Determination of the degree of ethylene vinyl acetate crosslinking via Soxhlet extraction: Gold standard or pitfall?

Since the beginning of PV module production, Soxhlet extraction has been the standard method for the determination of the gel content and the resulting calculation of the degree of crosslinking of the most common PV encapsulant ethylene vinyl acetate (EVA). While the method is deemed well establishe...

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Bibliographic Details
Published in:Solar energy materials and solar cells 2015-12, Vol.143, p.494-502
Main Authors: Hirschl, Ch, Neumaier, L., Puchberger, S., Mühleisen, W., Oreski, G., Eder, G.C., Frank, R., Tranitz, M., Schoppa, M., Wendt, M., Bogdanski, N., Plösch, A., Kraft, M.
Format: Article
Language:English
Subjects:
Crosslinking
Degree of crosslinking
Design of experiments
Ethylene vinyl acetate
Ethylene vinyl acetates
Extraction
Mathematical analysis
Photovoltaic cells
Photovoltaics
Round-robin test
Solar cells
Soxhlet extraction
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Summary:Since the beginning of PV module production, Soxhlet extraction has been the standard method for the determination of the gel content and the resulting calculation of the degree of crosslinking of the most common PV encapsulant ethylene vinyl acetate (EVA). While the method is deemed well established and several pertinent ASTM and ISO standards exist, in practise the actual procedures used in – and trusted by – both industry and R&D institutions vary substantially. To evaluate the reliability of the methods and the comparability of the results, a round-robin test involving seven independent European laboratories – one industrial PV module manufacturer and six R&D facilities – was conducted. The measurements were performed using homogenous, anonymized sample sets, each comprising five differently crosslinked EVA foils. The analysis showed that results obtained for the same samples may deviate significantly, but also that very different analytical procedures can yield comparable values. In a systematic study, the impact of various key parameters of the analytical process (extraction time and solvent, drying conditions, sample size and weight etc.) was investigated. Based on these findings, deviations observed in the round-robin study could be linked to their origins and the main pitfalls were identified. In conclusion, a suggestion for an optimised standard procedure was derived to ensure comparable results at all laboratories. •5 Crosslinked EVA foils analysed in 6 laboratories with Soxhlet extraction.•Analysis results differ by up to 50%, especially for weakly crosslinked EVA foils.•Individual results can be explained in accordance with the method applied.•The influence of the various experimental parameters was systematically analysed.•Optimised standard procedure has been proposed.
ISSN:0927-0248
1879-3398
DOI:10.1016/j.solmat.2015.07.043