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Part 3: Thermoset Versus Thermoplastic Membranes: What's the Difference?
By Reed Hitchcock
July 2008 -
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Single-ply roofing membranes fall into two classes — thermoset membranes, such as ethylene propylene diene monomer (EPDM) and Hypalon, and thermoplastic membranes, such as thermoplastic polyolefin (TPO) and polyvinyl chloride (PVC).
TPO and PVC are smooth, chemically resistant and water-resistant, and neither is easy to coat. For PVC membranes, high levels of liquid plasticizer seem to present a bigger challenge than their slick surfaces. The chemical resistance of these polymers also inhibits the development of suitable primers, and these systems do require special primers for coating. This is not a common application and is an area of ongoing research.
Managers should note that TPO and PVC membranes are easy to misidentify in the field. Because they require different primers and coatings, examining a test patch is a prudent precaution before coating an unidentified white membrane.
EPDM systems use carbon black to block UV radiation and are made from an inert material similar to tire rubber. Attempts to factory-coat EPDM have not succeeded, but managers have much to gain from coating an EPDM roof system with a white coating.
Hypalon is a chlorosulfonated polyethylene. When Hypalon is 15-20 years old, managers should have it coated directly to extend its useful service life. Applicators can use a white coating, provided the surface is prepared properly with a manufacturer-approved primer. Most manufacturers use a wash-primer.
Making a Match, System by System
Part 1: Roof Coatings: Options Abound for Metal Roofs
Part 2: SPF Roofing Systems Provide Extra Insulation
Part 4: Modified Bitumen and BUR Systems: Roof Coating Strategies