Effect of atmospheric pressure plasma pre-treatment and aging conditions on the surface of thermoplastics

Abstract

The application of atmospheric pressure plasma is a popular pre-treatment method in the field of joining technology. Its purpose is the improvement of adhesion between different materials, e.g., for adhesive bonding, painting, or injection molding. Especially, while bonding parts with lower surface energies, efficient surface activation can be achieved via this treatment method. The surface is specifically modified in its chemical, physical, and topographical properties. However, after surface treatment, it is possible that these new surfaces progressively change by aging and compensation processes. Knowing the time-variant surface state after pre-treatment is extremely important for interpreting the entire process chain. Additionally, the effect of the pre-treatment on other properties—such as the bonding strength of an adhesive bond—must be known. The aging behavior of an atmospheric pressure plasma-treated plastic surface has been demonstrated in several studies based on the surface properties (chemical composition, surface energy, etc.). Whether these property changes also significantly affect the bond strength of structural adhesive joints is to be clarified in the present analysis. In this study, the aging behavior of three typical thermoplastics (polyethylene, polypropylene, and polystyrene) is examined after a treatment with an atmospheric plasma jet. The aging is taken into account by two factors: ambient temperature and time. Kinematic parameters (distance and velocity) of the pre-treatment are integrated in the experimental design to include the effects of the pre-treatment method in the investigation. The dependence of these influencing effects on bond strength and surface energy is analyzed with the help of central composite experimental designs. The results show a change in surface properties over time and ambient temperature. An increase of temperature and time leads to a reduction of surface energy. Furthermore, the kinematic factors of the plasma treatment determine aging. However, a mathematical correlation between bond strength and aging parameters could not be detected in this study. The bond strength is time-invariant but significantly dependent on the parameters distance and speed. Additionally, a dimensional analysis was conducted in order to describe the dependency of surface energy and load factor on the investigated parameters. Therefore, several dimensionless parameters were developed. Regarding the load factor, it was possible to describe the ongoing processes based on the correlations discovered by dimensional analysis. This allows the prediction of the load factor based on process and storage parameters. A relationship between the investigated Π-numbers could not be detected for the surface energy, which means a functional relationship to describe the surface energy could not be found.