I am having a problem with one customer. They use a white powder 75/25 hybrid and manufactures evaporator shelves for coolers made all of steel. However there is a three-foot length copper wire attached to the part (this wire is the heat exchanger) that needs to be coated too.
When the part comes out of the oven the whole shelf looks good and it passes the MEK rubbing test including the copper wire. However if you bend the copper wire a little (less than 20-degree bend), the paint totally cracks and all the paint peels off. When you look into the paint that comes off, you can actually see a brown surface in the bottom of the paint flake.
Why is that? How can we improve the adhesion or cure, if this were the case? How can we improve this problem? We saw that increasing the part temperature doesn’t improve the performance, so we assume this is a not curing problem.
The part goes through a five-step cleaning process: Rinse, Phosphate, Rinse, Organic Seal. According to the clean supplier, the clean is very good. M.D.
There are two possible causes for your problem. The first, and most probable, is overcure of the powder on the copper wire. First of all, copper is a much better conductor of heat than steel (different specific heat value). Add to that the fact that the copper is much thinner (wire versus rod) and what you have is a high likelihood that the powder is overcured. This is further proved by your MEK rub test, which would be positive for overcure as well as proper cure. As you may know, overcured powder can be very brittle and provide poor flexibility results. This would explain the cracking when the copper wire is bent.
Secondarily, and less likely, the copper can have an oxide on the surface causing poor adhesion. The five-step pretreatment system is incapable of removing this oxide, as you have described it. Adding a deoxidizer in the rinse stage can help. Check with your chemical supplier for an appropriate solution.