First-generation, simple bisphenol-based phthalonitriles had high melting points and a short processing window. The Navy scientists addressed these issues in the new second-generation, oligomeric PEEK-like phthalonitrile. With no curing additive present, the resin or composite starts to soften and melt at about 60°C to 63°C. At 125°C, 150°C, 175°C, and 250°C, viscosity values were 1,940, 230, 80, and 30 centipoise (cP), Keller told us.
Although the curing temperature of both first- and second-generation phthalonitrile is the same, newer curing additives afford the ability to cure above 150°C, with faster curing reactions at higher temperatures. This makes it easily possible to control the time to cure and the speed at which the polymerization reaction occurs as functions of the amount of curing additive used and the curing temperature.
"The new phthalonitrile resin can be cured to a shaped solid or gelation at 200°C or 250°C, with curing occurring faster at the higher temperature," said Keller. "With some catalysts or curing additives that we've especially developed for the PEEK-like phthalonitrile, we can even cure to gelation at temperatures as low as 150°C. Of course, the shaped, cured component can be post-cured at higher temperatures to complete the cure, and the postcure can be done on the cured shaped polymer or composite in a high-temperature oven and in the absence of an autoclave or pressure."
Keller said the fact that the material does not soften or exhibit a glass-transition temperature at temperatures over 400°C during continuous modulus mechanical studies is novel. "Other polymers have not previously been reported with this characteristic and with the ability to retain shape under stress at temperatures up to 350°C for aerospace, oil drilling, and other domestic applications needing such stability. Moreover, the cured phthalonitrile is oxidatively stable for applications at these temperatures."
Aside from applications for oil rigs, aircraft, and ships, others may include robotic and autonomous firefighting systems, fire-resistant textiles, automotive components, wind turbine blades, structural composites, and battery casings.