Q: I’m having a big problem with my powder coating chipping off my customer’s parts. These are automotive seat assembly brackets.
First, we media blast the parts with aluminum oxide to remove any rust, then wipe clean with acetone. We next apply 2.0-3.0 mils of a zinc-based epoxy powder primer, curing per our supplier’s recommended bake conditions (20 min at 355°F), then topcoat with a standard bake polyester at about 3.0 mils. The polyester is cured per the 20 min at 375°F bake recommended on the technical data sheet provided by the powder manufacturer.
The loss of adhesion occurs between the epoxy primer and the polyester topcoat. Could we be applying the powder too thick? Is the powder undercured? Should we bake the powder longer or at a higher temperature?
A: By applying a two-coat system, you really are giving your customer a “Cadillac” coating that should extend beyond the life of the vehicle. When marrying two of more coatings on a substrate, there are certain procedures that can be used to ensure excellent adhesion and performance.
Let’s go through your process. Your metal preparation appears to be sound. Media blasting can provide an excellent surface for an epoxy primer. It is important to keep your media clean and to thoroughly blast the entire surface. A subsequent acetone wipe is a good idea as well, as long as you use clean, lint-free wipers.
You mentioned that you are using a zinc-rich epoxy primer. This technology can provide outstanding corrosion protection of ferrous substrates. It is important to note that epoxies are incredibly durable and quite hard when completely cured.
Because of this, it is wise to under-bake an epoxy-based powder if it is being used as a primer. By under-curing the epoxy primer, you are leaving a softer surface for the polyester topcoat to “bite” into.
Under-curing also provides a few unreacted functional groups that can be chemically bound by the polyester. These unreacted groups are compatible with the carboxyl functional groups in the polyester and provide a much more intimate bond between the primer and topcoat.
I would recommend under-curing the primer by about 25°F (about 15°C) or for 50% of the recommended time at temperature. I would then apply the polyester topcoat and ensure that it receives the full recommended bake.
It is also important to make sure the topcoat cures at a higher temperature than the primer. Otherwise, you risk the possibility of under-curing the entire coating system.