Surface modification of high performance polymers by atmospheric pressure plasma and failure mechanism of adhesive bonded joints

In this investigation, the effects of atmospheric pressure plasma treatment on the surface energy of polyetheretherketone (PEEK), carbon fibers (CF) and glass fiber (GF) reinforced polyphenylene sulfide (PPS) are studied. A substantial improvement in the surface energy of these materials is observed after the atmospheric plasma treatment. It is observed that the polar component of surface energy is responsible for the increase in total surface energy of these materials. To make a comparison of atmospheric plasma treatment and low pressure plasma treatment on the surface energy, PEEK surface is also modified by low pressure plasma. It is observed that the surface modification of polymer by atmospheric pressure plasma is more effective in comparison to low pressure plasma both in terms of improvement of surface energy and bonded joint strength. Scanning electron microscopy of untreated and atmospheric plasma treated specimens is carried out to examine the surface morphology. After atmospheric plasma treatment, increased surface roughness is observed which helps in improving the adhesion properties. The improvement in adhesion properties of these materials is correlated with lap shear strength of adhesive bonded joints. Bonded joints are fabricated by employing recently developed ultrahigh temperature resistant epoxy adhesive. Tensile lap shear testing is also carried out using PPS-CF and PPS-GF as substrate materials. Lap shear tests results for these materials show three to four times improvement in joint strength after atmospheric plasma treatment. Finally, the fractured surfaces of the joints were examined by scanning electron microscope to understand the failure mechanism.