LUMINESCENCE AND ABSORPTION CHARACTERIZATION OF ETHYLENE-VINYL ACETATE ENCAPSULANT FOR PV MODULES BEFORE AND AFTER WEATHERING DEGRADATION

Ethylene-vinyl acetate (EVA) copolymer films extruded from Elvax 150 pellets with additives are used in PV module encapsulation that involves lamination and curing steps. The Elvax 150 films (without additives) contain short alpha,beta-unsaturated carbonyl groups and a photosensitizer impurity that is also found in the pellets. The yellowing of unexposed, aged EVA films (with additives) is attributed to an interaction between the UV absorber, Cyasorb UV 531, and Lupersol peroxide curing agent. A relatively small change in the EVA structure results from the compounding and film extrusion below 120-degrees-C in the presence of additives. A significant structural modification occurs when curing at 140-150-degrees-C as new UV-excitable chromophores are formed. Discoloration of the weathering-degraded EVA films is attributed to the formation of polyenic chromophores of various lengths and appears to be closely related to the loss of Cyasorb UV 531. Discoloration increases from light yellow to dark brown as the extent of degradation increases, which also increasingly reduces the transmission of light. A mixture of three to four groups of polyenic chromophores, with some in conjugation with carbonyl groups, is present in the discolored EVA films that are derivatives from the initial chromophores in the cured EVA films. The majority of the polyenes in the yellow-brown EVA films are deduced to have three to six conjugations and some of the polyconjugations are longer than that of trans-beta-carotene.