Plasma treatment and adhesion properties of a rubber-modified polypropylene

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Abstract
In this study, cold, glow-discharge plasmas were used as a pretreatment method for the lacquering of rubber-modified polypropylene plates. This type of material is also referred to as thermoplastic polyolefins (TPOs). The effects of plasma treatments at radio- and microwave frequencies (RF and MW) and in combined MW-RF modes were studied, as were the effects of plasma power-to-gas flow (P/F) ratios and of discharges in oxygen, nitrogen, air, argon, and hydrogen. Surface characterization was carried out by contact angle measurements with water as the wetting liquid, and by XPS analyses. The adhesion between a two-component polyurethane (PUR) lacquer and plasma-treated TPO plates was evaluated by 180°-peel testing. The wettability of TPO surfaces was not affected by the plasma frequency or the P/F ratio, while the influence of the discharge gas was noticeable. Furthermore, no correlation between wettability and peel force could be found. Instead, lacquer adhesion was shown to be highly dependent on the P/F ratio and on the choice of discharge gas. The peel forces were found to be in the range of 0.1-35 N/15 mm, and the locus of failures was shown (by visual inspection or by XPS analysis) invariably to be in the TPO substrate. Electromagnetic radiation, most likely vacuum-ultraviolet (VUV) emission (<200 nm), was proposed to be a critical factor in plasma treatments. Attributed to VUV radiation was the creation of radicals in the TPO substrate; these lead to severe chain scission reactions and thereby govern the cohesive strength of the near-surface region of the substrate.