Significance and Use
Many important properties of crosslinked ethylene plastics vary with the gel content. Hence, determination of the gel content provides a means of both controlling the process and rating the quality of finished products.
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:
The filler is not soluble in either decahydronaphthalene or xylenes at the extraction temperature.
The amount of filler present in the compound either is known or will be determined by other means.
Sufficient crosslinking has been achieved to prevent migration of filler during the extraction. Usually it has been found that, at extraction levels up to 50 %, the extractant remains clear and free of filler.
A suitable antioxidant is added to the extractant to inhibit possible oxidative degradation at the extraction temperatures.
Before proceeding with this test method, reference shall be made to the specification of the material being tested. Any test specimen preparation, conditioning, dimensions, or testing parameters, or a combination thereof, covered in the materials specification shall take precedence over those mentioned in this test method. If there are no material specifications, then the default conditions apply.
1.1 The gel content (insoluble fraction) produced in ethylene plastics by crosslinking is determined by extracting with solvents such as decahydronaphthalene or xylenes. The methods described herein are applicable to crosslinked ethylene plastics of all densities, including those containing fillers, and all provide corrections for the inert fillers present in some of those compounds.
1.2 Test Method A, which permits most complete extraction in least time, is to be used for referee tests, but two alternative nonreferee Test Methods B and C are also described. Test Method B differs from the referee test method only in sample preparation; that is, it requires use of shavings taken at selected points in cable insulation, for example, rather than the ground sample required by the referee test method. Because the shaved particles are larger, less total surface per sample is exposed to the extractant, so this test method ordinarily yields extraction values about 1 to 2 % lower than the referee method. Test Method C requires that a specimen in one piece be extracted in xylenes at a constant temperature of 110°C. At this temperature and with a one-piece specimen, even less extraction occurs (from 3 to 9 % less than the referee test method), this method permits swell ratio (a measure of the degree of crosslinking in the gel phase) be determined.
1.3 Extraction tests are made on articles of any shape. They have been particularly useful for electrical insulations since specimens may be selected from those portions of the insulation most susceptible to insufficient crosslinking.
1.4 The values stated in SI units are to be regarded as standard. The inch-pound units in brackets are for information only.
Note 1—Method B of this test method is equivalent to ISO 10147.
1.5 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 and health practices and determine the applicability of regulatory limitations prior to use. Specific precautionary statements are given in Sections 6, 10, and 25.
15.1 This modification of the referee test method was developed particularly for wire and cable insulations and differs from the referee test method only in specimen preparation.
15.2 In using this alternative test method, it is recognized that the values obtained at times, are lower than those obtained by referee Test Method A for the reason given in 1.2.
20.1 Degree of crosslinking is determined by measuring the swell and extraction occurring in a solvent that attaches that portion of the polymer which is not crosslinked. Both measurements are obtained in one test. The degree of crosslinking is not expressed as a percent of total crosslinkability or similar expression, but is judged from swell ratio and percent extract based on experience with the particular polymer-solvent system under consideration.
20.2 This test method is applicable to crosslinked ethylene plastics of any density and including those containing fillers, provided that the fillers are insoluble in the extractant.
20.3 In using this alternative test method, it is recognized that the extraction values obtained will be lower than those obtained by either Test Method A or Test Method B for the reasons given in 1.2. However, this test method permits swell ratio to be determined.
2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.
ISO 10147 Pipes and Fittings Made of Crosslinked Polyethylene (Pe-X)--Estimation of the Degree of Crosslinking by Determination of the Gel Content
D297 Test Methods for Rubber Products--Chemical Analysis
D618 Practice for Conditioning Plastics for Testing
D883 Terminology Relating to Plastics
D1603 Test Method for Carbon Black Content in Olefin Plastics
D1998 Specification for Polyethylene Upright Storage Tanks
D3351 Test Method for Gel Count of Plastic Film
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
crosslinked ethylene plastics; gel content; insoluble fraction; swell ratio: Crosslinking; Decahydronaphthalene extraction; Ethylene plastics; Gels (fisheyes) content; Insulated wire and cable; Nonreferee method; Referee methods; Swelling; Xylene;
ICS Number Code 83.080.20 (Thermoplastic materials)
ASTM International is a member of CrossRef.
Citing ASTM Standards
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