Cathodic delamination measurement of Poly (vinyl butyral) coated steel after weathering exposure

Accelerated laboratory exposures are used to predict and understand the degradation of polymer coated metals in service. However, some discrepancies between laboratory exposure and field exposure are observed. A better understanding of the mechanism and the controlling factors of degradation is needed. The objective of this work is to propose an accelerated laboratory exposure that can reliably predict coating lifetime using a known acceleration factor.  Ozone is an oxidant that can decompose to form radicals under UV radiation.  Since ozone and UV radiation often exist in field environments, it is of interest to know how their reaction products react with organic polymer or metal surface. The effects of ozone, UV and RH on polymer-coated metal are studied. The samples are poly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate) on 1018 steel. The interface degradation between the primer and the substrate can be studied by measuring cathodic delamination with the Scanning Kelvin Probe (SKP). Exposure to UV (UVB radiation), O₃, and RH was performed for 4 days in a weathering chamber and the cathodic delamination rate was then measured in the SKP chamber exposed to lab air with 95% RH. Cathodic delamination was strongly accelerated when the sample was first degraded in the weathering chamber. Changes in ozone concentration below 0.25 ppm had no effect on cathodic delamination rate when the sample was exposed to UVB and 95% RH, but an increase to 1 ppm ozone accelerated delamination in the SKP chamber. A strong acceleration in the cathodic delamination was also observed when the sample was exposed to 0.1 ozone and 95% RH but with the UV radiation off. RH also affected the cathodic delamination rate, with a maximum observed at 60% RH when the sample was exposed to 0.1 ppm ozone and UVB.