Atmospheric pressure plasma surface treatment of thermoplastic composites for bonded joints

Glass fiber reinforced polypropylene samples were treated by an atmospheric pressure plasma jet using a rotary nozzle, in order to improve adhesive properties of the material surface. Thereby, the influence of the most significant operational parameters of the plasma treatment was studied. These parameters are e.g. the distance between the nozzle and the treated substrate, and the total plasma exposure time, varied by the axial velocity of the plasma jet. Two different parameter sets were selected to be compared in the present study. The surface properties after the plasma treatment were investigated by atomic force microscopy in combination with the contact angle measurements followed by the surface free energy calculation according to the OWRK method. The results were compared with those of the untreated samples to evaluate the induced effects. The surface free energy and hence the wettability were found to be increased significantly by both treatments. The surface topography was changed also in both cases, whereas one of the used parameter sets resulted in a higher roughness comparing to the untreated reference sample, and another one reduced it. Moreover, the adhesion properties of the treated surfaces were evaluated by lap shear tests. All treated samples exhibit a significant increased bonding strength in comparison to the untreated substrates. This improvement can be related both to the changes of the surface topography, induced by the thermal component of the plasma, and the cleaning and activation effects caused by the reactive plasma species during the treatment. However, significant differences concerning the total bonding strength were found between the used process parameter sets. Thus, a higher intensity of the plasma treatment led to a lower resistance of the final bond. In this case, the polymer surface seems to be partly degraded, forming a weak boundary layer, which indicates an overtreatment of the material.