Plasma and corona discharge pretreatment of polyetheretherketone for adhesive bonding

To enhance the bondability of polyetheretherketone (PEEK), surface treatment techniques of plasma and corona discharge have been evaluated. The results have shown that these two methods are effective and practical. The treated materials not only reach their highest possible joint strength, but also show very promising joint durability. In addition, the various environments, e. g. atmosphere, heat, water, and solvent, have little effects on the enhanced bondability of the treated materials. By using Contact Angle Measurement, Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and Time of Flight Secondaty Ion Mass Spectrometry (TOF-SIMS) to characterise the PEEK surfaces before and after treatment, it was established that the lack of active chemical groups, which if present can form strong interatomic and intermolecular forces across the adhesive/PEEK interface, is the main cause for the poor bondability of the untreated PEEK. Both plasma and corona discharge treatment introduce such active functional groups, for instance, hydroxyl, carboxylic acid, amine and etc., onto the surface of PEEK film and so greatly enhance the intrinsic adhesion at the interface between treated PEEK surfaces and epoxy adhesive, as confirmed by the TOF-SIMS interfacial analysis. It is deduced that low molecular weight molecules (LMWM) are formed on treated surfaces, which contain high concentration of oxygen and/or nitrogen, and can be removed by solvent washing. The removal of LMWM will drastically reduce the wettability of the treated surfaces, but does not impair the enhanced bondability. It has been found that both plasma and corona discharge treated surfaces are in a thermodynamically unstable state. When exposed to the atmosphere, the treated surfaces tend to lose their improved wettability and decrease their surface polarity. Increasing temperature can not only accelerate these processes but also change the surface chemical structures of the treated materials back to that of the untreated films to some extent, as revealed by the TOF-SIMS analysis. Water immersion, on the contrary, tend to reverse the above processes.