Troubleshooting Color Control
Posted on Tuesday, April 7, 2015
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
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.