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Degradation of poly(ethylene-terephthalate) under accelerated weathering exposures

Poly(ethylene-terephthalate) is one of the most critical materials in photovoltaic module backsheets due to its dielectric breakdown strength. Unfortunately, PET is susceptible to environmental stressors and subject to degradation over time. Stabilizers are added to increase its service lifetime; however, even these degrade over time and leave the material vulnerable. To study degradation of polyester films, various accelerated weathering exposures are applied to PET grades with different stabilizers. A lab-based experiment was performed utilizing a randomized longitudinal design where samples were assigned to four exposure types and followed over time with repeated measurements in order to determine the effect of common stresses like UV light and humidity on the main polymer and stabilizer additives. It was found that material yellowing is dominant with UV light exposures while moisture mostly causes hazing of the samples. Effects were more severe when light was coupled with moisture. UV-Vis absorbance, infrared, and fluorescence spectroscopy provided valuable insights into the chemical degradation mechanisms such as formation of degradation byproduct, chain scissions, and change in crystallinity under different exposure conditions.