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76 JANUARY 2017 COATINGSPROMAG.COM Science Behind It Tobacco Staining Through a White Roof Coating By Eric Thompson, Henr y Restoration Systems Territor y Manager for The Henr y Company T oday, white silicone roof systems are common in the coatings world. W hile silicone roofing materi- als are not new, they are becoming mainstream due to the performance on low-slope roofs, showing resistance to ponding water and ultraviolet (UV) degradation. As in any restoration, though, you are only as good as what you are going over. A main instance of this is if the roof is made of asphalt, most commonly seen as granulated or smooth modified bitumen. ese surfaces require an extra step in the process. ese materials are made with asphalt and solvents and, due to such, will migrate out of the roof material, turning a beautiful white silicone restoration muddy rather quickly. e solution to this problem is priming the asphalt surface prior to applying the silicone topcoat. is will help to maintain the white roof by stopping the asphalt materials from migrating. How It Works Asphaltic oils migrating through the coating is commonly referred to as "tobacco juicing" or "tobacco staining." at is because the migrating materials show up as a brown color that is exhibited on the surface of the silicone coating. To prohibit the movement of the asphaltic oils, the primer has to be able to block or encapsulate them within the cured film. On this project, the contractors used Pro- Grade Elite PGE966 as a primer. T his coating is made of several key components, including water to bind the materials and deliver them to the substrate; poly mer resins of the silicone; Titanium Dioxide for the white pigment; and C18 fatty acids. T he PGE966 cures by chemically cross-linking as opposed to solvent/water evaporation, which is another way some coatings cure. Imagine locking your fingers together tightly, cupping your hands, and placing them under a running faucet. You are now able to hold the water in your hands until you unlock your fingers. T his is similar to how the cross-linking technolog y in the PGE966, along w ith the C18 fatty acids, creates an imper vious barrier, preventing the asphaltic materials from bleeding through the film and into the silicone topcoat. Yet there is still a fine line in the application of the PGE966 as with many epoxies. Applying the primer too thinly to the substrate can still allow for those asphaltic oils to bleed through, as there is not enough material to complete the cross-linking. On the other hand, over application can cause delamination from the substrate or even the silicone topcoat to crack by continuing to cure after the application of the silicone topcoat. erefore, it is always important to follow the manufacturer's published literature regard- ing application. Age Is Just a Number Certainly, the age of the roofing membrane plays a factor in the amount of asphaltic oils that are left to migrate. However, age is not the only factor, and there are granu- lated modified bitumen roofs 10‒15+ years old that are still exhibiting tobacco staining through the silicone coating. is is most likely because the applicator either didn't apply a primer at all or under-applied the primer per the manufacturer's specifications. Unfortunately, adding more of a silicone topcoat will not fix this type of problem, as the asphaltic oils will continue to migrate through the coating. is is one example of how a primer should be used to help stop tobacco juicing. After all, your client paid for a white roof, so shouldn't it stay white? CP