Painting Viscosity vs. Temperature
One reader has something to add about paint temperature to Carl's response in the December 2012 column regarding spraying high-viscosity paints.
Q. We provide temperature controlling devices for paint application equipment and would like to add to your response to “Spraying High-Viscosity Paints” in the December 2012 issue. M.H. was having difficulty achieving even distribution of paint, resulting in striping on the parts. We agree with your position against solvent additions but think you may have missed the most obvious culprit and the one we deal with daily—paint temperature.
High-viscosity and high-solids coatings generally display a steep viscosity-versus-temperature curve. Therefore, small changes in temperature produce large changes in viscosity. This has a significant impact on atomization and spray pattern, which is probably why M.H. turned to solvent addition to adjust viscosity. In addition to the problems you cited with cost and regulatory requirements, adding solvent reduces the volume solids, requiring a heavier wet film to produce the desired dry film thickness. This heavier wet film and higher solvent content often results in finish defects, including reduced gloss, orange peel, run and sag, blistering, and even solvent pop.
Our company has performed a number of trials that clearly show the impact of temperature versus viscosity on spray patterns for a high-viscosity coating. These trials were performed with a robotic application system so the orifice, pressure, speed, distance, and angle to surface could be held constant, leaving temperature-related viscosity as the only variable. The trials were run at temperatures between 95 and 65°F. For this particular coating, at temperatures above 95°F, the fan pattern was wide, the resulting coating was thin, and print-through from the substrate was obvious. In the 90–95°F range, we saw a nominal 10 percent reduction in fan width, and the coating was smooth from edge-to-edge, providing complete coverage. In the 80–85°F range, we began to see the heavy edges that often result in the striping that M.H. described. These become even more pronounced as we moved down toward 65°F and the spray pattern became much more uneven. It is important to note that these sub-85°F temperatures are going to be the normal operating conditions for most of the country, most of the time. Though every coating behaves uniquely, it is quite likely that paint temperature is the root cause of M.H.’s problem. We have illustrated in our studies that controlling the temperature of coating materials in the spray application process is an excellent means to provide consistency and repeatability. M.B.
A. Thank you for your response, M.B. You are right. In my zeal to have M.H. comply with air quality standards and save money by not adding solvents, I missed the easiest way to correct his spray pattern problem, and that is heating the paint. Furthermore, the addition of an in-line heater would be less expensive than purchasing new spray guns and their attendant equipment.
I always welcome feedback from readers, especially when it provides tidbits that help me answer questions and look smart.
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