It is important to point out something at the beginning: all dyes get fainter, so you can’t expect a deep-dyed film not to faint. If the window film is not dyed, this is not an issue: there is nothing that can faint in a metallized film, therefore no colour difference will be present 10 years later, even when measured by a machine. This is not to say, however, that dyed films will always faint – the extent of fainting varies greatly, for the reasons detailed below.
In order to understand the fainting of dyed films, first we have to explore raw materials available and the production technologies applied. The colouring agent can be standard or with a higher light resistance, water-based or solvent-based. Regarding the manufacturing technology, we can speak of dying the adhesive, the laminating adhesive, deep-dying the material and granule colouring.
It is a basic requirement that the colouring agent be solvent-based and lightfast. You can’t expect of cheap films from the Far East that they fulfil even some minimum requirements, while American films (e.g. dyed layers of the SkyFol RS, HP and NT series) indeed do so. There are granule coloured films as well (e.g. the extruded-coloured layers of the NT Pro series), which have the strongest light resistance among coloured films and which practically don’t get fainter.
SkyFol window films will not get purple.
The manufacturing process is very often defined already by the quality of the basic film. The polyester film used has a distinct hole structure when seen through a microscope. In the case of a poor quality material, these holes are of different sizes and scattered, while with high quality films, they have the same size and are situated in a regular way. Structural colouring is not an option with lower quality films, so only their adhesive can be coloured – this is the manufacturing technology for the vast majority of Chinese and Korean films. Most quality window films are deep-dyed in their material with appropriate colouring agents and a good structure of layers; this is one of the most durable technologies nowadays.
The film gets purple if the colouring agent doesn’t get fainter in a uniform way and only certain shades leave the film, while others are retained. Black and bluish shades are always more colour stable, while reddish and yellowish shades get fainter more easily. This is why the film will become purple – these shades remain in the film, while others are fainted completely. A purple fill is always an indicator of a problem with the colouring technology.