Update of quality control tests for new PV encapsulation materials

Although ethylene vinyl acetate copolymer (EVA) is still the dominant PV encapsulant, polyolefins (PO) have gained market share in recent years. Like EVA, PO can be mixed with a cross-linking agent and chemically reacted during the lamination process to form polyolefin elastomers (POE). The cross-linked and therefore insoluble fraction of the encapsulation can be determined by extracting the soluble part of the material with a solvent. The degree of cross-linking, or gel content (GC), is determined from the mass ratio before and after extraction of the samples. The analytical methods for determining the GC value are described for example in IEC 62788-1-6 and the Chinese national standard GB/T29848-2018. We observe a significant difference between the test results of these two standards, with the GC values resulting from the GB standard test being significantly higher. This is particularly true in the low GC range. The main reason for this difference is the extraction time of 5 h specified in the GB standard, which is insufficient to ensure complete extraction of the non-crosslinked polymers at lower GC values. However, for certain POE materials the required minimum GC (50%-65%) is exactly in this range.