Synergetic Effect of Grit-Blasting and Atmospheric Cold Plasma Pretreatments on the Surface Free Energy of a Fibreglass/Epoxy Vinyl Ester Composite

For the first time, the efficiency of different surface pretreatment approaches prior to adhesive bonding of a fibreglass-reinforced epoxy vinyl ester thermoset composite has been investigated. It was found that grit-blasting generally had a negligible effect on the surface free energy (SFE) calculated using the Owens, Wendt, Rabel, and Kaelble method, as well as the Lifshitz–van der Waals/acid-base (LWAB) approach. However, contrary to abrading, grit-blasting has shown its efficiency to flatten sharp surface irregularities and introduce surface roughness features suitable to adhesive bonding processes. With or without a previous grit-blasting step, argon gas atmospheric cold plasma treatment has shown a slight to moderate efficiency in increasing the SFE polar component of the composite. However, it was found that the addition of 0.07% oxygen to the argon plasma readily allows an important gain in the SFE polar component. Indeed, when processed at a speed of 30 m/min on a previously grit-blasted composite surface, the Ar/O2 atmospheric cold plasma treatment increases the surface free energy to values >73 mJ/m2, making the surface condition optimized for structural adhesive bonding. An oxidation mechanism of the composite surface exposed to atmospheric cold plasma was suggested on the basis of correlations established between the polar part of SFE obtained from the Owens et al. method, acid/base components calculated using the LWAB approach, and ATR infrared spectroscopy signatures obtained for a model polyolefin material.