Determination of Gel Content and Swell Ratio of Crosslinked Ethylene Plastics

The gel content or insoluble fraction produced in ethylene plastics by crosslinking can be determined by extracting with solvents such as decahydronaphthalene or xylenes.This process has been standardized under ASTM D2765-16 and can be carried out by a number of test methods. These are similar but vary principally in the way the sample is prepared prior to extraction.

Test Method A
This permits the most complete extraction, and uses sample that has been ground to between 30 and 60 mesh (0.231-0.5mm)

Test Method B
This is used when cable insulation needs to be tested and uses shavings taken from selected points in the cable insulation. As the test sample pieces are larger, with lower sample area for the pieces, this method ordinarily yields extraction values 1-2% lower than for Test Method A.

Test Method C
This uses samples in single pieces or larger “chunks” and with even lower surface area than for Test Method B the extraction values are typically 3-9% lower than for Test Method A.

This method also allows the measurement of swell ratio – a measure of the degree of cross-linking in the gel phase.

In each case a small mesh packet containing the sample is suspended vertically in the reaction vessel such that it is entirely immersed in the solvent. The sample is then boiled in the solvent, typically for a number of hours, before being reweighed and the Gel Content calculated.