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 p ellets with additives are used in PV module encapsulation that involve s lamination and curing steps. The Elvax 150 films (without additives) contain short alpha,beta-unsaturated carbonyl groups and a photosensi tizer impurity that is also found in the pellets. The yellowing of une xposed, aged EVA films (with additives) is attributed to an interactio n between the UV absorber, Cyasorb UV 531, and Lupersol peroxide curin g agent. A relatively small change in the EVA structure results from t he 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. Dis coloration of the weathering-degraded EVA films is attributed to the f ormation of polyenic chromophores of various lengths and appears to be closely related to the loss of Cyasorb UV 531. Discoloration increase s from light yellow to dark brown as the extent of degradation increas es, which also increasingly reduces the transmission of light. A mixtu re of three to four groups of polyenic chromophores, with some in conj ugation with carbonyl groups, is present in the discolored EVA films t hat are derivatives from the initial chromophores in the cured EVA fil ms. The majority of the polyenes in the yellow-brown EVA films are ded uced to have three to six conjugations and some of the polyconjugation s are longer than that of trans-beta-carotene.