Evaluation of the aging behavior of ethylene copolymer films for solar applications under accelerated weathering conditions

Ethylene copolymers based on acrylic acids and acrylates are an interesting alternative to ethylene(vinylacetate) (EVA) for photovoltaic (PV) encapsulation. These materials provide similar or better mechanical and optical properties and a slightly better aging behavior, but without the formation of corrosive acetic acid during aging, which is particularly of importance in PV applications. The focus of the research work was to evaluate and screen the aging behavior of ethylene copolymers containing different types of comonomers for solar applications. To investigate the intrinsic weathering behavior of the materials, unstabilized films with comonomer contents around 10% were exposed to temperature, humidity and solar radiation. Special attention was given to the optical and mechanical properties. All investigated films showed high transparency in the solar range with hemispheric transmittance values above 91%. Regarding mechanical properties, the ethylene copolymer films exhibited a highly ductile behavior and high flexibility. Similar degradation behavior could be observed for all investigated films. Due to formation of chromophoric degradation products, yellowing could be observed and hemispheric transmittance values dropped slightly to values between 88.5 and 90.5%. The unstabilized films showed significant embrittlement due to weathering. After 750 h of weathering both strain-at-break and stress-at-break values of all ethylene copolymer films dropped significantly below 50% of the initial values.