Damp heat induced physical aging of PV encapsulation materials

Apart from its chemical structure, mechanical properties of polymers depend mainly on polymer morphology, which is determined by the processing conditions. Exposure of the materials at elevated temperatures lead to changes in polymer morphology. The main objective of this study was to investigate the change in thermo-mechanical properties of different encapsulation materials (EVA, Ionomer) after damp heat testing. Therefore the materials were exposed for 1000 h to three different temperature and humidity levels and the thermo-mechanical properties of unaged and aged materials were measured by dynamic mechanical analysis (DMA) and by differential scanning calorimetry (DSC). At temperatures below -20°C, EVA and Ionomer showed high elastic modulus values around 1000 MPa, which decrease significantly to below 10 MPa at temperatures higher than 20°C. All films showed an increase in elastic modulus values over the whole temperature range due to damp heat exposure. Using DSC, recrystallization was found to be the dominant physical aging mechanism. Higher temperature levels during exposure caused greater changes in the polymer morphology, which led to stiffening of the polymer. The significant increase in film stiffness under application relevant temperatures could cause some severe problems during the service life time of a PV module, starting with delamination and a reduced ability to withstand mismatches in thermal expansion and eventually cracking of the cell or the wiring.