Blocking/Deblocking Resin Systems

This technology (AERoBOND) enables the assembly of large-scale, complex composite structures while maintaining predictable mechanical and material properties. It does so by using a novel barrier-ply technology consisting of an epoxy resin/prepreg material with optimal efficiency, reliability, and performance. The barrier-ply materials prevent excessive mixing between conventional composite precursors and stoichiometrically-offset epoxy precursors during the cure process by forming a gel early in the cure cycle before extensive mixing can occur. The barrier ply is placed between the conventional laminate preform and the stoichiometrically-offset ply or plies placed on the preform surface, thus preventing excessive mass transfer between the three layers during the cure process. In practice, the barrier ply could be combined with the offset ply to be applied as a single, multifunctional surfacing layer enabling unitized assembly of large and complex structures. The AERoBOND method is up to 40% faster than state-of-the-art composite manufacturing methods, allows for large-scale processing of complex structures, eliminates the potential for weak bond failure modes, and produces composites with comparable mechanical properties as compared with those prepared by co-cure.

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.

RTM370 imide resin was developed to address the limitations of conventional imide resins, which are generated from commercially available symmetrical biphenyl dianhydride and oxydianiline (ODA). These resins form symmetrical dianhydride or diamine compounds that result in a substance with much higher viscosity than is viable for RTM, RFI, and VARTM. RTM370 harnesses the unique properties of asymmetric biphenyl dianhydride (a-BPDA) used in combination with a kinked ODA and a 4-(Phenylethynyl) phthalic anhydride endcap to form a mixture that can be melted without the use of solvents, and achieve the desired low-melt viscosity. RTM370 displays a high softening temperature (Tg = 370°C) and can be melted at 260-280°C. It can then be injected into fiber preforms under pressure (200 psi) or through a vacuum (VARTM) to form composites with excellent toughness. The resin can also be made into powder prepregs by melting the resin powders so that they fuse onto fibers. Recently, carbon fiber filled RTM370 imide resins have been fabricated into composites by laser sintering. This exciting advancement in additive manufacturing represents a new frontier for high-temperature composites. Not only are RTM370 composites lightweight, durable, and impact-resistant, they also possess outstanding abrasion resistance and significant thermo-oxidative stability (as demonstrated in long-term isothermal aging at 288°C for 1,000 hours). In summary, this groundbreaking approach yields a vastly superior resin for fabricating high-quality composites with improved performance, durability, and adaptability. RTM370's unique, solvent-free melt process is simpler, more environmentally friendly, and more cost-effective than competing systems, lending it broad appeal for a variety of Earth-based applications.

The FMLs resulting from the NASA process have similar properties to traditionally produced metal/composite hybrid laminates including, as compared to either the composite or metal only structures, improved load carrying capability, lighter weight, improved stiffness, improved impact resistance and damage tolerance, and improved permeation resistance. The NASA process can be applied to various FML types, including GLARE (glass, aluminum, epoxy), and TIGR (titanium, graphite). Typical manufacturing processes are costly and complex shapes are hard to produce, whereby the NASA process enables use of these kinds of laminates without an autoclave or press, thus increasing the size that can be produced and decreasing the cost. The resin pathways in the foils enable connection between the plies that can improve the interlaminar strength of the final part. Functionally the NASA process creates resin columns in the transverse direction of the plies. NASA is working to optimize the final properties by varying the size and distribution of the pathways.

This unique approach modifies the phenolic polymer network by adding thermoplastic molecules with flexible segments such as aliphatic carbon and siloxane. The thermoplastic molecules are terminated with bifunctional groups that can directly react with the phenolic under the curing condition to form chemical bonds. Further incorporation of these segments can be facilitated by a relay reaction of a second molecular component which can bond with both the first flexible segments and the phenolic network. The selections of flexible, thermoplastic segments are based on desired properties, which include flexibility, ablative, an charring ability, heat resistance, and low catalycity. The modified phenolic is a truly molecular composite in which flexible segments are connected with the phenolic network through strong chemical bonds and are uniformly distributed among the networks. This feature renders a uniform toughening/strengthening effect without compromising the lightweight nature of the materials. The process is also feasible to scale up and amenable for manufacturing.