Metal Finishing Guide Book


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Note These regulations have been written for the sole purpose of reducing air pollution; therefore, you should avoid adding thinners unless it is really required. There is one other critical point that must be borne in mind. When an EPA or state inspector takes a sample, it is sent to an analytical laboratory where the VOC content is determined. In the analytical test, a few drops (approximately 0.3–0.5 g) of the mixed coating are weighed into a small aluminum dish and heated to 230°F for 1 hr, after which the sample is weighed again. The loss of weight between the first and second weighing is due to the loss of VOC from the mixture. At this relatively high temperature, it is possible for some of the coating resin and other ingredients to evaporate. According to the EPA definition of VOC, everything that evaporates, with the exception of water, inorganic compounds, and a few select number of exempt organic compounds, is considered to be VOC. Therefore, even though you calculated the VOC content of the mixture to be 3.47 lb/gal, it is possible that if you subjected a sample of the mixture to the laboratory procedure, the VOC content could be higher than 3.47 lb/gal. In fact, it could exceed the regulatory 3.5 lb/gal limit, which would cause you to be in violation of the regulation. For this reason you should always be cautious when adding thinners to ensure that you stay well below the regulatory limit. In the case of this problem, add less than 1/2 gal of thinners, thereby playing it safe. Solution Using Algebra This method assumes that the amount of thinners added is the unknown "y." Set up the table as before. The result is shown in Table V. VOC = (15.9 + 6.4y)/(5.0+y) Because you are constrained by the regulatory limit of 3.5 lb/gal, you can solve for y. 3.5 = (15.9 + 6.4y)/(5.0+y) y = 0.55 gal 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 815

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