3D-Printed Composites for High Temperature Uses

Next generation aircraft are anticipated to be largely made with composite components, requiring significant increases in manufacturing rates of composites to meet the demand for a new fleet of aircraft. The higher rate manufacturing will require multiple advances, including rapid curing and lower processing temperatures. These requirements can be enabled by new processing methods such as isothermal rapidly cured composite parts. NASA has developed materials and methods that meet those stringent requirements for high-rate manufacturing. The innovators have demonstrated at least two families of new resin formulations that meet the expected high-rate manufacturing needs. These new formulations have been engineered to be infused and cured at the same (i.e., isothermal) temperature, below that of commercially available materials. The materials can then be removed from the mold while still hot without distorting the shape, thereby reducing the processing times by eliminating the need for cooling to occur in the mold. After a post-cure process - which takes 4 hours or less and can be performed in batches - the mechanical properties of NASA's next-gen composites. The related patent is now available to license. Please note that NASA does not manufacturer products itself for commercial sale.

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.

Many researchers have attempted to use polymer-ceramic composites to improve the thermal and dielectric performance of polymer insulation for high voltage, high temperature electronics. However, using composite materials has been challenging due to manufacturing issues like incomplete mixing, inhomogeneous properties, and void formation. Here, NASA has developed a method of preparing and extruding polymer-ceramic composites that results in high-quality, flexible composite ribbons. To achieve this, pellets of a thermoplastic (e.g., polyphenylsulfone or PPSU) are coated with an additive then mixed with particles of a ceramic (e.g., boron nitride or BN) as shown in the image below. After mixing the coated polymer with the ceramic particles, the blended material was processed into ribbons or films by twin-screw extrusion. The resulting ribbons are highly flexible, well-mixed, and void free, enabled by the coated additive and by using a particle mixture of micronized BN and nanoparticles of hexagonal BN (hBN). The polymer-ceramic composite showed tunable dielectric and thermal properties depending on the exact processing method and composite makeup. Compared to the base polymer material, the composite ribbons showed comparable or improved dielectric properties and enhanced thermal conductivity, allowing the composite to be used as electrical insulation in high-power, high-temperature conditions. The related patent is now available to license. Please note that NASA does not manufacturer products itself for commercial sale.

This NASA invention enables several benefits that mitigate limitations associated with conventional ATP systems, including the following: (1) avoids obtuse head rotation or cross-tool translation when laying adjunct tape plies, (2) simultaneously places tape on both sides of a part via two robots, (3) eliminates external anchoring frame requirements, and (4) translates parts during build while also translating the applicator head. The ability to perform simultaneous layup on opposite sides of the component, as well as reduction of head rotation reversal during bidirectional tape layup, offers increased layup speed. The invention offers increased placement accuracy as a result of reduced movement between tape layup operations and the eliminated need for an anchoring frame (facilitated by simultaneous pressure extrusion of prepreg by the two robots). NASA’s automated tow/tape placement system has two key unique features: the use of two opposed ATP cars to enable a tool-less process, and an on-the-fly reversal tape/tow laydown tooling head. The system uses two opposing (i.e., underside-to-underside) ATP cars, and can build parts vertically, horizontally, or at any other angle, depending on the workspace available. The ATP die wheels can be reversed or turned to draw the composite back and forth at different angles to create a layer-by-layer composite structure. Both cars can dispense TPC tape – thus, either car can function as an opposing tool surface while the other performs prepreg lay-up. For structures that do not vary in thickness, both cars can lay tape at the same time – doubling layup speed. Current ATP robots must rotate the large tooling head, or traverse panels without layering tape to achieve bidirectional layup, where each additional movement introduces alignment error. To increase layup rate while simultaneously minimizing misalignment, NASA’s system incorporates an on-the-fly reversal tape/tow laydown tooling head to enable efficient bidirectional layup.

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.