Q: We are a multi-substrate powder coater with both batch and conveyorized paint lines. A typical day of powder coating might show we change colors 10-15 times, spraying everything from TGIC, polyester, super-durable, and 70% fluoropolymer, spray to waste. We have about 25-30 stock colors, but we might have somewhere in the neighborhood of over 400 different colors total. We could get an order for a special match color, buy 55 lbs of the color and only spray 30 lbs of it. Then, the leftover powder will sit on a shelf for months/years until the possibility of using it again comes around.
We have sprayed three-, four-, and five-year-old powders with no problems, but we have also run into a few issues recently. Just within the past year, we have started 100% MEK checking our products to ensure full cure. Since then, we have had six jobs where our MEK check showed insufficient cure. We re-bake the parts, but it typically doesn’t improve the MEK cure check. The powder softens quickly and often wipes down to the base metal in 10-15 rubs.
These six jobs have all been lighter colors like whites, beige, light tan, pink, etc. and have all come from boxes of powder over three years old, from multiple vendors. It seems the darker colors, even over five years old, still can be used and will cure out. Do you have any idea what might be causing this?
We have multiple powder suppliers, and we have seen similar results with all vendors. Our powder storage area is not temperature controlled. The powder storage area will see hot southern days of 100°F or more and cold winter months below 30 easily.
What exactly is causing the powder to not cure? Is there some specific constituent in the powder that doesn’t age well to temperature fluctuations and moisture? I realize that all powder has a life expectancy and that we are going way over this usage date, but what other factors should we be paying attention to?
A: These are fascinating issues that you have been observing. First of all, let me commend you for the versatility and breadth of your operation. It takes a high level of discipline and planning to handle so many colors and chemistries in one shop. Kudos to you and your team.
Now to the questions. As you note, powders as old as five years can be successfully used. Some points that are critical to powder coating longevity:
1. Keep your powder dry. You needn’t have to keep the storage area at a certain humidity range, but it is imperative to keep the bag (liner) that contains the powder sealed when storing the powder. Powders are supplied in boxes or barrels that have 3-4 mil plastic liners. These liners essentially isolate the powder from environmental humidity. Opened bags allow the powder to absorb humidity and can cause serious clumping.
2. Keep the powder cool. You mention that your storage area can see southern temperatures of 100°F+. While most powders can endure this exposure, any low-temperature-cure materials will undoubtedly clump and quite possibly pre-react at these elevated temperatures. This pre-reaction will cause an increase in texture that may be unacceptable. Pre-reacted powder can also impair adhesion due to inferior flow and leveling of the powder in the oven.
It is wise for you to check the MEK resistance of your jobs. It is important to note that every powder coating will not provide the same MEK resistance. Polyesters typically soften after 25 double rubs, whereas epoxies and polyurethanes are virtually unaffected with this test. Hybrids fall somewhere in between polyesters and epoxies, depending on how they are formulated.
Your observation that light colors of aged powder are more likely to fail MEK resistance than dark colors is perplexing. There may be a couple reasons for this. First, the original fresh powder may not be very resistant to MEK. If you just recently started to check MEK resistance, you may not have the data regarding the fresh powder. The other reason may be a general lack of cure due to either a higher bake requirement or denser parts/higher load in the oven. Have you checked the actual time and temperature of parts of the various jobs that you run through your ovens? Higher oven loads either due to heavier parts or more parts tax your burners and can keep parts from reaching temperature. It is essential to verify part temperature and time at temperature to ensure full cure.
Regarding the failure after 10-15 MEK double rubs, this is a serious issue. All powders will surpass this test if fully cured. I am somewhat surprised that rebaking parts doesn’t improve this performance. I would spray a part and cure it for a very long time at temperature to verify the poor performance. One long bake may provide better cure than two shorter bakes.
Regarding color differences equating to different levels of cure, this only makes sense when using infrared as a source of heating. Light colors reflect a high percentage of IR and therefore heat more slowly than darker counterparts. Alternatively, dark colors readily absorb infrared and heat up much more quickly than lighter colors and metallics. I can’t think of a reason why light colors would age more quickly than dark colors.
In summary, I think you are on the right track by checking the MEK resistance of all your powder coating jobs. Poor curing may be due to inadequate time at temperature rather than the age of the powder. Carefully check the suppliers’ recommended bake parameters and verify them with actual part temperature measurements. Keep your powder dry by sealing unused containers and keep your powder relatively cool, especially if it is a low-temperature-cure product.