Effect of global damp heat ageing on debonding of crosslinked EVA- and POE-glass laminates

Polyolefin elastomers (POE) have been established as a promising alternative to commonly used peroxide crosslinked ethylene vinyl acetate copolymer (EVA) photovoltaic encapsulation materials. The main objective of this work was to assess the long-term ageing behavior of double-glass laminates based on UV-transparent EVA and POE under damp heat (85 degrees C, 85 %rh) conditions for up to 10,000h. Ageing induced changes were characterized by microscopy, UV-visible-near infrared (UVVISNIR) spectroscopy, compressive shear testing (CST), differential scanning calorimetry (DSC), macro-and microscopic Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). Within the laminates, the formation of birefringent spheric microdomains (maltese-crosses) was confirmed after an exposure time of more than 2,000h. The compressive shear behavior of EVA laminates was not significantly affected by damp heat ageing. Interestingly, the POE laminates revealed an even improved compressive shear performance. Whereas the tested EVA laminates failed in an interfacial manner independent on damp heat ageing time, POE laminates exhibited a failure mode transition from interfacial (unaged) to combined interfacial and cohesive (10,000h). On the interfacially fractured surfaces of EVA and POE laminates, carboxylic acid groups were ascertained. Deprotonated silanol and water bands were discernible on the opposite glass sides. Damp heat exposure of POE laminates for 10,000h resulted in pronounced glass ageing close to the interface associated with a partly cohesive failure within glass. This phe-nomenon was quite different to EVA and was attributed to the alkaline characteristics of the hindered amine light (HALS) stabilized, less-polar, UV-transparent POE encapsulant.