Metal Finishing Guide Book


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1, 2 and 3 as shown in Table II. Following the same steps given in the previous example, you simply divide the total number of gallons of the mixture into the total Emissions. Therefore, the VOC of one gallon of the mixture is: VOC = 15.9/5 = 3.18 lb/gal Once again, if someone were to sample the coating, the VOC content would be 3.18 lb for 1 gal of the mixture. Because the VOC content of the mixture is below 3.5 lb/gal, you are allowed to add thinners, if necessary, but under no circumstances are you allowed to exceed the 3.5 lb/gal limit. CALCULATING THE VOC OF A TWO-COMPONENT COATING MIXTURE PLUS THINNERS Suppose you do want to add thinners having a VOC of 6.4 lb/gal to the coating mixture in Table II. How much can you add before exceeding the regulatory limit? There are two methods for making this determination. The first is quite simple and does not require any knowledge of algebra. The second is slightly more difficult, but a whiz for those who still remember their high school math. Here is the simpler of the two methods. Suppose you were to add only one quart (0.25 gal) of thinner, would you exceed the VOC limit? Solution Using Trial and Error The results of the first trial are shown in Table III. Following the same steps as given in the previous examples, VOC = 17.5/5.25 = 3.33 lb/gal Because the VOC content of the mixture is still below 3.5 lb/gal, you can afford to add slightly more, if necessary, but remember that under no circumstances may you exceed the 3.5 lb/gal limit. Repeat the calculations by adding another quart (0.25 gal) of thinners. In recalculating 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. 814

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