Impact of UV Light Intensity on Photodegradation of PV Backsheets

Polymeric accelerated weathering tests are often performed by increasing the intensity of the ultraviolet (UV) radiation to shorten the testing time. One critical question is whether the results from high UV intensity tests can be extrapolated to in-service exposure levels. We have investigated the influence of UV intensity on the photodegradation and validity of the reciprocity law for a commercial polyethylene terephthalate (PET) and ethylene-co-vinyl acetate (EVA) based backsheet, called PPE. The free-standing films of PPE were exposed to different UV intensity levels ranging from 60 W/m
²
to 180 W/m
²
(300 nm - 400 nm), at 85 °C/0 % RH and 45 °C/0% RH, respectively. Chemical degradation, yellowing and surface morphological changes were measured as a function of time and UV dose. A mathematical model was developed to describe the degradation kinetics under different UV intensity levels. The results have indicated that the reciprocity law was obeyed for samples aged at 85 °C/0 % RH, but not 45 °C/ 0 % RH.