High-density cross-linked polyethylene offers superior value for storing a wide range of critical chemicals. Gel level testing is the ASTM D-1998 method used to test the cross-link density of the polyethylene. The ASTM standard establishes a minimum for the percentage of cross-linked or polymer connected chains. This article and the downloadable technical bulletin will help to explain this standard.
ASTM Standards for Gel Level Test
ASTM provides standards to test both cross-linked and linear polyethylene tanks to ensure the tanks are properly processed. For example, an impact test is conducted at -20 degrees F to ensure the tank will perform as designed and provide the longevity expected. ASTM requires that cross-linked polyethylene have a gel test. The gel level test of the poly samples measures the percentage of cross-linked chains. The ASTM standard is 60% or greater percentage of cross-linked or connected polymer chains.
How The Gel Level Test Is Conducted
First, a cross-section of the plastic is removed from the processed tank. The sample is then carefully weighed. Next, the sample is boiled in xylene. Xylene dissolves the linear polyethylene from the sample section. The sample is then re-weighed. If the resulting matter's weight is greater than 60% cross-linked, ASTM standards are achieved. Whether the gel results are 60% or higher, the cross-linked tank will be equally strong. It is the minimum of 60% that needs to be achieved.
The Myth of Longer Curing
A common misconception is that simply curing or cooking the tank longer leads to higher gel level percentages. The virgin material resins used today are specifically engineered to achieve a predetermined gel level percentage. As a result the final gel level test percentages remain nearly the same and quality standards are met in the processing of the tanks.
A thermocouple system monitors internal air temperatures throughout the cooking process. This ensures complete peroxide consumption during the curing process as well as consistent and correct gel levels.