Determining the degree of crosslinking of ethylene vinyl acetate photovoltaic module encapsulants—A comparative study

A total of 16 analytical methods, spanning from classical solvent extraction over different thermo-analytic and mechanical approaches to acoustic and optical spectroscopy, have been evaluated as to their ability to determine the crosslinking state of ethylene vinyl acetate (EVA), the prevailing encapsulant for photovoltaics applications. The key objective of this work was to create a systematic and comprehensive comparison, using a unified set of traceable test samples covering the full range of realistically occurring degrees of EVA crosslinking. A majority number of these tested methods proved fundamentally suitable for detecting changes in the polymer properties during crosslinking based on the effect e.g. its mechanical properties or its crystallinity. Interestingly, when investigated in detail, most of the methods showed mutually different dependencies on the lamination time, indicating a complex range of effects of the chemical crosslinking on the properties and behaviour of the material. Furthermore, Raman spectroscopy could be identified as a potential new method for measuring the degree of crosslinking in-line in the PV module manufacturing process, thus providing an interesting approach for improving process control in PV module processing.