Atmospheric pressure plasma sources are new devices for modifying the surface condition of engineering materials such as thermoplastic and thermoset-based composites. Because they operate at ambient conditions, these plasma systems can be used on a production line as a pre-treatment solution prior to painting or adhesive bonding to significantly improve adhesion strength. However, their efficient use requires sound understanding on how they modify the surface state of materials and, by the same token, how these modifications can be detected and quantified as regards their ability to provide high-strength adhesive joints. Polypropylene, since it is one of the most difficult-to-bond thermoplastic polymers and, at the same time, one of the most interesting polymers for the automotive industry (due to low cost, widespread use in the formulation of composites, lightweight and recyclability), was used in this paper as a model polymer. Because adhesive bonding opens the door to the design of hybrid structures composed of virtually any combination of dissimilar materials, often impossible to esthetically marry otherwise, the efficacy of polypropylene surface modifications using two plasma systems was tested on adhesively bonded aluminum-polypropylene hybrid assemblies. Once pre-treated using atmospheric plasma sources, the polypropylene surfaces were also analyzed using surface science techniques, namely contact angle measurements, attenuated total reflectance infrared spectroscopy, and X-ray photoelectron spectroscopy. From the correlations established between the mechanical strength of adhesively bonded joints and surface features using surface science approaches, the role of surface and adhesive chemistries in providing semi-structural adhesive joints was determined. Indicators of performance for adhesive joints, based on fast infrared spectroscopy analyses, were found to be efficient for the systematic identification of promising adhesive-surface treatment combinations. The adhesion mechanisms of plasma-processed polypropylene are also presented.
