Challenges in Measuring the Degree of Crosslinking of Ethylene Vinyl Acetate
Creators: name: Oreski, G.
name: Rauschenbach, A.
name: Hirschl, C.
name: Neumaier, L.
name: Kraft, M.
name: Pinter, G.
Publisher: name: WIP
Year: 2014
Subjects: OPERATIONS, PERFORMANCE AND RELIABILITY OF PHOTOVOLTAICS (from Cells to Systems)
PV Modules

description: 29th European Photovoltaic Solar Energy Conference and Exhibition; 2457-2461
description: Ethylene vinyl acetate (EVA) is the dominating material for the encapsulation of solar cells. During Photovoltaic (PV) module lamination a three dimensional network is formed by chemical cross-linking of the polymer chains in order to increase the thermal stability of the material and prevent the material from melting within application relevant temperatures up to 100 °C. The crosslinking reaction is the time-determining step of PV module lamination, which is discontinuous and can take up to 30min, depending on the EVA type. A better understanding of the reaction kinetics, especially at the beginning of PV module lamination, could lead to promising approaches for shortening of PV module lamination times but also for optimization of the EVA formulation. Therefore, the main aim of this study is to investigate the cross-linking behavior of EVA during PV module lamination. Three commercially available EVA encapsulation films with different stages of crosslinking were prepared for the investigations. The specimens were then investigated by Soxhlet extraction, Differential Scanning Calorimetry (DSC), Dynamic Mechanical Analysis (DMA) and tensile tests and a corresponding degree of cross-linking was derived for every method. As expected, every method revealed different reaction kinetics as well as varying sensitivities were observed. These differences can be explained by the typical scheme of radical chain reactions. At first, radicals are formed by the decomposition of the peroxides, which can be seen by DSC, followed by the formation of polymer radicals. In the next step, the polymer radicals react with each other or other polymer chains, and long chain branched polymers are formed. Although having increased molecular mass, the long-chain branched polymer molecules are still soluble and therefore cannot be detected by Soxhlet extraction. Also tensile test does not show any significant changes within the first three minutes of PV module lamination. The post-yield behavior of the material is highly influenced by the cross-linking density of the material. Comparable to Soxhlet extraction after 3 minutes a steady increase in stress level at 800 % strain was observed, which indicates the starting formation of a 3-dimensional network. The most sensitive method to describe the initial stages of cross-linking proved to be DMA, where any slight change in the molecular mass can be seen instantly. Already after 5 minutes of cross-linking time no significant changes in the thermo-mechanical properties were observed anymore. Furthermore, the measured modulus curves indicate the elastomeric character of the material, with no significant decrease of modulus after 65°C due to melting of the material. It can be concluded that a sufficient 3-dimensional cross-linking network was formed, even though only about a third of the initial peroxide has reacted up to this time. Currently, a degree of cross-linking higher than 70% obtained from Soxhlet extraction, is used as quality control standard in PV industry. Thermo-mechanical properties of the investigated EVA films demonstrate a sufficient state of cross-linking already after 5 minutes, which corresponds to a Soxhlet value around 50%. Summing up, a significant reduction of lamination time up to 5 minutes seems viable. Nevertheless, the effect of the remaining, still reactive peroxide under service relevant conditions cannot be neglected.