- January 31, 2004, Dr. Richard M. Podhajny, Ph.D. Contributing Editor
Jet ink printers dominate the growth of digital printing. One of the most promising applications of jet ink technologies is radiation-curable jet inks used in package and label printing.
Ultraviolet (UV)-curable inks are not new to printers; the technology has been around since the early 1970s. However, its application to jet inks is a recent development.
Combining UV radiation and jet ink technology is a natural extension of the evolution of these two technologies. UV inks require high-energy UV light to change the ink from a liquid to a solid print. One key advantage of UV inks is they keep the jet ink nozzles clear without plugging due to ink drying. UV inks can be cured rapidly under high UV energy and generate low or no volatile organic compounds.
Physical advantages of UV jet inks include improved rub and solvent resistance and high gloss. Ink adhesion can be formulated to match the most demanding of substrates, and UV inks can provide excellent print quality with little or no dot gain.
The application of UV jet inks is well suited for drop-on-demand (DOD) piezo technology. Unlike the bubble DOD process, UV inks are not exposed to high temperature in the printhead. Like other jet inks, UV jet inks must use sub-micron colorants and additives and must perform at relatively low viscosity.
There are different UV jet ink technologies. UV-curable inks can be formulated in either water-based or 100% solid versions that use organic materials as diluents.
The advantage of water-based-formulated UV jet inks is that these formulations produce low-viscosity inks. The inks are formulated with UV-curable resin emulsions and use water as the diluent. However, there are disadvantages. Water-based UV jet inks tend to have undesirable dot gain as well as feathering on some substrates. Poor wetting of film substrates can limit the ink adhesion.
White pigmented UV water-based jet inks have been formulated successfully and used commercially. This is significant since acceptable ink viscosity, low pigment settling, and curing rate were shown to be commercially acceptable. The key to this advance was the ability to stabilize the low-particle-size pigments using appropriate dispersants as well as treated pigments.
UV ink cure can be inhibited by oxygen, and inert gas can be used to overcome this phenomenon. Often nitrogen is used, but other inert gases can be used as well. The advantage of using inert gases is the UV ink achieves higher cure level as well as the through cure.
UV jet inks can be formulated as “hot-melts” to produce low viscosity and can be used commercially on Piezo-type print heads.
Adhesion to substrates such as polyolefin films usually requires a surface treatment. Polyolefins that have been treated to 42 dynes/cm or higher typically will show better ink adhesion. Corona as well as flame treatments produce similar results.
There are still challenges to the successful implementation of UV jet ink technology. Many of these challenges are similar to the hurdles UV inks faced in other printing applications such as lithography, screen, and flexography.
One challenge to UV jet ink growth is the high cost of the UV jet inks compared to conventional jet inks. UV-curable inks are several times the cost of conventional jet inks due to the higher cost of the raw materials. In most other UV printing applications, the cost of UV ink is offset by improved productivity. In the case of UV jet inks, productivity may not be as important as improvements in product resistance.
The early acceptance of conventional UV inks in many market sectors was delayed due to skin irritation or sensitization. Today's UV formulations have come a long way to reduce undesirable characteristics and improve the ink performance.
Experience with commercializing conventional UV inks clearly points to the need for training printers in this technology. Close cooperation among printer, substrate manufacturer, ink supplier, and UV lamp supplier is critical to the successful implementation of UV jet inks.
All in all, UV jet inks are one of the more exciting technologies being developed in packaging applications.
Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 267/695-7717; email@example.com