Significance and Use
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 30 %, 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 test method differs from Test Methods D2765, ISO 10147 and Test Method D7567 which also describe procedures for determining the gel content of crosslinked polyethylene. It allows for the use of naphthenic hydrocarbon blend, isoparaffin solvent, or light aromatic solvent naptha as alternatives to xylenes. Xylenes are the only solvent allowed to be used for referee tests. The preferred method of sample preparation in this test method 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
Scope
1.1 The gel content of pipe and tubing produced from crosslinked polyethylene plastics as described in Specification F876 and other pipe or tubing standards is determined by extracting with solvents such as xylenes. A test method for quantitative determination of gel content is described herein. The method is applicable to PEX pipe and tubing of all densities, including those containing fillers, and provides correction for the inert fillers present in some of those compounds.
1.2 Continuous extraction (see definition in Section 3) is used in this method to test the gel content of crosslinked polyethylene specimens. Continuous extraction when used for testing gel content has the advantages of decreased cost of testing, increased accuracy and consistency of results, and decreased test time. This is because extraction with a pure solvent is more efficient than extraction with a partially saturated solvent.
1.3 While extraction tests may be made on articles of any shape, this test method is applicable for determining the gel content of crosslinked polyethylene pipes and tubing.
1.4 This test method makes use of xylenes or alternative solvents. Alternative solvents either have lower toxicity than xylenes or allow decreased extraction times. The alternative solvents are also potentially beneficial from an economic and environmental viewpoint. Xylenes are used for referee tests.
1.5 The values stated in SI units are to be regarded as standard. The inch-pound units in brackets are for information only.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
