Achieving Enhanced Fracture Toughness of Adhesively Bonded Cured Composite Joint Systems Using Atmospheric Pressure Plasma Treatments

Fibre reinforced composite materials are widely used in products ranging from automobile bumpers to aircraft wings and sports equipment. The objective of this research was to compare the performance of both helium / oxygen and air atmospheric plasma treatment systems for enhancing the bond strength between the composite materials and adhesive layer. The atmospheric plasma systems investigated were a reel-toreel helium plasma system called Labline™ and an air jet system called PlasmaTreat™. The plasma in the case of the former system is formed using a helium / oxygen gas mixture, while the PlasmaTreat™ system uses an air plasma. The research investigated the effectiveness of these two atmospheric plasma sources at activating cured composite surfaces, their effect on surface chemistry, in addition to the fracture toughness and failure mechanism of treated and adhesively bonded cured composite joints. The level of surface activation was examined using water contact angle measurements. Surface hydrophobic recovery of the treated composites was monitored over a period of 12 days. Surface chemistry was examined using X-ray photoelectron spectroscopy (XPS). The failure mechanism of the adhesively bonded composite joints was studied by double cantilever beam (DCB) tests. It was observed that for both plasma treatment systems the locus of failure changed from interfacial between composite and adhesive layer to either cohesive or interlaminar in the composite, depending on the adhesive employed. The more intense PlasmaTreat™ air plasma was found to significantly reduce composite treatment times and the treated surfaces exhibited reduced levels of hydrophobic recovery compared with that obtained with the Labline™ system.