Metal Finishing Guide Book

2012-2013

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calculating the problem, a total of 1/2 gal of thinners has been added. (See Table IV). VOC = 19.1/5.5 = 3.47\ lb/gal This is as far as you should go. By adding any more thinner to the coating, you will overshoot the 3.5 lb/gal limit. It is important to understand that you should only add thinners if you cannot properly atomize the coating. By adding thinners you might be able to overcome a problem such as orange peel, or excessive film build (dry film thickness), but the trade off is that you are adding to air pollution. 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 869

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