Q: We powder coat welded components in a high-gloss red TGIC polyester that consist of a box made from 11-ga. metal and has section of ¼-in. wall pipe welded to it with a 1-in.-thick flange at the end of the pipe. We are having trouble that, after cure and as the part begins to cool, a haze will develop on the thick flange.
The process for these parts is grit blast, eight-stage iron phosphate pre-treat with a DIP sealer, TGIC polyester powder, cured at 375°F for 23 min.
The haze can be wiped off, and the powder is glossy underneath. We also noticed it on some other parts as well (all were high-gloss TGIC powders and mostly on heavier surfaces). Can you explain what is causing the haze and how to prevent it?
A: What you are observing is a textbook phenomenon known as blooming. This occurs with some polyester TGIC formulas when the coating is exposed to an elevated (but not necessarily high) temperature for a long period of time. The parts you describe are relatively thick gauge, which will take a long period to cool from baking temperature to room temperature.
It is this long cool down that allows the polyester resin to exude a cyclic oligomer comprising the main two monomers used to make polyester resins for powder coatings. This phenomenon is very common with virtually all standard-grade TGIC polyester powder coatings.
Eliminating the root cause of this defect can take two paths. One would be to eliminate the long cool down period of the parts exiting your oven. This could involve quenching the parts with water or a high-velocity cooling tunnel. Neither of these options would be very economical.
The best approach would be to contact your powder coating supplier and have them provide a non-blooming polyester powder coating. This technology is well-known by the better formulators. If your powder supplier doesn’t have non-blooming grades of polyester powder coatings, it may be time to solicit alternate suppliers.