The present work is concerned with adhesive bonding of thermoplastic composites used in general aerospace applications, including polyphenylene sulfide (PPS), polyetherimide (PEI) and polyetheretherketone (PEEK) carbon fibre composites. Three different surface treatments have been applied to the PEEK, PPS and PEI-based composites in order to enhance the adhesion: atmospheric plasma, ultraviolet radiation (UV) and isopropanol wiping as a control. Water contact angles and free surface energies were measured following the standard experimental procedure based on the employment of three different liquid droplets. Infrared spectroscopy and X-ray photoelectron spectroscopy (XPS) were subsequently performed to characterize the surface chemistry of the samples after treatment. The single lap joints were manufactured and bonded by an Aerospace grade epoxy-based film adhesive originally developed for use on metals but with the ability to bond treated thermoplastics to good strength (supplied by Henkel Ireland). Quasi-static (QS) tests were conducted. The lap shear strength was evaluated, and the failure mechanisms of the different joints were examined for the range of surface treatments considered. It was found that the performances of the PEEK and PPS joints were considerably improved by the plasma and UV treatments resulting in cohesive and delamination failures, while PEI was unaffected by the plasma and UV treatments and performed very well throughout.@en

Poly-etherether-ketone (PEEK) fibres (average diameter 30μm) were surface-activated by a UV-irradiation technique, and then used as interlayers of carbon fibre/epoxy composites. The results of a flatwise tensile test demonstrated a significant improvement in the PEEK fibre/epoxy adhesion upon the UV-treatment, i.e. the ultimate strength increased from 0.6–0.7MPa to 7.6MPa. Accordingly, interlaying UV-irradiated PEEK fibres resulted in considerable increases in the maximum values of open-hole tensile strength, Charpy impact strength and mode-I fracture energy, i.e. of 12%, 131% and 293%, respectively. However, it also decreased the flexural strength by 29%, owing to the thickness increase caused by adding interlayers. Fortunately, the load carrying capacity (the maximum failure load under flexural bending) was largely unaffected, and moreover, an average residual strength of 475 ± 23MPa still remained after the damage at the maximum load. The results demonstrated significant benefits of using longitudinal UV-irradiated PEEK fibres as interlayers of CFRPs.@en

The compatibility between the majority of thermoplastic veils (TPVs) and epoxies is typically poor, owing to the inherently low surface energies of thermoplastics. This tends to largely affect the toughening performance of TPVs as interlayer materials of carbon fibre/epoxy composites. The traditional methods for surface activation of thermoplastics, such as corona discharge, plasma treatment and acid etches, are not applicable to TPVs as they could cause significant damage to the thermoplastic fibres with nano-/micro-scale diameters. Herein, a UV-irradiation technique was proposed to active the surfaces of polyphenylene-sulfide (PPS) veils, that effectively improved their adhesion with epoxies. Consequently, the effects of an improved veil/epoxy adhesion on the mode-I and mode-II fracture behaviour and corresponding fracture mechanisms of the interleaved laminates were investigated. It was found that an improved veil/epoxy adhesion significantly enhanced the toughening performance of the PPS veils for the laminates manufactured by resin transfer moulding of non-crimp fabrics, by introducing additional carbon fibre delamination and significant PPS fibre damage during the fracture process. In contrast, the increased level of veil/epoxy adhesion inhibited PPS fibre bridging during the fracture process of the laminates produced from unidirectional prepregs, and caused considerable adverse effects on the fracture performance.@en

Carbon fibre reinforced poly-etherether-ketone (PEEK) and poly-phenylene-sulfide (PPS) composites were rapidly surface-treated by high-power UV light, and then adhesively bonded to aluminium 2024-T3 and carbon fibre/epoxy composites. The results of a single lap-shear joint test demonstrated that a UV-treatment lasting for 5 s was sufficient to prevent joint failure occurring at the composite/adhesive interfaces in all cases, e.g. it increased the failure strength of the PPS composite/aluminium joints from 11.1 MPa to 37.5 MPa. Moreover, the composite/adhesive interfaces performed well upon an exposure of the joints to an environment of high humidity and temperature for 8 weeks. Additionally, an investigation lasting for 6 months showed no degradation of the surface functionalisation from UV-irradiation. Overall, this work highlights high-power UV-irradiation a very promising method for surface preparation of thermoplastic composites (TPCs) for adhesive joining, i.e. TPC adhesive joints with excellent structural integrity can be obtained by using this rapid, eco-friendly and low-cost surface-treatment method.@en