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Ask the Expert Question-and-Answer Archive

by Ron Joseph

December, 2001

Paint Curing and VOC Emissions

Q. I am trying to help a client obtain an air emissions permit for a drying oven. Aerospace components are spray coated in a paint booth and allowed to air dry to tack free condition, usually around 30 minutes. Coated parts are then moved to the drying oven. The air district (SCAQMD) requires that we quantify the emissions from the drying oven. Your methodology works fine for quantifying total facility emissions from coatings, but does not identify how much of that comes from the oven. So my question is how much VOC is left in the coating at various intervals after the part has been coated. Obviously most of the VOC comes off during the initial drying period while the part is still in the paint booth. Everyone agrees on that, but nobody has any empirical data for how much VOC remains in the coating after a specific amount of time. Any help you could provide in the way of hard numbers or addition resources would be greatly appreciated.

A. If I were required to get data on the emissions at various stages of drying I would do the following:

  1. Perform a Method 24 analysis on the coating, but instead of curing the coating in a laboaratory oven, cure them in the actual production oven at the time and temperature that all the workpieces are cured. This test is very easy to perform. I have done it in painting operations numerous times. DO NOT USE MSDS DATA. ONLY USE THE ACTUAL MIXED COATING THAT IS BEING APPLIED IN OTHER WORDS, TAKE IT OUT OF THE PRESSURE POT AT THE TIME OF PAINTING. DO NOT MIX THE COATING YOURSELF IN A LAB.

    The SCAQMD loves statistical data; therefore conduct the test on a sufficient number of sample so that you can get a reasonably good mean and standard deviation.

  2. Apply the coating to several metal panels in exactly the same manner that the workpieces are coated. Insure that you use the same mixed paint, a sample of which was earlier taken from the pressure pot.
  3. Remove a few of the panels at each stage of the curing process and immediately transfer them to the oven. Determine the weight loss for each set of panels. Calculate your statistics.
  4. If you do this right, you will know how much VOC is given off in the oven.
  5. If for any reason you get erroneous results, you might need to hone your procedure some.
Again, I've performed these tests numerous times to determine the transfer efficiency of a paint application, and I know that it works.

Good luck.

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