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From the GMA Techline
Subject: Testing — What methods are available to test seams on HDPE and EPDM to ensure quality of joins in the field that should be part of the installation quality assurance? (Masson, New Zealand)
Reply: You ask an interesting question in that HDPE is a thermoplastic polymer that can, and is, thermally welded in the field. The methods are either hot-wedge or extrusion-fillet. Our GRI GM19 Specification covers it, LLDPE and fPP geomembranes. It is on our Web site: www.geosynthetic-institute.org.
EPDM is in the category of a thermoset polymer and cannot be thermally bonded other than vulcanization in the factory. Field seams are made using double-sided tapes supplied by manufacturers. In this regard, you should contact some manufacturers to get their specific information.
It is really difficult to compare them with one another on either the short- or long-term.