5.1 Many important properties of crosslinked ethylene plastics vary with the gel content. Hence, determination of the gel content provides a basis for controlling production processes and a means of establishing the quality of finished products.
5.2 Extraction tests permit verification of the proper gel content of any given crosslinked ethylene plastic and they also permit comparison between different crosslinked ethylene plastics, including those containing fillers, provided that, for the latter, the following conditions are met:
5.2.1 The filler is not soluble in the solvent used in this method at the extraction temperature.
5.2.2 The amount of filler present in the compound either is known or can be determined.
5.2.3 Sufficient crosslinking has been achieved to prevent migration of filler during the extraction. It has been found that, at gel content above 308201;%, the solvent remains clear and free of filler.
5.3 Since some oxidative degradation of the material and solvent may occur at the reflux temperature of the solvents, a suitable antioxidant is added to the solvent to inhibit such degradation.
5.4 This test method is normally used for specimens consisting of an equal representation of the entire cross section of the product, but may also be used to examine specific portions of a product for differences in extent of cross-linking when compared to either a product standard or another sample.
5.5 This test method is intended for testing crosslinked polyethylene compounds that are not hygroscopic. If compounds that are hygroscopic are tested using this method, specimen conditioning before and after extraction is required.
5.6 This standard differs from test methods Test Methods D2765, and Test Method D7567 which also describe procedures for determining the gel content of crosslinked polyethylene. It allows for the use of naphthenic blend or isoparaffin solvent as an alternative to xylenes. Xylenes are the only solvent allowed to be used for referee tests. The preferred method of sample preparation in this standard is to use a lathe to create thin ribbons of PEX material. This standard requires the use of a continuous extractor in order to provide consistent results and to allow for reliable solvent re-use. Specialized specimen holders are used to minimize variability resulting from loss of specimen particles.
Note 1: Pressurized extraction techniques have been found to yield useful results in a shorter time frame, however not all grades of PEX tolerate the elevated extraction temperatures without substantial degradation. For this reason pressurized extraction techniques are recommended for control tests only if it is possible to determine that the crosslinked matrix of the PEX does not break down at the temperature of extraction