Secondary bonding of CF/PEKK thermoplastic composites with a PEI film adhesive in a press welding process

High-performance poly-ether-ketone-ketone (PEKK) thermoplastic (TP) composites, which are beginning to gain interest in the aerospace industry, require a joining procedure to integrate the individual parts. Amorphous TP bonding has been recently proposed to address the limitations of commonly used joining techniques for composites, such as mechanical fastening and adhesive bonding. In this study, we investigated the adhesive properties of a polyetherimide (PEI) film applied by hot press welding as an amorphous polymer interlayer of PEKK laminate. The temperature range of the bonding process was determined via differential scanning calorimetry analysis. The bonding strength was evaluated through single-lap shear tests. The effect of process temperature on the bonding strength was qualitatively analyzed via fracture surface morphology analysis. The interface between PEKK and PEI was analyzed via optical microscopy. The adhesive strength was as low as about 5 MPa up to the bonding temperature of 280[Formula: see text]C but then started to improve as the temperature increased, which is attributed to the interdiffusion of the PEKK matrix and PEI adhesive film. The maximum bonding strength of 23 MPa was confirmed at a bonding temperature of 340[Formula: see text]C. A mixed failure mode consisting of a relatively high ratio of fiber-tear failure to light-fiber-tear failure is observed at this process temperature. This failure mode can be attributed to the bonding temperature condition at which PEKK begins to melt.