Analysis MethodPermeation of water vapour through polyethylene terephthalate (PET) films for back-sheets of photovoltaic modules

Different polyethylene terephthalate films were investigated as to the permeation and diffusity for water vapour and as to structure-property correlations. Water vapour transmission rate, permeability and diffusion coefficient were measured at temperatures 23, 38, 60 and 80 °C using a special test device with a mass spectrometer detection unit. While the permeability was significantly dependent on temperature and specimen thickness, the effect of TiO2 white pigmentation was negligible. Based on the empirical data a model for the temperature and film thickness dependency of the water vapour permeability was established and validated.