Why Do Dyed Window Films Turn Purple?

Almost everyone, whether in our industry or not, has seen window film that has turned purple from exposure to the sun. Unfortunately, often this is the first thing that comes to mind for many people outside of our industry when they think about window films. However, many people, even many in our industry, do not know is why a window film turns purple. This week I plan to 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 simplified, the following will only address basic dyed window films offered by most manufacturers as an entry-level automotive film.

First, let’s discuss what is happening that changes the film to a purple color. For this, I am going to need all of you to think back to when we all learned about primary colors and the color spectrum in grade school. The three primary colors are red, blue and yellow. All colors in the spectrum come 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 primarily a combination of the red and blue primary colors.

As mentioned above, all colors are derived from a mixture of the three primary colors. The window film you use, whether it is smoke, charcoal, neutral, etc., 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 will fade at a faster rate than blue or red. So, if you are following along, you can guess what happens next. 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 as you would expect from a mixture of primarily red and blue dyes.

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

The first thing that can be done to give a dyed film better longevity is the use of UV absorbers in the construction of the film. 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 that the 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 the specification of 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.

The benefit of designing a film that fades on color is that while the film will still fade overcar-window 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 as shown in this picture. 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 was not meant to go into deep detail about all the things that can play a part in film construction and fade resistance. Use of metalized layers, pigments, etc., 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. However, I hope this article gives you a basic understanding of the fading process that occurs with dyed window films and why some films may react differently than others. I would be happy to answer any clarifying questions you might have in the comments area below.

Credit: Patric Fransko, Window Film Magazine