FACTORS THAT AFFECT THE EVA ENCAPSULANT DISCOLORATION RATE UPON ACCELERATED EXPOSURE

Several factors that affect the discoloration rate of the ethylene-vin yl acetate (EVA) copolymer encapsulants used in crystalline-Si photovo ltaic (PV) modules upon accelerated exposure have been investigated pr imarily by employing UV-visible spectrophotometry, spectrocolorimetry, and fluorescence analysis. A variety of film samples including the tw o typical (unprimed) EVA formulations, A9918 and 15295, were studied. The films were laminated, cured, and exposed to either a concentrated 1-kW Xe or an enhanced-UV light source. The results indicate that the extent of EVA discoloration can be affected by factors of two general categories: chemical and physical. In the chemical category, the degra dative factors include (1) EVA formulation, (2) presence and concentra tion of curing-generated, UV-excitable chromophores that depend on the type of curing agent used, (3) loss rate of the UV absorber, Cyasorb UV 531(TM), (4) curing agent and curing conditions, and (5) photobleac hing reactions due to diffusion of air into the laminated films. In th e physical category, the factors involve (6) UV light intensity, (7) U V-filtering effect of glass superstrates, (8) gas permeability of poly meric superstrates, (9) film thickness, and (10) lamination-delaminati on (maybe chemical and/or mechanical effect, too). Photodecomposition of the Cyasorb was first verified in cyclohexane solutions and then in Elvax 150(TM) (EVX) films (the copolymer without any additives and cu ring agent). Cyasorb decomposition rates in cyclohexane solutions are exponentially proportional to the light intensity, but can be greatly reduced by a free-radical scavenger, Tinuvin 770(TM), and furthermore by an antioxidant, Naugard P-TM. The discoloration rate of EVA increas es with Increasing loss of Cyasorb UV 531 and is faster for the EVA A9 918 films that have a greater concentration of UV-excitable chromophor es generated from a slower curing than for the EVA 15295 films that ar e fast cured. In general, the loss rate of the UV absorber and the rat e of discoloration from light yellow to brown follow a sigmoidal patte rn. A reasonably good correlation for changes in transmittance at 420 nm, yellowness index, and fluorescence peak area (or intensity ratio) is obtained as the extent of EVA discoloration progresses. No discolor ation was observed for the laminated EVX films that contain no stabili zers and curing-generated chromophores. The discoloration rate of both types of EVA can be largely reduced by UV-filtering glass superstrate s that remove UV < 320 mn. Photobleaching reactions are responsible fo r the non-discoloration of unlaminated EVA, the visually clear perimet er around the edges of laminated samples, and the EVA films laminated with gas-permeable polymer film superstrates. Delamination of EVA film s from the top glass superstrate was observed after prolonged UV expos ure.