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A Primer on UV-Curable Inkjet Inks
By Jeff Burton, Digital Printing Analyst, SGIA
Digital printing today is the de facto method for short-run, wide- and grand-format or variable-data print production. Technologies driving these applications include toner (electro-photography) and inkjet-based printing systems. Major trans-promotional marketing players generally utilize toner-based systems and include such names as Xerox IGen3, Kodak Nexpress, HP and HP Indigo, Océ, Konica-Minolta, Ricoh, Kyocera and Xeikon.
In the inkjet arena of narrow format, there is Screen (Dainippon), HP, Kodak Versamark and Agfa Dotrix. All are steadily improving in speed and quality. These systems share the common substrate of paper, as opposed to large-format — .9-m to 2.5-m (36-inch to 100-inch) width — or grand-format inkjet printing (more than 2.5-m or 100-inch width) that uses predominantly vinyl or other petroleum-based substrates.
Wide- and grand-format printing applications have flourished because of the flexible nature of inkjet technology and adaptability of inkjet chemistries to various substrates. Ink research and development for inkjet printing has never been greater, with a migration from the original water-based dye and pigment inks to the presently popular mid- and high-solvent and UV-curable inkjet ink types. Driving the shift to UV is its ability to almost immediately change the ink state from a liquid to an extremely resilient hard film.
The capability to control this change only when the printer needs it to happen makes the ink appealing to anyone occupied in the use or formulation of inks. Within the last two years, there has been a incredible rush toward the use of UV-curable inks in most of the printing markets. This increase has been driven by new developments in ink technology and demand from printer manufacturers and end users.
UV-curable ink technology has been in commercial use by printers since the 1970s. UV-curable inks, which were already being used extensively in the screen printing field, are replacing much of the solvent inkjet ink currently in use. Solvent- and water-based inks dry by evaporation and upwards of 80 to 90 percent of those ingredients go into the atmosphere as vapors during the printing process.
UV inks do not evaporate, and you actually use less ink per square printing unit. UV inks cure, or set, by chemical reaction from exposure to ultraviolet light. Ultraviolet light is electro-magnetic radiation, situated between 200 and 380 nm of the light spectrum. The advantages of UV-curable inks overshadow any disadvantages, marking them as the dominant ink system for the future of industrial inkjet printing.
Sales of UV inks have been given considerable momentum by new European Union legislation on the emissions of volatile organic compounds (VOC). This came fully into effect in October 2007. The regulations aimed to cut VOC levels to 50 percent of those of 1990. UV-curable inks are on par with solvent-based inks for the majority of applications. The significant uniqueness of UV-based inks is seen in a couple of areas. The first is that it has no VOCs, offering benefits to worker safety and the environment.
Second, UV-curable inks can be printed at higher speeds in both narrow- and wide-format sizes without the need for cumbersome drying systems. Lastly, the flexibility now being seen in the UV ink formulations allows roll-to-roll capability, opening up fleet graphics applications in the areas of wide- and grand-format printing.
Coupled with that flexibility is the wider range of rigid substrates that these inks have superior adhesion to. Compared to solvent inks, UV does not dry up in the inkjet head and exhibits a lower rate of nozzle failure caused by blockage. But there are downsides, including UV-light exposure hazards and possible sensitization issues related to the handling of uncured UV ink. Let’s go over some of the verbiage associated with UV inks, and how the UV curable process works.
UV Ink Components and Functions
The types of photoinitiators most commonly used in inkjet inks have been of the free-radical nature. Ink formulators work with photoinitiator suppliers to develop inks that are compatible with the UV output of medium-pressure, mercury-vapor bulbs found in most curing systems for inkjet printing. Disadvantages from this type of light source include excessive substrate heating, high power consumption and the need for scheduled lamp replacement.
Stabilizers are used to help with the ink’s shelf-life and increase the tolerance to heat, which is important at higher jetting temperatures. Simplistically, stabilizers neutralize or absorb reactive molecules in the ink during storage and prevent polymerization.
Free-Radical UV Ink
Free-radical UV inks use an acrylate or urethane resin, which polymerizes rapidly when exposed to UV radiation. This stimulates the inks’ photo-initiator chemicals. These initiators then emit a distinctive odor. They can cure rapidly when exposed to UV-light radiation.
The curing is reduced by the presence of oxygen and ceases in the absence of UV radiation. Wide-format, inkjet printing systems rely on intense UV exposure to cure the inks. There are limitations to free-radical UV inks, which include oxygen inhibition, poor adhesion to difficult substrates and residual odor.
Free-Radical, Cure-UV Lamps
This heat issue limits the kind of substrates that can be utilized with this particular ink/lamp system. UV curing lamps age, and with subsequent changes in output, they may reach a point in which inadequate energy is emitted to fully cure the ink layer. If the ink layer is not cured completely, the ink will not reach its intended hardness potential.
A UV lamp’s peak intensity will have a drop off in performance that is related to the bulb type, usage and duty cycle of the printer. Dust and ink residues on lamp windows and reflectors also can have an impact on the lamp unit’s output, requiring periodic maintenance to prevent any reduction in cure efficiency.
Adhesion also is variable with substrate and UV-exposure duration. UV-curable inks are essentially on top of the substrate (unlike solvents that etch into the substrate), and the interface between the ink and the substrate is not as strong as solvent inks. Moreover, the effects of exposure time to UV radiation are cumulative: If an ink is over-cured, it can become fragile and flake off. Brittleness depends on the media type and ink color.
Even though the UV-curing process is fundamentally an immediate process — inks are certainly “dry to the touch” right after printing — some post-curing does take place. This may lead to a delay of a few hours until the full resistance properties are achieved.
The curing process is best described as the following: The protons, generated from the photoinitiator during UV exposure, continue to be active after exposure. Cross-linking continues after exposure to UV light. That means a coating does not need to be fully cured after it leaves the light source, just dry to the touch.
The cure rate depends on the ink’s formulation, light source used and initiator contents. Experience has shown full-ink film characteristics are usually developed within 24 hours. Cationic inks are sensitive chemistries, and can react with the bases and acids present in inkjet media, resulting in poor adhesion.
These inks typically have better adhesion to the traditional “difficult” substrates, like expanded plastics and glass. The excellent adhesion to difficult substrates is related to the ink’s reduction in shrinkage (due to heat), which is roughly one-third of that experienced by free-radical UV ink systems. This feature enables UV inkjet systems to be used on substrates that, until now, had been considered unsuitable, including vehicle graphics, uncoated glass materials and other slick or heat-sensitive substrates.
Cationic Cure Lamps
The arrival of digital printing has completely revolutionized printing as we know it. As with all new technologies, UV is not a cure-all. Stay tuned as HP brings on water-based latex inks for outdoor printing and a .9-m (30-inch) wide, web-fed inkjet for high-volume production of books, transactional/trans-promotional mail and direct-marketing materials.
Jeff Burton, Digital Printing Analyst, has been with SGIA since 1998. He provides solutions to digital imaging production, computer and workflow issues as well as digital equipment/vendor recommendations. His extensive background in digital imaging, electronic pre-press for print, professional photography and computers, serves members by supplying individualized solutions to their daily business problems. Burton received his Bachelor of Science in Photographic Science from the Rochester Institute of Technology. email@example.com
This article appeared in the SGIA Journal, 2nd Quarter 2008 Issue and is reprinted with permission. Copyright 2008 Specialty Graphic Imaging Association (www.sgia.org). All Rights Reserved.
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