Diffusion of UV Additives in Ethylene-Vinyl Acetate Copolymer Encapsulants and the Impact on Polymer Reliability
Robert Heidrich, Marius Lüdemann, Anton Mordvinkin, Ralph Gottschalg
Abstract
The reliability of solar modules is largely determined by the encapsulation materials used and their inherent additive composition. Although the use of polymer additives generally leads to better encapsulant characteristics, the diffusion of these species must be considered. While UV additives protect the encapsulant from photo degradation, their interactions with other additives or with formed radicals can lead to undesirable effects such as browning. Within this work, the diffusion of UV additives and their impact on the degradation behavior of encapsulation material based on ethylene-vinyl acetate copolymer (EVA) is investigated. The fabricated test specimens (glass/EVA/PTFE/glass) were prepared with an artificially created UV additive concentration gradient and weathered under IEC 62788-7-2-A3 conditions. Test points with included UV additives showed no degradation effects, while test points without UV additives showed strong degradation. Surprisingly, the UV additives used could diffuse up to 4 cm within 1500 h weathering. They were able to effectively protect an area of 1.3 cm next to the concentration gradient. Therefore, minor inhomogeneities during the manufacturing process can probably be compensated by intrinsic UV additive diffusion. However, this simultaneously represents a high risk of reaction with residual crosslinking peroxides leading to the UV stabilizer depletion and eventually to browning.