Loss of Gloss Over Aluminum Bonnets

Q. I am the paint line manager of a company situated in the Middle East. The temperature here gets up to 55°C (131°F). We paint parts for motor vehicles. We have a problem with loss of gloss on painted parts. Why do we encounter this problem of loss of gloss when using waterborne basecoat system on aluminum bonnets? Thanking you and waiting for your response. L. C.

A. According to my engineer’s handbook, 55°C translates to 131°F, which is hot on any scale. Incidentally, in the United States, we call an automobile bonnet a hood. Your problem can best be solved by the paint supplier or his formulator. When a paint film is applied to a substrate, a number of thing happen. One of these is flow. A formulated paint has additives that, among other things, control flow and therefore, gloss. As you would expect, a Newtonian fluid decreases in viscosity and has increased flow at elevated temperatures. However, most paints are Non-Newtonian because they contain flow control additives that limit sagging. At elevated temperatures, solvents evaporate faster and paints dry faster and don’t have enough open time to flow and develop their intended gloss. I suggest you take up the problem with your paint supplier. He can reformulate it to have the correct amount of open time and flow at elevated temperatures to give you the desired gloss.