Case Study: Roof Coatings Flunk Out
From D+D Online, September 2012
| by Cynthia L. O’Malley |
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Premature coating problems on the standing seam roofs of two school buildings were an eyesore for the community and an issue for the school board that was trying to improve the school’s image....
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Tagged categories:
Adhesion;
ASTM;
Coating failure;
Disbondment;
Failure analysis;
Peeling;
Schools
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Comment from Robert Burke, Burke Painting Company in Wilmington, DE, (9/21/2012, 10:27 AM)
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And here I thought the problem was just poor adhesion of just the top coat due to poor prep. great Job Cynthia!
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Comment from VCBud Jenkins, (9/21/2012, 12:08 PM)
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Epoxy primers are hard to get good adhesion to once they are cured longer than 2 days or so, especially in the hot sun. It could be that the horizontal, sunlit areas were hotter and got a more complete cure, keeping them from giving good adhesion to the topcoat which explains why the vertical areas gave better adhesion. If the topcoat is waterblasted off, I recommend coating with another coat of epoxy primer and topcoating immediately after the epoxy cures to prevent delamination. Or, sand the whole surface and apply a bonding promoter solution.
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Comment from peter gibson, (9/24/2012, 11:23 AM)
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Another example of stupid contractors not having any technical, coatings knowledge.
Why was'nt the mfg asked these questions.Mr Jenkins,the problem was not curing,but UV stability.
Great report from the coatings front - Cynthia.
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Comment from Phil Kabza, (10/22/2012, 9:47 AM)
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I assume in reading the article that the "4-year old roof" consisted of factory-coated metal rather than a field-applied coating, as is typical for PVDF-based applications. If this was a field-applied re-coating project, it would have been identified as such. When specifying the finish system on new metal roofs, we specify that the coater and the coating material manufacturer take responsibility for selection of compatible primers along with proper metal substrate preparation. Given the millions of square feet of factory PVDF-coated metal roofs that have provided excellent service for over 30 years, that approach appears to work. The article does not pinpoint what specifically happened in this instance that differs from normal practice, resulting in this non-typical failure. Was the PVDF formulation deficient? Are there PVDF pigment systems that are not recommended for certain applications? Was the wrong primer formulation selected? Are changes to industry standard specifications called for based upon the investigation? We would benefit from further comment by the author, if available.
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Comment from Cynthia O'Malley, (10/22/2012, 10:29 AM)
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The "4- year old roof" referenced in the article was factory-coated metal. As mentioned in the article, the system included a primer that was not UV resistant and a topcoat that had insufficient UV barrier properties to function as an exterior system in the given environment. The PVDF formulation was not deficient on it's own but as part of this system in this environment the primer and topcoat as a system was insufficient. So yes the wrong primer was selected.
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Comment from William Talbott, (10/23/2012, 5:15 AM)
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Great article! Thank you for sharing.
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Comment from Stephen Burdick, (10/24/2012, 9:34 AM)
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I can't agree completely with the conclusion on this failure. The age of the painted metal? Most, if not all coil applied PVDF's are over polyester urethane primers. Epoxy primer has been out of the picture for a long time. PVDF topcoat, d.f.t is rather high. Most specifications are for 0.7 to 0.9 mils d.f.t.?
UV transmittance thru the PVDF is not the issue. This is a green with pigmentation that should adequately block UV from reaching the primer. I've measured UV in the 300 to 400 nm range using a dual beam UV/VIS spectrophotometer on coil coatings. A color of this nature should have 0% transmittance between 300 and 360 nm and less than 0.1% transmittance in the 360 to 400 nm range. There are still a few questions that the article doesn't answer, but if I were to speculate, I'd be suspect of the cure on the epoxy. Over cured epoxy can/is very difficult to adhered too.
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