By V.C. “Bud” Jenkins
When paint fades uniformly, people don’t usually notice. There’s little or no noticeable surface contrast as the fading occurs over time.
Where it does get noticed most is in situations where a poster or picture is taken down and the paint behind it looks bright and colorful, while the rest of the room is “washed out” and weak in color. Or there’s the case outside, where a board has been leaned up beside the house for a year or so and one day it’s removed, showing the original bright color behind it.
Fading is especially noticeable when the roof overhangs the wall a foot or two and the original, bright color is visible under the eaves but is faded on the major part of the wall that the sun hits with full force. Also, on surfaces such as tops of cars parked outside, the paint will be faded more than the sides. Machines left outside will be faded on the sun-exposed side, but when they are turned over, the paint is still bright as new.
From these examples, it’s plain to see that sunlight is the cause. But less obvious—and more complex—are the factors that cause some colors and some paints to fade faster than others.
Color in paint is mainly produced by pigments and dyes that are trapped in the resin (polymer) matrix of the coat of paint. These pigments and dyes reflect certain colors to the eye of the observer, depending on what their molecular makeup happens to be.
Unfortunately, some pigments and dyes are less “stable” over time than others. When some type of chemical activity such as that resulting from sunlight, heat or reactivity occurs, the pigments can lose their ability to maintain their color.
The mechanism of color fade
Chromophores are the part of the pigment (or dye) molecule that causes its specific color to be seen. They work by being in sync with a certain wavelength of the visible spectrum, which results in the rest of the wavelengths being reflected and seen as color. Sunlight can attack these chromophores and change their structure, causing the pigment to lose its color and turn into a transparent, colorless pigment.
Also, some pigments react with chemicals in the environment (oxygen, nitric acid, ozone) to cause fading. If this reaction takes place at the chromophore part of the pigment, the color is changed, or turned colorless, since the chromophore is not present any longer and is not able to absorb light rays in the visible spectrum. For those wanting more information on chromophores, Wikipedia gives a good explanation, with chemical structure and physical mechanisms.
Common pigments that are most susceptible to sunlight fading are the bright yellows, oranges, and reds found in most house paints. But since the fading takes place at the surface of the paint film, it is possible to maintain a bright red color if the only pigment used in the formulation is a bright variety. This is because at the surface, the red pigment will be slowly turning to a colorless state, but the observer will be looking through it to the bright red pigment that remains below.
What causes the most extreme fading is a combination of the bright red (or yellow or orange) with white pigment (typically titanium dioxide) to give the paint better “hiding power.” This mixture is applied to the wall of the house, and the red pigment gradually turns colorless at the surface, causing the white pigment to be the predominate color seen. It’s akin to adding more white pigment to the color in the first place!
This effect can be demonstrated by scratching the faded surface with a knife, revealing the bright red color beneath it. This technique is used by art restorers when they scrape the surface of an old work of art, bringing back the original look of the painting.
Cadmium pigments will lose their chromophores to an acid environment and turn colorless. Houses, structures or signs that face a freeway and are painted with cadmium yellow, for example, will be exposed on the freeway side to nitrates. These nitrates dwell on the paint surface, and the humidity in the air creates a very dilute nitric acid. This can create a dramatic effect when cadmium yellow is mixed with a blue pigment to make a green. When the environment turns the yellow into a clear state, it causes the green color to turn into a blue, since the blue did not react and becomes the predominant color in the mix. Alas, the customer who wanted a sign painted green looks at it later and sees that it has turned blue.
The addition of UV absorbers and light stabilizers to the pigmented paint has not been shown to help reduce fading, since the photon of UV light passes through the uppermost layers of the film before being intercepted. Thus, the damage is done to the surface before these additives can have an effect. A clearcoat over the colored basecoat containing these additives (called HALS/UVA for hindered amine light stabilizers/ultraviolet absorbers) will work better at reducing fading than if the additives are formulated into the pigmented paint, but then the exterior exposure of the clearcoat will come into question. Without pigments to protect the carbon-carbon bonds of the resins, the clearcoat will eventually crack, turn hazy, and flake off.
Some ideas for minimizing fading problems
So, what’s the formulating secret to preventing fading in paint?
First and foremost, it’s advisable to stay away from bright reds, yellows and oranges unless they are made from more stable, automotive-quality type pigments, which can last for 15 to 20 years without fading. Recommended pigments for these colors are yellow iron oxide and red iron oxide as the primary sources. These “inorganic” pigments essentially come from the earth, and are extremely stable and UV resistant.
One slick technique in making a less-fading bright yellow paint is mixing a bright yellow colorant with yellow oxide. That way, when the bright color fades away, the remaining color is mostly yellow and is less likely to be the target of a complaint about fading. If the color still isn’t bright enough for the customer, it helps to use an accent base with just a touch (0.1%) of white or transparent red oxide in it for some protection against sunlight, and use only one pigment for the bright red, yellow or orange. It might be necessary to use a white primer or basecoat to get adequate hiding and uniformity, since these types of pigments don’t hide very well.
Informed consent in paint
Just as in medical procedures, where the patient is asked to give his or her consent to the doctor before undergoing medical treatment, the paint maker or painting professional should provide the customer with information about the dangers of bright colors when painting trim, exterior walls, signs, or even interior walls exposed to bright sunlight.
It might even be a good idea to get something in writing that the risk of using bright colors has been disclosed, that these colors can be susceptible to fading, and that the customer has accepted the risk.
This way, there are no surprises and no angry calls when the fading ensues…only a request to repaint.
