Upside-Down Thinking by Patric Fransko
by Patric Fransko
June 16th, 2014

Why Do Dyed Window Films Turn Purple?

purple film

Almost everyone, whether in our industry or not, has seen window film that has turned purple from exposure to the sun. Unfortunately, this is often the first thing that comes to mind when non-industry people think about window film. Yet even some in our industry do not know why window film turns purple. So let’s discuss the factors at work that cause dyed films to turn purple and some of the manufacturing strategies used to address this problem. In an effort to keep this article basic, it will address only common automotive window film.

First, think back to what you learned about primary colors and the color spectrum in grade school. The three primary colors are red, blue and yellow. Every color in the spectrum comes from a combination of these three colors. A complete absence of any primary color will give you absolute white, and a combination of equal parts all three primary colors will give you absolute black. As you can see in the picture, purple is a combination of the red and blue primary colors.

The window film you use, whether it is smoke, charcoal, neutral, or anything else, has the color derived from a mixture of these three primary colored dyes. While all three of the primary colors are susceptible to fading from the sun, yellow typically fades at a faster rate than blue or red. As the film is exposed to sun and the yellow dyes fade out at a faster rate than the other primary colors, the mixture of the colors becomes primarily red and blue. As this occurs, the film begins looking more purple—a color you would expect from a mixture of primarily red and blue dyes.chart

It is important to note that ALL dyed window films will fade over time. While I realize that some films have warranties against color change and fading, there is simply no way to stop the fading process in a dyed film completely. Now that we understand what changes a film to appear purple, let’s look at some manufacturing processes that can slow down or address the effects of this fading.

First, the use of UV absorbers in the construction of the film can give dyed film a longer color life-span. UV absorbers will absorb the damaging UV rays and dissipate them in the form of heat. UV absorbers can be added to the adhesive layer, the core polyester film or a combination of both. The higher the quality and concentration of UV absorbers used in a film, the more protection those dyes have against the UV exposure. The more protection the dyes have against the UV exposure, the better the resistance to fading and color change the film will have.

The second thing that can be done to offset the effects of this fading is for the manufacturer to specify a particular mix of dyes designed to fade uniformly so that the film will “fade on color.” In order to keep a film from turning purple, you need to keep the yellow dye from fading out faster than the red and blue. So, care is taken to select a set of red, blue and yellow dyes that are matched for susceptibility to fading so that they will all fade at the same rate.

While the film will still fade over time, it will still retain its original color. So, your 35-percent film may look like a 45-percent film after a period of time, but it will still retain the original film color instead of turning purple. You can often see the effects of this when you look at older film that has a top edge that went inside the window frame or was protected by a sun visor. The portion of the film that was protected by the window frame or the visor retains the original darkness, while the exposed film has faded to a lighter shade of the same color.

This article is a basic one; it does not dive into deep detail about all the things that can play a part in film construction and fade resistance. Use of metalized layers, pigments and other materials in a film’s construction can all have an impact on how a film reacts to the effects of exposure to the sun over time.

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