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.

 

WHICH INK TO USE?

Water-based pigmented inks, water-based dye inks, water-based latex or resin inks, true solvent, mild-/eco-solvent inks and UV-curable inks—which should I chose? How long will they last? The key here is the system and not the ink type. First, there are the pigments. The ones chosen are selected for different reasons by each vendor. If an ink set is primarily for indoor use, very bright pigments are selected since “color sells.” The pigments might not be the most lightfast, but that might not be needed if the prints are not expected to be exposed to intense UV light. 

 

When used outdoors in demanding applications prints will show UV fading irrespective of whether they are water-based, solvent based or UV-cured. If they are protected by a protective layer with UV absorbers they will last longer. Yellow is the most difficult to protect, and bright yellows are not very light fast. If you want lightfast yellow then pick a greenish yellow with less brightness so you have much better lightfastness. 

 

The tradeoff is in the reds. They are not as good with lightfast yellows. Second is the substrate/binder combination and how it bonds together. There is very little binder with traditional water-based inks. There is just enough to hold the pigments to the coating on the media. The media must provide the durability. For more than just short-term outdoor applications, a protective layer is required. 

 

With Latex/resin, UV-curable and solvent and mild-solvent inks there is significant binder present. With UV-curable you have the most binder since all of the fluid becomes part of the cured image. One might think that because there is more binder with UV-curable inks that they will be more durable. Not necessarily. 

 

Along with the binder, one must consider the adhesion of the binder/pigment combination to the substrate. A strong durable binder is no good if it doesn’t adhere to the substrate. 

Perhaps the best adhesion is with strong solvent inks. They have ingredients that smell bad, are not very people or environmentally friendly and need to be ventilated in your shop—but they bond really well, especially to vinyl. In addition, their binder is very durable. They were initially designed for vinyl billboard applications so were never intended for indoor use. 

 

Mild solvents/eco-solvents are a compromise and generally do not perform as well. To make them people-friendly, they trade-off the aggressive solvent components for less aggressive solvents. UV inks frequently suffer from incomplete curing. If this occurs, then adhesion and outdoor durability will suffer. Black and yellow pigments absorb UV light very effectively and make it difficult to get a complete cure unless the printer is run slowly. With UV inks it is sometimes difficult to tell if the cure is complete so it can be a mixed bag.

 

SUMMARY

So my conclusion to how long will they last? Well, it just depends! There is no magic answer here. Only by arming yourself with the knowledge of what factors affect the durability of the print and the knowledge of how it is expected to be used can you determine what to expect from your print. Don’t be afraid to use manufacturer type warrantees as a guide on what combinations of materials will provide you with the solution your customer needs.