THE PLASMA TREATMENT OF THERMOPLASTIC FIBER COMPOSITES FOR ADHESIVE BONDING

The adhesive bonding of three unidirectional carbon.fibre composites - with thermoplastic polyetheretherketone, thermoplastic polyphenylene sulfide and, for comparative purposes, thermosetting epoxy matrices - has been examined. The adhesives were a room-temperature-curing epoxy paste and a hot-curing epoxy film; composite substrates were pretreated by light abrasion and solvent cleaning, followed by either an oxygen-plasma or corona treatment. It is shown that good adhesive bonding of the thermoplastic composites can be achieved if a plasma treatment is used prior to bonding. For the oxygen-plasma and corona treatments employed, the adhesive fracture energy, G(c), for the thermoplastic composite joints increases steadily with the intensity of the treatment until a plateau value of G(c) is reached. On the other hand, the thermosetting-based composite needs only light abrasion and solvent cleaning to give high values of G(c). Without the oxygen-plasma or corona treatment, the thermoplastic composite joints fail at the adhesive/composite interface at a very low applied load. The oxygen-plasma and corona treatments lead to an increase in the surface concentration of polar, oxygen-containing groups, resulting in both increased wetting by the epoxy adhesive on the substrate and greater intrinsic adhesion across the adhesive/composite interface. The enhancement in intrinsic adhesion is reflected in the locus of failure of the joints moving away from the adhesive/composite interface, and in far higher values of G(c).