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US Navy develops new high impact, high integrity polymer

18 April 2013

A new polyetheretherketone (PEEK)-like phthalonitrile-resin offers superior high temperature and flammability properties for defence and domestic applications.

NRL's proprietary phthalonitrile resins were measured to have dielectric permittivity and loss tangent characteristics that make them ideally suited for use in radomes like the one on this E-2C Hawkeye aircraft (photo: DoD/US Navy)
NRL's proprietary phthalonitrile resins were measured to have dielectric permittivity and loss tangent characteristics that make them ideally suited for use in radomes like the one on this E-2C Hawkeye aircraft (photo: DoD/US Navy)

The resin, developed by scientists at the US Naval Research Laboratory Chemistry Division, can be used to make composite components by established industrial methods such as resin transfer moulding (RTM), resin infusion moulding (RIM), filament winding, prepreg consolidation, and potentially by automated composite manufacturing techniques such as automated tape laying and automated fibre placement.

Phthalonitrile-based polymers constitute a class of high temperature thermosets that remain strong at temperatures up to 500 degrees Celsius (C), and that are easily processed into shaped fibre reinforced composite components by low-cost non-autoclave techniques. They are also nearly fireproof.

"The NRL-developed phthalonitrile-based polymeric composites exhibit superior flammability, low water absorption, and high temperature properties that do not exist in the current marketplace," says Dr Teddy Keller, head of NRL's Advanced Materials Section. "The fully cured phthalonitrile does not exhibit a glass transition temperature and can be used for structural applications in oxidising and thermally hostile environments."

The many attractive features of the phthalonitriles make these resins excellent candidates for numerous marine, aerospace, and domestic settings. The fire performance of phthalonitrile-carbon and phthalonitrile-glass composites is superior to that of any other thermoset-based composite currently in use for aerospace, ship, and submarine applications, and opens up many more applications than could be realised for other resin systems.

"Furthermore, a low melt viscosity and a larger processing window exhibited exclusively by the new second generation phthalonitrile technology are useful for fabrication of thick composite sections where the melt has to impregnate into thick fibre preforms," Dr Keller explains.

NRL's PEEK-like phthalonitrile resins were also measured to have excellent dielectric permittivity and loss tangent characteristics for potential high temperature radomes and other applications requiring radio frequency transparency. The ability to cure to a shaped solid or composite below 250 C and the superior physical properties relative to other high temperature polymers such as polyimides enhance the importance of the phthalonitrile system.

Due to their low water absorption, processing temperatures comparable to common epoxy resins, and superior thermo-oxidative stability at temperatures in excess of 375 C, the second-generation PEEK-like phthalonitrile-based polymers can revolutionise the use of composites in applications ranging from lightweight automobiles to fire-resistant building materials.

A notable aspect of this second generation technology is the ability to melt the resin and to control its initial cure to the shaped solid below 200 C. This permits NRL's phthalonitriles to be processed in the same way as ordinary commercial resins, using standard industrial composite manufacturing methods.

The oligomeric PEEK-like phthalonitrile is a liquid above 50 C and polymerises to a thermoset occurring above 150 C, giving it an ample processing temperature window. The rate of polymerisation is controlled as a function of temperature and the amount of curing additive, and the fully cured void-free phthalonitrile polymer does not exhibit a glass transition temperature (does not melt or soften) when post-cured to temperatures greater than 375 C. NRL's phthalonitrile resins/prepolymers or resin prepregs have an indefinite shelf life without the need for refrigeration.

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