A new high-performance resin developed by the US Naval Research Laboratory (NRL) could prove key to the use of polymers and composites in naval and aerospace applications, among others. Its resistance to high temperatures, flammability, and impacts makes the oligomeric PEEK-like (polyether ether ketone) phthalonitrile resin a candidate for better composites using standard processes like resin infusion molding (RIM) and resin transfer molding (RTM).
Phthalonitrile resins are high-temperature, almost-fireproof thermoset polymers. They retain mechanical strength at temperatures up to 500°C, yet can be easily shaped into composite components using less costly production methods. These processes are faster than the traditional composite production methods typically used in military and aerospace applications. Those require autoclave ovens, which are big, slow, and expensive, as we've previously discussed.
Scientists at the NRL's Chemistry Division developed the new resin to have loss tangent characteristics and excellent dielectric permittivity, both desirable in applications where RF transparency is required, such as high-temperature radomes that shield radar antennae. Some qualities, primarily a similar chemistry, make it similar to PEEK, a much-used engineering plastic that's also typically chemical-, flame-, and temperature-resistant while maintaining excellent mechanical strength.
The material's superior flammability, low water absorption, and high-temperature properties are better than those of the current class of high-temperature thermoset polymers and composites, such as polyimides, Teddy Keller, head of the NRL Chemistry Division's Advanced Materials Section, told Design News in an email. For example, when heated to 1,000°C under inert conditions, the fully cured phthalonitrile-based polymer exhibited a high char-yield of 80 percent to 90 percent, with little outgassing or volatility to ignite. This outstanding resistance to flammability indicates it could be an excellent candidate for composite and fire-barrier applications below deck on ships.
The fully cured phthalonitrile polymer showed less than 2 weight percent water uptake when soaked in water for about 20 days, and water uptake leveled off after approximately 40 days. "This is a significant advancement over other high-temperature polymers such as polyimides," Keller told us. "The water absorption is so low even after soaking for extended periods that the mechanical and structural integrity is not affected on exposure to elevated temperatures. The wet and dry physical properties are essentially the same." Also, the fully cured material does not soften or exhibit a glass transition temperature at temperatures over 400C, and can be used for long-term applications above 350°C.
The phthalonitrile resin has a low melt viscosity and a wider processing window than the current class of high-temperature thermoset polymers and the composites made from them, such as polyimides, which are currently used for aerospace, ship, and submarine applications. These characteristics are especially important in the material's ability to be used in composite fabrication for thick sections of a component where there are many fibers. It can be challenging to ensure that the melted polymer fully impregnates the thick portions of a preform. These characteristics also make it possible to fabricate composites using less expensive processes, such as RTM, RIM, prepreg consolidation, and filament winding. Automated composite production methods like automated fiber placement and automated tape laying are also possibilities.