In this review we will discuss some of the important developments in graphics printing over the last several decades that have led to the development of the various inks available today and a look ahead at potential improvements in inks for tomorrow.
Water-Based Inkjet Inks
In the mid-1990s, after many users experienced the disappointing durability of dye-based inks for sign and graphic applications, water-based pigmented inks were introduced by Encad as their Graphic Outdoor inks.
This ushered in a new era in digital inkjet technology. Over the years that followed, nearly all water-based inks introduced for graphics were based on pigment ink technology. First they were offered for thermal inkjet printers largely from Hewlett-Packard followed by piezo printhead-based printers using Epson print heads. By the 2000s, tens of thousands of these printers were producing everything from fine art reproductions to posters and vinyl banners.
Solvent Inkjet Inks
Beginning with low-resolution valvejet and airbrush technologies in the 1980s in billboard applications, solvent inks have been around the longest. These are pigmented inks dispersed with a binder in organic solvents. Initially these were only useful for outdoor signage since residual solvent odor made them unsuitable for indoor applications. With the development of industrial piezo printheads, resolution continued to improve. Today it is competitive with the best water-based ink print quality. Because the solvents initially used had serious odor, chemical hazard, flammability and VOC issues, ink designers went to work to improve on all these issues.
Light- or eco-solvent inks were developed to increase the flashpoint, reducing the explosion hazard, a major safety issue. With that development also came decreased performance in the form of lower gloss and color gamut and poorer adhesion to vinyl—the principle substrate used with solvent inks. Many of these inks still had an odor problem and to improve adhesion some included additives that were health hazards.
Today the odor problem has been overcome to the point that there is little odor from solvent types of ink. This has presented another problem: “If there is no smell, it must not be there.” Some sales people suggest that ventilation is not necessary. This is not the case and solvents should always be ventilated or absorbed by filtration. Health considerations should not be taken lightly. The compromise still is present between the presence of harsh chemicals in the ink and performance. Work is continuing to improve these properties while improving environmental and health performance.
Water-based Latex / Resin
To offer an alternative to solvents and UV-curable inks—and to combat the high cost of coated media and laminates required for these inks in many applications—resin-based versions of water-based pigmented inks were introduced. HP Latex ink technology is notably the most commercially successful implementation of resin ink.
The inks incorporate, in a printer/ink solution, a binder that for many applications allows water-based inks on uncoated media to achieve durability approaching that of printers using solvent or UV curing technology.
Continued improvement in resin ink technology has made it more and more competitive. Besides the fact that this technology is the safest ink technology, it also provides one of the “greenest” solutions. Higher productivity and continually improving performance makes this the primary water-based technology for signage and other applications where durability is paramount. For posters, maps, architectural renderings and the like where speed is most important, new single-pass wide-array printers are coming to market.
Today these printers use either pigment (HP) or dye (Memjet) water-based inks where very high speed and very small drop size is required. These inks are formulated to dry instantly since all drops are printed virtually simultaneously. New additives were required to aid in maintaining nozzle health and drop performance, both major advances. As the reliability of these full-width-array printers continues to improve, more analog printing will go digital. So, water-based technology inks with resin and without resin will continue forward with improvements and a wide range of applications.
UV-Curable Inkjet Inks
As with screen printing, there has been a move in inkjet—particularly in grand-format printing—from solvent to UV curable inks. Liquid UV inks are organic liquids composed of oligomers, monomers, and photoinitiators, which are reactive hazardous chemicals. To jet them, they must be low in viscosity and stable. From a safety standpoint they must be treated with caution because they can cause damage if they come in contact with eyes or skin until they are completely cured. These inks are only used with piezo printhead printers, which can tolerate higher viscosity.
Performance can be variable depending on the substrate being printed and the end use for which the printed material is intended. Inkjet can be printed on anything, but the performance depends on both the ink and the substrate. Adhesion and durability are two of the most obvious concerns. Much research has been done to improve adhesion to a variety of substrates. For grand-format printers, most of the substrates are vinyl-based, so the inks are designed to wet well and bond well to the vinyl structure. On porous materials like paper and mesh, the problem is not the bonding, but curing. If the UV light does not reach all of the ink, it will not cure. This causes durability issues.
Initially the lamps used for UV curing were all arc lamps that emitted a wide spectrum of UV light—and a lot of heat. This made curing heat-sensitive substrates problematic, causing surface damage or deformation of the substrate. These arc lamps also produced free radicals and ozone as a byproduct of the process. These materials are hazardous to humans and must be ventilated or filtered out of the air leaving the printer.
In recent years the development of LED (Light Emitting Diode) UV-cure lamps has improved this situation greatly. First, in screen printing applications, it was used to “pin” the UV ink so that it was immobilized before finishing the curing using an arc lamp. This allowing for control of dot spread and faster printing speeds.
Today LED-based curing technology has been developed in many units to handle the entire curing process. Companies like EFI Vutek have gone one step further by using a nitrogen flush system to eliminate oxygen from the curing process, allowing even more effective curing from LED-based UV-curing systems. Oxygen competes in the curing process with the photoinitiators, reducing the effectiveness of the process. Eliminating it allows more freedom in ink design and faster curing, thus faster printing speeds. Work continues on better, lower-cost LED lamps and inks to enable even higher printing speeds.
UV inks are still not equal in performance to strong solvent inks when it comes to gloss and durability. Improving durability is problematic because to improve abrasion resistance the ink needs to be harder. This works well with rigid board applications where stretching is not a concern, but on flexible substrates the ink may crack or delaminate. Making the ink stretchable also makes it more susceptible to scuffing. So the one-size-fits-all ink is unlikely to give optimum properties on all substrates. For gloss, the ink layer needs to form a smooth film. Solvent inks, like paint, can do this prior to drying; however, UV ink when quickly cured does not. So, work continues in developing durable glossy inks with good adhesion.
Hybrid UV Inkjet Inks
Several companies have recently announced hybrid UV inks. These inks may contain solvent or water as well as the ingredients required for UV curing. The solvent or water is first evaporated before the UV curing takes place. The development of this technology requires the redesign of the UV curing materials to be compatible. The result is a smooth highly glossy yet durable ink layer.
Printer companies like Mimaki and ink companies like Sun Chemical and Fujifilm Specialty Ink Systems have described this technology. Durst last year introduced a new soft signage printer that employs water-based hybrid inks and the newly introduced “Durst Water Technology.” Durst uses a combination of infrared and UV curing in this unique system. These are very promising technologies, and when fully implemented should come the closest yet to solving the remaining deficiencies of UV inkjet ink performance.
Textile Inkjet Inks
One of the fastest growing areas in inkjet is textile printing. With dye-sublimation transfer printing inks provided by Sawgrass and others, print service providers were given a way to color synthetic fabrics and plastic films and coatings with inkjet graphics. This was followed by companies like DuPont and BASF who brought the dye chemistries of the analog textile printing industry to inkjet. All of the chemistries, acid dye, disperse dye, reactive dye as well as pigment printing have been applied to inkjet inks.
Today, textiles of all materials can be printed by inkjet using water-based dye and pigment chemistries. Dye sublimation is probably the most used in graphics printing while the others are primarily used for more traditional textile applications. The move from transfer printing to direct-to-fabric printing has happened rapidly and has become the most popular for textile graphics. While the ability to control drop penetration and dot spread is clearly better with transfer, the simplicity of direct-to-fabric printing for graphics applications is very attractive.
Initially ink suppliers of dye sublimation inks modified printers, especially Mimaki printers, to use their dyes and handle fabrics. Today Epson, Mimaki, Roland and others offer dedicated printers designed for direct-to-fabric dye-sublimation printing of graphics. The chemistry is now available from many more ink companies, and working with the printer manufacturers, they have created a much better solution. This trend will continue and soft signage should continue to grow as an alternative to plastic films and vinyl.
The world of inkjet continues to grow and prosper as companies continue to improve the ink chemistries and come up with new and better ways to achieve the results required in the graphics market.