Metal Finishing Guide Book

2013

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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 VOC than "x" lb/gal, less water. In the case of a 3.5 lb/gal limit, the regulation would specifically state that the coating may not contain more than 3.5 lb/gal, less water. The following calculations will demonstrate this concept. Example 1: Single-Component Solvent-Borne Coating Plus Water A state regulation limits you to a coating for which the VOC is less than 3.5 lb/ gal, less water. You purchase 1 gal of solvent-borne coating, which contains no water, but has a VOC content of 5.0 lb/gal. To bring down the VOC content you decide to add 1 gal of water. What is the VOC content, less water, of the mixed coating? Would it now meet the regulatory requirement? Regardless of how much water is added, the VOC content remains 5.0lb/gal, less water, and continues to be in violation of the regulation. Example 2: Two-Component Solvent-Borne Coating Plus Water What is the VOC content of the following two-component epoxy? Is it in compli816

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