We have 30-plus steel light poles on Main Street with a silicone coating. We want to blast the coatings off and have them powder coated. Before we do this, do you know of a field test we could do to determine if there is silicone contamination on the surface prior to powder coating?
The presence of silicone on a surface can usually be detected using analytical methods such as FT-IR and EDX spectroscopy, but the sample size is usually a challenge. The test requires that the surface be mounted on a tiny jig that fits into the analytical instrument.
I recommend that you completely strip the light poles mechanically and then clean the surface vigorously with a decent solvent (I suggest acetone). I would then coat one light pole and inspect the finished product before coating the rest of the poles. By following these steps, you should be able to successfully refinish the poles.
Is it possible to powder coat plastics? If so, what are the limitations? I have a part that I am trying to have molded, but getting the different colors I need will mean a separate molding operation, more assembly, more inventory, etc. My hope is to have this part molded from polyethylene, and then do either a coating or some other process. Any help would be appreciated.
Plastics can be powder coated. However, the plastic must be able to withstand the powder bake temperature without distorting. Materials such as sheet molding compound (SMC), polyamide, polycarbonate, polyetheretherketone (PEEK) and most engineered plastics can take the heat. Polyethylene (PE) comes in two generic types: low density (LDPE) and high density (HDPE). The softening and melting points of both are too low for powder. In addition PE is notorious for causing paint adhesion problems. You may want to consider a two-component or one-pack urethane with careful surface preparation. Some urethanes are specially formulated for plastic.
For plastics that can be powder coated, you must either make the plastic conductive to apply the powder electrostatically (tribo-charging works well), or preheat the part and apply the powder while the part is still hot.
Not all powders work well. Low-temperature-cure powders are recommended. There are epoxies, polyesters and polyurethane products that cure at temperatures between 250 and 275°F. If you want to complicate your life and can tolerate the addition processes and cost, you may want to consider ultraviolet (UV) curable powders. These powders are applied, heated to melt and flow, and then exposed to high-intensity UV light. It’s a complex process, but it works well for simple part geometries such as boards and cylindrical parts. The advantage is the powder does not require high temperatures or long dwell times to cure.
We powder coat our aluminum brake calipers either red (using a polyester urethane) or black (using a polyester TGIC). These components frequently see temperatures of around 300°F and are subjected to hot DOT 3 brake fluid. We perform a “hot pot” test, where we submerge the calipers in hot DOT 3 brake fluid at 250°F for two hours, remove them and let them cool, wiping off excess brake fluid, and then bake them at 375°F for 20 minutes. After that time the red is still rock hard, but the black is soft and tacky. What would cause that?
--Craig Steffen, Wilwood Engineering
Brake fluid is one of the most aggressive solvents a coating can face. Your red polyester urethane has more chemical resistance than the black polyester TGIC. It’s all a matter of what we call crosslink density - how close together the chemical links are spaced in a cured powder coating. In most cases, a polyester urethane will possess higher crosslink density than a polyester TGIC when fully cured.
By the way, the best performance for brake fluid resistance is usually achieved with a good epoxy powder.
Ask Joe Powder is a regular feature of Finishing Today magazine. Please send your questions to firstname.lastname@example.org.
FINISHING ANSWERS: Ask Joe Powder
August 1, 2007