of individual components can be followed separately by joining/curing of components together. The ability to delay and control the co-cure joining step provides ease of manufacturing of multi-part composite structures, without compromising joint integrity. There are significant cost savings associated with eliminating fasteners and joint surface preparation steps. To date, the focus of the NASA development effort has been on novel epoxy-based prepreg formulations though other types of thermosets could be considered as well. The AERoBOND+ innovation provides an added adhesive layer to the AERoBOND joint design to improve the ability to join composite surfaces when these surfaces are less tightly matched. Conventional adhesives, e.g., film, paste, etc., are employed. By including an adhesive between the offset stoichiometric prepreg plies, the adhesive can fill the gaps between the bonding surfaces while maintaining reflowable AERoBOND layer interfaces. Since all interfaces are reflowable, they are much more tolerant of surface contamination, thereby mitigating a primary challenge for conventional adhesive bonding.

This technology enables the fabrication of co-cured structures without the need for an autoclave or oven large enough to contain the full-scale structure. Instead, sub-components can be prepared in a less expensive, smaller autoclave or oven with co-cure plys applied to the faying surfaces. A continuous, assembled structure can be prepared using a subsequent curing process in a heated press. The co-cured structures can be designed to meet FAA certification criteria for composite structures because no adhesive bond or mechanical fasteners are needed. The structure can be treated as a single, joint-free component.

NASA's new technique for generating recyclable feedstocks for on-demand additive manufacturing employs the high-yield reversibility of the Diels-Alder reaction between maleimide and furan functionalities, utilizing the exceedingly favorable interaction between specific chemical functionalities, often termed "click reactions" due to their rapid rate and high efficiency. Integration of these moieties within a polymer coating on epoxy microparticle enables reversible assembly into macroscopic, free-standing articles. This click chemistry can be activated and reversed through the application of heat. Monomer species can be used to incorporate these functionalities into polyimide materials, which provide excellent mechanical, thermal, and electrical properties for space applications. Copoly (carbonate urethane) has been shown to be a viable coating material in the generation of polymer-coated epoxy microparticle systems and is amenable to being processed through a variety of approaches (e.g., filaments and slurries for 3D printing, compression molding, etc.). The polymeric materials are grown from the surfaces of in-house fabricated epoxy microparticles. The thermal and mechanical properties of the microparticles can be readily tuned by changes in composition. There are a number of potential applications for this NASA technology ranging from use of these materials for recyclable/repurpose-able articles (structural, decorative, etc.) to simple children's toys. More demanding uses such as for replacement parts in complex industrial systems are also possible. For long term space missions, it is envisioned that these feedstocks would be integrated into secondary spacecraft structures such that no additional concerns would be introduced due to in-space chemical reactions and no additional mass would be required.

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.

NASA's technology is the first successful 3D-printing of high temperature carbon fiber filled thermoset polyimide composites. Selective Laser Sintering (SLS) of carbon-filled RTM370 is followed by post-curing to achieve higher temperature capability, resulting in a composite part with a glass transition temperature of 370 °C. SLS typically uses thermoplastic polymeric powders and the resultant parts have a useful temperature range of 150-185 °C, while often being weaker compared to traditionally processed materials. Recently, higher temperature thermoplastics have been manufactured into 3D parts by high temperature SLS that requires a melting temperature of 380 °C, but the usable temperature range for these parts is still under 200 °C. NASA's thermoset polyimide composites are melt-processable between 150-240 °C, allowing the use of regular SLS machines. The resultant parts are subsequently post-cured using multi-step cycles that slowly heat the material to slightly below its glass transition temperature, while avoiding dimensional change during the process. This invention will greatly benefit aerospace companies in the production of parts with complex geometry for engine components requiring over 300 °C applications, while having a wealth of other potential applications including, but not limited to, printing legacy parts for military aircraft and producing components for high performance electric cars.