Q: We received a sample panel from our powder supplier for our reference. For trial purposes, I also received a quantity of the same powder from this supplier.
I baked the powder according to the supplier’s technical data sheet (TDS), but I am seeing shade variation when compared to the reference panel from the supplier. What’s the solution for overcoming this problem?
A: When investigating shade variations between different representations (panels or powders) of a powder coating product, you must consider two things: chemistry and process. Is the chemistry inconsistent, or does the process invite variation in color?
Look first at your process. You mentioned that you cured the powder per the TDS instructions provided by the powder supplier. Is your process in control? Your oven may be set at the specified cure temperature; however, you need to verify that the part or test panel that you coated experienced the same cure temperature and time as specified on the TDS.
You should ask your supplier how the sample panel was cured. Was it cured in a laboratory oven? Was this oven electric or gas-fired? Is the oven that you used gas-fired or electric? Electric ovens produce cleaner heat that typically produces a cleaner, brighter, and less yellow color.
Gas ovens can be variable in oven atmosphere, especially if the oven is inadequately exhausted. In addition, a dirty or fouled gas-fired oven can produce a hazy film on the surface of the coating.
If you are satisfied that there is not a significant difference in your oven process and that of your supplier, then you should investigate film thickness as a possible influence on shade difference. Bright yellows, oranges, and reds appear as significantly different colors if applied too thin to achieve opacity.
I recommend that you spray the powder sample at a variety of film thicknesses, ranging from about 30 microns up to 100 microns. Measure color until it is consistent at successively thicker films. Then compare the film thickness of the sample panel to the powder panels you prepared.
If the film thickness investigation does not resolve the shade variation issue, then I suggest you ask your powder coating supplier for sample panels and powder samples from at least four production lots of this product. How large is the shade variation between the lots of powder? Is the difference you’re seeing consistent with the variation in the different lots of your supplier’s powder?
One note regarding the color stability of powder coatings: some powder chemistries are more prone to color variation. Epoxies and hybrids (epoxy-polyester) have the least color stability, while polyesters and polyurethanes have much better inherent color consistency.
If these approaches do not resolve your problem, you should consider seeking an alternate powder coating supplier.