Troubleshooting Color Control

Powder coatings are used to protect and beautify.  Formulators by their nature us mainly on film performance and durability.  It must  be part of their chemistry training.  However, the first thing the customer sees is the appearance of the product.  If the color does not meet a customer's expectations because of inconsistency of variation, then finishing operation has failed.  This article offers an overview on color control and how to troubleshoot variations.

First, let’s cover film thickness. Variation in film thickness is probably the most common source of color discrepancies in a powder coating operation. Opacity (commonly known as “hiding”) is the property responsible. Hiding is achieved by the incorporation of opaque colorant pigments. Each pigment has a characteristic degree of opacity. This is governed by the chemistry of the pigment (inorganic or organic), its refractive index and also the particle size of the pigment. Refractive index describes the ability of a compound to bend light.

Formulators typically incorporate just enough pigment to achieve acceptable hiding at a reasonable film thickness. “Reasonable” translates to somewhat below the expected coating thickness applied to the parts. Smart finishers and their customers express film thickness requirements in a formal specification. For example if a film thickness specification is 2.0 to 3.0 mils (51 to 76 microns) the coating provided should provide adequate hiding at 1.7 mils (43 microns).

If a powder is applied at a thickness below the opacity threshold of the coating light will penetrate the coating and reflect the color of the substrate. Consequently too thin a film will look darker over steel substrate and probably lighter over an aluminum substrate.

Some colors are more sensitive to opacity issues whereas others will almost never cause a problem. In particular bright yellows, oranges and reds are typically the most difficult to achieve acceptable hiding. Two reasons account for this. The bright intensely colored pigments used to create these colors are more transparent than other colorant pigments. In addition these pigments are relatively expensive and therefore formulators tend to use as little as possible to minimize overall product cost.

Worst of all colors for opacity are neon hues. This is because neon “pigments” are not actually pigments at all, they are transparent dyes dissolved onto a resin and ground into fine particles. Hence the need to use a white basecoat under neon colored powders to achieve acceptable hiding.

Colors least susceptible to poor hiding are deep darker colors based on strong tinting pigments. These include vibrant greens, blues and violet colors. Black powder coatings are usually colored with rich carbon black pigment which provides exceptional hiding and are therefore able to be applied at relatively thin coating films. (See Figure 1.)

Next, let’s talk about lighting. Lighting conditions will affect the perception of color. The wavelength of the light source will determine how the light absorbs, reflects and scatters color on the surface of a pigment particle. Under different light some colorant pigmentations create an imperceptible difference whereas other pigments produce a significant color disparity. A significant difference in color caused by different light conditions is described as metamerism.

Metamerism was more common in surface coatings (including powders) when incandescent lamps were popular.

powders) when incandescent lamps were popular.