Metal Finishing Guide Book

2012-2013

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If you wanted to add thinners right up to the limit, you could add 0.55 gal, which is slightly higher than the 0.5 gal that was calculated by trial and error, but it is strongly advised not to go so close to the limit as you can easily overshoot the mark and find yourself with a costly violation. More importantly, since pollution prevention is now the name of the game, it is good practice to keep your addition of thinners to a minimum. PROBLEMS THAT INCLUDE WATER The following calculations will demonstrate how the EPA and states differentiate between solvent-borne and waterborne coatings. If the coating is solely based on organic solvents, there is only one VOC content to report, such as 3.5 lb/gal. If you are dealing with a waterborne coating, such as a latex or water reducible, most EPA or state regulations require that the coating contain less than, e.g., 3.5 lb/gal, less water. What this means is that if you were able to remove the water from the coating, then the new ���hypothetical��� coating would need to have a VOC content less than 3.5 lb/gal. To clarify this, imagine that you have a 1-gal can of waterborne coating, which contains a small amount of VOC, and a large amount of water. The coating is represented by Figure 1a, for which the volume solids is, say 25%. Assume that the ratio of VOC to solids is 1:10 by volume. Now suppose you had another 1-gal can with identically the same solid ingredients (resins, pigments, extenders, and additives), and maintained the VOC to solid ratio at 1:10, such as in Figure 1b. Imagine now that you wanted to paint two identical large walls so that you would deposit exactly 1 mil (0.001 in.) of solid coating onto each. The first wall will be painted using the waterborne coating, and the second with the solvent-borne. In each case, you would stop painting as soon as the entire surface had a coating film of exactly 1-mil dry film thickness. Which of the two coatings will emit more VOC into the air? They will both emit exactly the same amount of VOC, because in each case the amount of solid coating deposited is identical, and in each case the ratio of VOC to solid is the same. For instance, if you were to apply 1 gal of solid of the waterborne coating you would emit 0.1 gal of VOC (ratio of VOC to solid is 1:10). In the case of the solvent-borne coating, you would also need to apply 1 gal of solid coating to achieve the 1 mil thickness, and for the same reason you would emit 0.1 gal of VOC. In other words, regardless of how much water is in the coating, the amount of VOC emitted will solely depend on the ratio of VOC to solid, in this case 1:10. For this reason, the regulations are written such that the coating may not contain more 870

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