Degradation of polyester film exposed to accelerated indoor damp heat aging

The aim of this paper is to investigate the effect of indoor accelerated damp heat aging conditions on the degradation behavior of polyester film commonly used in PV backsheets for c-Si PV modules. Combinations of different accelerated aging conditions from 3% RH (dry heat) to 95%RH and temperatures below and above the glass transition temperature (Tg) of polyester film have been investigated. Degradation of polyester film is probed using mechanical tensile testing, intrinsic viscosity measurements as well as acid end group analysis. Results indicate hydrolysis of PET is a major factor leading to degradation of properties in PET under damp heat aging. Additionally, hydrolytic degradation is significantly influenced by the glass transition temperature of PET. Below the Tg minimal hydrolytic degradation is observed even at high relative humidity levels of 95%RH but increasing temperature above the Tg results in significant hydrolytic degradation.