Thermal properties of high performance thermoplastic composites based on poly(ether ketone ketone) (PEKK)

The innovative LDF material system employing 'aligned' long discontinuous carbon fiber with high performance poly(ether ketone ketone) (PEKK) matrix has been demonstrated for manufacturing cost effective thermoplastic composite parts for aerospace applications. This paper reviews recent progress in characterization of thermal properties of PEKK matrix and its composites such as crystallinity, melt rheometry, pressure-volume-temperature relations, thermal expansion, thermal conductivity and heat capacity. These properties are relevant in thermoforming to achieve optimum mechanical performance for the LDF or continuous fiber reinforced composite parts. Crystallization kinetic measurements showed that PEKK matrix and AS-4/PEKK composites have fast crystallization rate and may crystallize in a wide range of cooling rates during thermoforming process. The 'optimum' isothermal crystallization temperature for the PEKK-based composites is 255°C with a minimum 'peak-time' of 8 seconds. No relevant difference in isothermal crystallization kinetics was found between PEKK neat resin and AS-4/PEKK composites. In differential scanning calorimetry (DSC) analysis, PEKK exhibited double-melting transition behavior after isothermal annealing, or slow cooling, probably attributed to the melting of two crystalline regions. PEKK has a higher Tg, a lower equilibrium melting temperature and lower melt viscosity than poly(ether ether ketone) (PEEK).