This question has been asked for as long as color inkjet printing has existed. With the first color wide-format inkjet printers introduced in the mid-1990s people tried to use these water-based dye inkjet ink printers to produce prints for outdoor applications. They were very disappointed. Every time a new technology arrives it is one of the first questions asked. The big problem is that there is no single answer other than “it depends.” Here we will describe some of the factors that lead to better image permanence in outdoor applications in some of today’s inkjet prints.
PIGMENTS VS. DYES
The first introduction of color into wide-format inkjet printing utilized organic dyes dissolved in water/co-solvents. The inks printed onto various absorbent substrates, but the dyes tended to fade rapidly. There are many reasons for this. Chemical reactions within the coating on the substrate can result in a bleaching of the dyes. Pollutants like ozone in the air (in even very small quantities) can bleach them as well. Light (particularly UV) can cause fading by decolorizing the dye, rendering them colorless by severing the chemical bonds in the dye molecules that give rise to the color. And the continued penetration of dye into the absorbent layer of the substrate can cause the substrate to “hide the dye” inside.
To overcome these fading mechanisms, several improvements were made. Probably the most important was the development of pigment dispersions for use in inkjet inks replacing dyes. These pigments are very small transparent color particles made of similar organic dye molecules as in dye-based inks. Black pigmented inks are the exception because they are made using carbon black, which is not a dye molecule. In addition to providing a black color, it needs to be opaque and not transparent. If the pigments are the right size—small enough to be transparent but large enough to be lightfast and not penetrate too far into the substrate—they can overcome many of the deficiencies of dyes in outdoor applications. They offer many molecules in a particle, so if several are destroyed by UV light or chemical attack there are redundant molecules left behind to maintain the color of the print.
PROTECTION SYSTEMS
The life of an inkjet print in outdoor applications can be extended by adding a protective layer over the print once produced. Laminating films and liquid laminates can offer protection from pollution, from weather (which can cause erosion of the printed image) and from UV light (by using UV absorbers, which reduce the amount of UV light that encounters the print). In addition, lamination is required if prints are subjected to physical abrasion in the intended use. So this technique can help, but there may be a price to pay in performance.
The color gamut of the print is dependent first on the molecules chosen to give the color and second the reflector under the ink. In general, the whiter the substrate on which the print is made the better the color gamut that can be achieved. To make the substrates white, optical brightening agents are often used. These are dye-like molecules that absorb UV light and reflect it in the visible spectrum. This gives the appearance of a “whiter than white” substrate and thus a very high color gamut because the light passes through the colored pigments twice. If a UV absorbing protective layer (such as a laminate) is applied over the print, this mechanism is stymied and the UV light cannot get to the substrate to be reflected as visible light. So, after the protective layer is applied, the print can appear somewhat duller and perhaps even yellower than prior to the layer’s application. However, it should have better image permanence.
3M and Avery, among others, use inks, media and protective layers in sets for which they provide image permanence information. These sets are a way to be able to make a scientific prediction of the durability of the combination of materials under certain environmental conditions. Since environmental conditions vary widely this is only a guide to what you might experience.