Linking Accelerated Laboratory And Outdoor Exposure Results For Pv Polymeric Materials - A Mechanistic Study Of Eva

Linking accelerated laboratory tests to field performance for predicting the service life of polymeric materials is being investigated at NIST using a reliability-based methodology. Based on this methodology, a successful linkage between the laboratory and field exposure data for a model polymeric material has previously been made. Recently, for the first time, this methodology has been applied to the lifetime assessment of PV polymeric materials. In this paper, a mechanistic study of the degradation of three unstabilized model ethylene vinyl acetate (EVA) systems -uncured EVA, cured EVA and laminated EVA - was carried out under accelerated laboratory exposure and outdoor exposure conditions. The NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) was used for the accelerated laboratory tests, and the outdoor exposure was conducted in Gaithersburg, Maryland. Simultaneous multiple stresses, including temperature, relative humidity and UV radiation, were applied individually or in combination during SPHERE exposure. The effects of the environmental factors on the main degradation mechanisms of different EVA systems were investigated. The results showed that the UV radiation was the most important factor in the degradation of EVA, and that there was synergism between UV radiation and relative humidity. A slower degradation rate was observed for the laminated system as a result of limited diffusion of O-2 and H2O into EVA. It was also found that the substantial chemical changes of the uncured EVA system did not result in yellowing, which was dramatically different from the peroxide cured EVA system. Additionally, the chemical degradation modes of the three EVA systems exposed outdoors appeared to be similar to those exposed to the SPHERE. The implication of this work on the current test standards was discussed.