Relative Lifetimes of IR Absorbing and IR Transmitting Black Resins in Accelerated and Natural Weathering 2019-01-1274
Degradation processes driven by UV exposure, and manifested for example as polymer gloss loss or coating failure, are generally accelerated at elevated temperature, or conversely, their rates are reduced at lower temperature. In a weathering environment comprising IR irradiance, IR transmitting black resin tends to be cooler than an otherwise comparable sample of IR absorbing black resin. Accordingly, slower UV-driven degradation, and longer weathering lifetime, is expected for IR transmitting black resin relative to IR absorbing black resin, commensurate with their temperature difference in a given weathering environment and the sensitivity of the degradation process to temperature.
This paper reports temperature differences of IR absorbing and IR transmitting black resins as measured in two Xenon-arc accelerated weathering instruments for several black panel and chamber temperatures, several configurations of light and dark witness samples in one of the instruments, and two commercial filter combinations that differ in their IR transmittance. These differences under filtered Xenon-arc are compared with the temperature difference of the same samples measured outdoors in southeast Michigan. The temperature difference under the various Xenon arc treatments generally exceeds the difference outdoors, the more so when irradiance-weighted average temperatures are considered. Thus, the difference in UV exposure to failure in the Xenon arc instruments overstates the expected difference in UV exposure to failure outdoors. The degree of overstatement is relatively sensitive to black panel and chamber temperatures and to filter combination, is less so to configuration of witness samples, and can be estimated for some cases of practical interest where the degradation process is characterized by an activation energy.