ASTM D2765 Standard Test Methods for Determination of Gel Content and Swell Ratio of Crosslinked Ethylene Plastics

Get Certified ASTM D2765 Testing for Reliable Crosslinked Ethylene Plastic Verification 

Wire and cable insulation, pipes, and photovoltaic module encapsulants are crosslinked ethylene plastic products that require an appropriate crosslinking level to achieve the expected thermal, mechanical, and chemical resistance. The ASTM D2765 test assesses crosslinked polyethylene to verify that it contains the gel content specified in the standard, based on unique extraction procedures with a specified solvent. Accredited laboratory testing is used to ensure the quality of cable insulation, crosslinked polyethylene pipe verification, and PV encapsulant crosslinking assessment. ASTM D2765 certification confirms that a cross-linking process has the desired effect before entering service in the manufacturing environment. 

The scope of ASTM D2765 includes:

• Material type: Crosslinked ethylene plastics of all densities, compounds filled with fillers.
• Purpose: To determine the gel content and the swell ratio to confirm the crosslinking degree for quality control and comparison of materials. 
• Properties measured: Gel content (Insoluble fraction) and, in Test Method C, the swell ratio of the extracted specimen.
• Method: Solvent extraction is by decahydronaphthalene or xylene; Test Method A (ground sample, referee method), Test Method B (shaved sample, nonreferee method), or Test Method C (single piece extraction at 110°C, nonreferee method).
• Result: Gel content (in percentage) as insoluble material after extraction, swell ratio (when determined). 
• Test Limitations: Test Method B is generally estimated to give extraction values 1-2% lower than the referee method because of larger particle size and particle surface area being less exposed. 
• Environmental conditions: The extraction temperature, solvent type, and method of sample preparation (ground, shaved, or single-piece) influence the gel content results determined. 
• Applications: Quality control of wire and cable insulation, verification of XLPE pipe, crosslinking assessment of EVA encapsulant for PV module, development of crosslinking process. 

What are the Uses of ASTM D2765 Testing?

The ASTM D2765 test is used to test manufacturers to ensure the extent of crosslinking in ethylene plastic products following a standard solvent extraction. Quality control engineers then use these results to verify a crosslinking process is meeting the specification. This standard- 

• Calculates the amount of gel (insoluble content) generated by cross-linking in ethylene plastics.
• Measures swell ratio under Test Method C, as an extra measure of crosslink density.
• Checks the appropriate gel content of a crosslinked ethylene plastic sample for a specific specification.
• Allows comparison of other crosslinked ethylene plastics, such as filled plastics.
• Helps with quality control for manufacture of cable insulation, including use of samples from installed insulation in Test Method B.
• Supports the measurement of the crosslinking degree of PV module encapsulant materials. 

What Materials Can Be Tested Under ASTM D2765? 

ASTM D2765 covers crosslinked ethylene plastics with all densities as well as compounds containing fillers, where the filler is not soluble in either decahydronaphthalene or xylenes at the extraction temperature. This standard applies to crosslinked polyethylene as an insulation material in wire and cable, pipe, and ethylene vinyl acetate encapsulant in photovoltaic modules. An analyst first verifies the availability of the sample form prior to initiating a test program to determine which of the three test methods (A, B, or C) is most applicable. Test Method B requires shavings from selected areas, including cable insulation, rather than the ground sample required by Test Method A, the referee method. 

Why is ASTM D2765 Important? 

ASTM D2765 establishes standard procedures for determining the extent of crosslinking obtained in ethylene plastics by a crosslinking process. The gel content affects many of the important properties of crosslinked ethylene plastics; therefore, it is important to determine it to ensure that a material has the thermal, mechanical, and chemical resistance properties desired for a properly crosslinked product. Extraction tests are used to confirm the proper gel content of any particular crosslinked ethylene plastic and are also used to compare different crosslinked ethylene plastics, as long as the filler material does not dissolve in the extraction solvent. Manufacturers in the wire and cable, pipe, and photovoltaic industries use this standard to check the quality of crosslinking and ensure consistent product performance.

ASTM D2765 Testing Process and Data Collection

Technicians weigh the prepared specimen before extraction, and then extract the specimen under the following conditions: Test Method A is for a ground sample; Test Method B is for shavings taken from selected portions of a cable, including its insulation; and Test Method C is for the entire sample from a single portion of a cable. For Test Method C, the constant temperature is 110°C. Following extraction, the insoluble residue is recovered and dried, and the weight is determined to calculate the amount of gel (as % of original). Corrections are made for any inert filler present. In Test Method C, the swell ratio is also calculated for the extracted specimen. Laboratories collect this information to verify the crosslinking process and support material quality control.

Common Challenges and Troubleshooting

Common issues with ASTM D2765 testing include inconsistent sample preparation, insufficient drying of the residue in the test sample, and interference from the solubility of the filler. These problems can cause the measured gel content to be skewed and make it difficult to compare samples. If a sample preparation method is not suitable for the test method (e.g., shavings for Test Method A instead of a ground sample), the results may not be directly comparable to those obtained by referees. If the recovered residue is not completely dried, the apparent gel content may be elevated. If the extraction solvent is decahydronaphthalene or xylenes, the solvent will be soluble in the filler, which will negate the correction assumptions used in the gel content calculation. These problems are minimized through consistent sample preparation corresponding to the test method selected, careful drying of the residue, and confirmation of the insolubility of the filler. 

• Laboratories report gel content as the percentage of insoluble material remaining after solvent extraction.
• Laboratories record which test method (A, B, or C) they used based on the available sample form.
• Laboratories also measure the swell ratio using Test Method C when measuring crosslink density.
• Analysts typically obtain extraction values from Test Method B that are 1 to 2% lower than those from Test Method A, the referee method, due to differences in sample preparation.
• Laboratories use consistent sample preparation and extraction procedures to achieve reliable and comparable gel content results. The results indicate whether the tested material meets the specification’s required degree of crosslinking.
• Quality control analysts use gel content data to evaluate crosslinking processes for cable insulation, pipes, and encapsulants. 

Link to ASTM D2765