Demand for radiation-curing inks and coatings is increasingly relatively quickly in Europe, particularly in sectors like packaging where UV and EB (electron beam) applications are helping to boost brand awareness.
However, UV and EB’s expansion into printing processes where they have tended to have only limited applications is posing a major technological challenge, not just for ink producers but also equipment manufacturers and makers of chemicals such as surface modifiers.
Once some of these difficulties have been resolved, UV inks are poised to replace water-based products as being both the most environmentally friendly and the most robust.
In some cases, UV inks still have problems and drawbacks like odors, adhesion to substrates, deficiencies at high printing speeds and, above all, costs. These handicaps have been hampering the advance of UV since the technology was first introduced on a commercial scale in the 1970s.
Water-based inks have been regarded as an answer to the restrictions on solvent emissions imposed by the latest European Union legislation on emissions of volatile organic compounds (VOCs). However, these inks have been increasingly criticized by printers for falling below their quality expectations.
Meanwhile, suppliers of inks, equipment and auxiliary products have been working together to tackle some of the inadequacies in UV/EB systems. These have helped to make radiation curing both more efficient and more cost effective.
Radcure technologies still accounts for less than 10 percent of the total printing ink market, mainly because of the difficulties they have had expanding beyond their strongholds in offset and narrow web flexo.
UV and EB inks and coatings account for approximately 65 percent of the total offset inks and coatings sales in the packaging sector in Europe, according to a report on radcure in packaging being published by Pira International, the UK-based consultancy. This is equivalent to about half of the total UV/EB inks and coatings sales in packaging.
With UV flexo printing, 80 percent to 90 percent of consumption is in packaging, a large proportion of which is in narrow web applications for label printing, the Pira report estimates.
However in printing processes like gravure, digital and screen, UV and EB have relatively low shares of ink sales.
Nonetheless, with the help of new technologies, particularly in the area of substrate adhesion, UV/EB is steadily beginning to gain ground in segments where it has previously only had a peripheral role.
Eltex of Germany, a specialist in electrostatic control, has been developing systems to counteract undesirable side-effects of static electricity in printing. These have, for example, improved the efficiency and costs of UV in sheetfed and web offset applications by cutting down the required amounts of photoinitiators and reducing the inhibiting effects of oxygen on the surfaces of substrates.
The Eltex’s innovation is that it requires a lower amount of photoinitiators than a lot of UV systems – around 1 percent to 3 percent, while the content can be as high as 10 percent.
The company has also been extending its Innocure technology to gravure applications. In publishing gravure, it claims Innocure enables the production of UV-varnished magazine covers on gravure printing units rather than on separate heatset offset machines. The UV coating can also be done at speeds typical of those for the printing of cover pages at approximately 420 meters per minute, or 25,000 covers per hour.
Publishing is seen as an area presenting expansion opportunities for radiation curing because of a trend toward glossier magazine and catalogs, especially with covers.
Thomas Litterst, an Eltex sales director, told a recent meeting on packaging organized by the European Rotogravure Association (ERA) that Innocure in packaging and other applications also helped to raise the competitiveness of using UV in inert atmospheric conditions.
“The sort of innovations being developed by Eltex are being welcomed by the gravure sector, particularly when they help extend the use of UV in gravure printing of packaging,” said Ajit Vaidya, an ERA technical coordinator. “Brand owners want printers to use UV on packaging because it helps to create a stronger visual effect.”
In the digital sector, where UV has been making only limited headway, recent technological advances have been helping radiation curing to be more widely employed in ink jet printing.
Sericol has introduced the Uvijet range of UV curable ink jet inks, which can be printed on paper and board and a variety of polymers, including PVC, polycarbonate, polypropylene and polyethylene.
One of the benefits of UV inks in digital printing is that since the inks do not dry until exposed to UV light, there is no risk of their clogging printheads, whose lives are thus prolonged.
Among the disadvantages are delays of up to a day before printers can be certain the curing has been effective. Also, extra ink sometimes has to be fed through the printheads to ensure thorough curing, which can slow down the printing process.
The digital printing market also has the structural handicap which results in the printing equipment manufacturers (OEMs) determining ink formulations. This leaves less room for the development of customized UV and other types of inks.
In screen printing, equipment suppliers have developed mesh and stencil systems which make UV inks more effective. These have enabled UV to provide the fine four-color images required for printing CDs and DVDs.
Improvements of UV-cured screen inks in the CD/DVD segment has helped push radcure’s share of the screen printing market to more than 25 percent, with an annual growth rate of 5 percent to 10 percent.
Ink producers and chemical companies have both been commercializing surface modifiers which allow UV inks to adhere more effectively to substrates. Lack of adhesion can be a big complication with radiation curing inks, particularly with polymers.
Ciba Specialty Chemicals has recently introduced a primer system, called Prime IT, under which a photoinitiator is grafted onto a polymer, increasing the tension on the substrate surface. The photoinitiator reacts during UV light exposure so that a bond is formed between the ink and polymer surface.
SICPA has launched a Dual Bond product comprising an isocyanate pre-polymer which under UV light become an integral part of the substrate with molecular bonding with the ink.
Nanotec srl, Cosenza, Italy, has been developing an aerosol system comprised of gas, solvents, liquids and solids to produce layers of nanometer thickness on substrates like films. Unlike many UV curing methods, the layers also have low shrinkage.
Flat webs, sheets and plates are coated with the aerosol from a nozzle dispenser. The aerosol can also be blown through a rotating stencil mask so that there is only a partial coating of the substrate.
On plastic materials such as polyethylene, polypropylene and PET, a multifunctional polymer can be applied by means of an aerosol to a corona-treated substrate.
Nanotec officials concede that in the packaging sector, more work needs to be done on the aerosol system to enable it to be used more effectively at printing speeds of more than 200 m/min. A lot of printing of packaging is done at speeds of more than 350 m/min.
Currently, the biggest complication with radiation curing is that difficulties like its effectiveness at high printing speeds and adhesion to substrates cannot be solved together.
A possible solution to the lack of adhesion of UV products is to raise the viscosity of radcure inks. However higher viscosity can mean slower printing speeds.
“Inks with a higher molecular weight do not shrink under exposure to UV light, which is one of the main causes of poor adhesion, nor do they tend to emit an odor,” said Georg Bolte of Bolte Consulting, Halle, Germany, who wrote the Pira report on radiation curing.
“The higher molecular inks, which usually comprise acrylic binders, also have a better resistance to high temperatures, which helps the curing process,” he said. “Ink suppliers don’t like to try to market high viscosity ink because printers with the need for faster printing speeds have got so used to low viscosity inks.”
Various auxiliary products available for improving adhesion have their drawbacks. Some, for example, contain solvents which generally do not operate well with UV inks, and could also require printers to invest in solvent abatement equipment to comply with the EU legislation on emissions of VOCs.
The best prospects for expansion in radcure in printing is probably in the medium web flexo segment, where UV could become as dominant as it currently is in the narrow web area.
BHS Druck & Veredelungstechnick, the German-based manufacturer of flexo presses, said that in response to demand, the vast majority of its medium web machines are now supplied with UV facilities. Five years ago the proportion was nearly 10 percent.
“The new medium-web flexo presses have electronically controlled motors,” said Mr. Bolte. “As a result, they have a much better management of temperature levels, which enables the printer to use thin polyethylene film, for example, as well as UV curing.”
For radiation curing to become more widely applied throughout printing, however, much more progress will have to be made in dealing with issues such as adhesion. Water-based inks ironically have similar troubles with adhesion, particularly on polymer substrates.
“UV has many advantages over water-based systems, especially its higher temperature resistance, which make it potentially more effective with the production of laminates,” said Mr. Bolte. “Once these problems with adhesion have been sorted out, radiation curing in printing has a bright future.”