In sectors such as flexible food packaging, growth of UV is now being held back by renewed fears about the safety of radiation-cured inks.
Although ink producers have made a lot of progress in recent years in closing the cost gap between UV and solventborne inks, the price of curing inks is still deterring some printers from investing in radcure equipment. Meanwhile, UV ink prices are still as much as 50 percent higher than those for solvent inks.
The higher prices for UV inks does, nonetheless, compensate for the relatively low level of their volume sales. In some segments, ink producers could be facing a dilemma of how much to sacrifice margins in order to bolster sales of UV products.
Sales of UV inks have been given considerable impetus by new European Union legislation on emissions of volatile organic compounds (VOC), which is due to come fully into effect in October 2007. The regulations aim to cut VOC levels to 50 percent of those of 1990.
Nonetheless, printers are still finding inks with low solvent content or waterborne inks a more attractive alternative to radiation curing.
“More has to be done to make UV inks less expensive,” said Georg Bolte, a consultant in UV printing at Halle, Germany. “Also, ink producers have still not managed to eliminate the unpleasant odors from UV inks, which continues to be a major complaint among printers.
“Another major challenge is achieving the right balance between the thickness of ink layers in some printing processes and UV penetration to obtain adequate curing,” he continued. “A lot of improvements could be made if UV inks could be applied to substrates with thinner layers and more appropriate viscosity levels.”
In Europe, offset and screen printing still account for the vast majority of sales in UV inks. They have big shares of niche segments like screen printing of DVDs and offset printing of certain plastic materials.
In flexographic printing, UV has been making inroads into the labels sector. But in the flexo market as a whole, radcure still continues to struggle to have a major impact.
“The main problem facing UV inks in the mainstream flexo printing is still the same as it was 20 years ago,” said Mr. Bolte. “The thickness of the ink layers required in flexo make UV inks too expensive for the printers. One solution would be to raise the viscosity of the inks but at the same time to reduce the thickness of the ink layers and increase the concentration of pigments. This is a big challenge for ink producers, but could be achievable with a step-by-step approach.”
Opaque whites are still a predicament for users of UV inks in some applications. Since whites account for approximately 40 percent of the total volume of printing inks, they can be a significant barrier to the effectiveness of radiation curing because of their inherent reflectivity.
For ink formulators, the dilemma is how to adjust the opacity of the whites without making the viscosity inadequate and undermining the adhesion of the ink to the substrate.
Inks with cationic curing systems can be more efficient in dealing with higher opacity levels than conventional types of UV inks because the radiation does not need to penetrate the entire ink layer.
However, ink makers are still striving to eliminate difficulties with cationic systems. Without these complications, cationic inks could be considered to be superior to the traditional and dominant free-radical UV-curing technology. The advantage of the cationic system is that the energy has only to penetrate the surface of the ink, after which it triggers a reaction which results in the cross-linking of its polymeric components through to the substrate.
The curing of cationic inks takes place gradually – sometimes longer than 24 hours after UV exposure. The slow cure rate ensures that the ink adheres well to the substrate, which at the same time is far less likely to shrink.
“The big drawback with cationic inks is their high cost, which seems to have held back the availability and development of more effective components for the inks,” said a technical manager at one ink company. “For the moment they will continue to remain a niche product, particularly for applications where good adhesion is required and opacity is high.”
Ink producers have been making progress in tackling these challenges to enable radiation curing to gain a much bigger share of some key segments.
With label printing, for example, UV inks have now established themselves as a mainstream technology which can help to strengthen the visual impact of packaging.
“We are constantly innovating new solutions addressing all the technological challenges in UV inks as its inherent performance and added value is very appreciated in the entrepreneurial market of narrow web,” said Niklas Olsson, global brand manager of XSYS Print Solutions, part of Flint Group. “We have recently shown, together with one of the big UV flexo press producers in this field, free-radical UV inks running at 1,000 feet per minute on 20 micron substrate achieving adequate cure rate and adhesion. We’re continuing to develop UV flexo technology for shrink sleeves and rotary silk screen as well as looking at the possibility of developing UV inks for label printing by gravure, even if there are some fundamental technical and commercial aspects we need to solve before we know if it is feasible.”
In screen printing, UV has become the first choice in many applications due to a series of improvements in raw materials, ink formulations and in the printing machines. One of the most influential changes in the European screen printing sector has been the introduction of water into UV inks.
This combination of water and high solid content prevents the buildup of substantial amounts of ink which could slow down the printing process. Printers have taken time to master the hybrid system while retaining quality standards and ensuring effective curing.
Among raw material suppliers, a recent innovation has been the use of dendrimers or hyperbranched aliphatic polymers to improve the flow properties of UV formulations. They enable high-solid UV inks to be converted to a stable emulsion while still achieving relatively rapid curing.
Much has also been done in other ways to minimize exposure times to UV radiation so that heat intensity is curbed. This has enabled the greater use of temperature-sensitive substrates like plastic in UV applications.
Electron beam (EB) curing systems have the advantage of using electrons rather than UV radiation to start the curing process. They are able to cause crosslinking in much thicker and opaque ink films as they have much more penetrative power than UV rays.
Despite considerable reductions in their costs in recent years, EB machines still remain more expensive than UV equipment – as much as 20 times more in some cases.
An emerging alternative to conventional UV lamps is light emitting diodes (LEDs). Among the benefits of LEDs are an instantaneous UV output without the need for warming up the equipment and a long-service life. However, LED equipment is expensive, although the cost will almost certainly come down as the technology is more widely used.
A big advantage of EB curing is that it does not require photo initiators because the electrons deliver sufficient energy to start the crosslinking reaction. Companies such as Ashland Specialty Chemical have been developing UV resins which initiate the cross-linking reaction themselves without the need for a chemical additive.
In Europe, photoinitiators are becoming an issue for the future of UV inks following a health scare last year over isopropylthioxanthane (ITX), a photoinitiator, traces of which were found by a laboratory in Italy in infant milk products made by Nestle. The chemical had migrated into the milk through the packaging made by Tetra Pak, the Swedish-based packaging multinational.
The Italian health ministry obtained a court order requiring the company to withdraw two million liters of its Nidina and Latte Mion brands from the Italian market. This was despite the fact that ITX is not on the European Union’s negative list of banned in foods, nor is it included on the World Health Organisation (WHO) list of substances considered detrimental to human health.
Soon afterward, Tetra Pak announced it would stop using ITX in packaging of certain types of foods and would switch to non-UV inks for these products.
An investigation by the European Food Safety Authority (EFSA) concluded that “the presence of ITX in foods, while undesirable, does not give cause for health concern at the levels reported.” It also found that ITX seems to be attracted to fat-containing foods as well as pulp-containing fruit juices. It was virtually non-existent in clear fruit juices.
The EFSA noted that inks in food packaging are not covered by any specific EU legislation although they would be subject to general food safety rules on prevention of “unacceptable changes in composition or characteristics of foodstuffs.”
The European Commission, the EU executive, said earlier this year it was considering bringing in a regulation that would require companies to comply with guidance on good manufacturing practice in respect of chemicals used in food packaging materials. The objective would be to prevent migration of chemicals through the packaging.
The European Printing Inks Association (EuPIA) has since set up a task force of representatives from ink producers, printers and end-users to draw up a protocol on the manufacture and application of UV inks in food packaging. “Hopefully this will make any legislative restrictions by the EU unnecessary,” said one source close to the task force.