ABSTRACT
Injection-moulded plates of four commercial thermoplastic polyolefins (TPOs) were subjected to oxygen plasma treatment. The modified surfaces were analyzed by water contact angle measurements and X-ray photoelectron spectroscopy (XPS), and the adhesion properties of the plates were evaluated by a 90° peel test after being lacquered with a two-component polyurethane lacquer. The study included treatments in two different plasma reactors operating at different frequencies. The influence of certain processing parameters, such as discharge power, flow rate and gas pressure, was investigated, as was that of frequency (using the same reactor). While the results revealed that oxygen plasma treatment indeed led to improved wettability, the degree of surface modification was not highly affected by changes in the processing conditions. In contrast, there was a great effect on the lacquer adhesion, in particular by changes in discharge power and gas pressure. The results also showed that the TPOs were sensitive in different ways towards changes in the processing conditions. It was also found that, regardless of the absolute peel force, the failures occurred in the substrate at some distance below the oxidized layer. These observations were attributed to a VUV-induced formation of radicals which, in the case of polypropylene-based materials, predominantly lead to ß-scissions. As secondary radicals have a higher tendency to form crosslinks that can compensate for chain scission reactions, the difference in the sensitivity of the TPOs was proposed to be related to the amount and distribution of ethylene in the materials.
