The continuing evolution of the printing and packaging industries and the demand for improved product quality and production efficiencies have in turn placed greater challenges on UV curing technology. New film substrates that provide both physical and aesthetic properties and that also reduce material thickness and costs have been developed in recent years bringing with them new challenges. In the main new substrates and ink and coating chemistries designed to work with them can be efficiently cured with conventional UV systems coupled with the availability of various forms of heat management such as cooling rollers and design improvements in reflector technology. With certain applications, in particular direct food contact packaging, or with specific ink and coating weights curing has to be carried out under inert conditions that reduce oxygen to obtain the required degree of cross-linking and meet the challenges of the end user.
Malcolm Rae, Managing Director of GEW (EC) Ltd. discusses how the company is addressing the challenges of new applications and technologies with its inert atmosphere curing system that minimises the amount of oxygen at the surface of the ink or coating at the curing zone.
Replacing oxygen with nitrogen
Oxygen in ambient air reduces cross-linking in photoinitiator driven reactions. Inks and coatings are formulated to overcome this with the addition of aggressive photoinitiators that combine with oxygen. Curing of very thin coatings such as some silicones can only be achieved if the oxygen is excluded from the curing zone. Under these conditions, cross-linking is substantially improved on conventional inks and coatings. New applications in printing and packaging mean that there is now an increasing need for inert gas UV curing. Benefits include having the capability to process a wide variety of substrates, better chemical resistance and adhesion, faster cure speeds, thinner coating weights, lower photoinitiator levels, increased production speeds, reduced energy consumption and more consistent curing.
Inert gas generation
Either carbon dioxide or, more usually, nitrogen can be used to displace oxygen from the curing chamber, and gas can be delivered in compressed or liquid form. For all but the smallest installations, liquid gas has been the preferred source of nitrogen. There are differences between curing with carbon dioxide and nitrogen, but they are small. Liquid nitrogen is usually stored on site in large vertical tanks with an evaporator to boil off the gas. Liquid gas costs vary widely depending on delivery distances and volumes used. Typically liquid nitrogen costs in the region of 45 Eurocents per cubic meter of gas with approximately another 30% for tank rental, constant flash off and delivery charges. An alternative and cost effective source of nitrogen is the nitrogen generator. This unit chemically removes oxygen from compressed air to leave nitrogen with a purity of around 20-30ppm. These units can generate nitrogen for around 20 Eurocents per cubic meter. The cost of these depends on the volume requirements. However, they only need compressed air to operate. There is an upfront capital cost for this equipment, but maintenance is low: there are no consumable parts and lifetime is at least 15 years.
The effect of inert gas curing
Excluding oxygen significantly improves cross-linking and hence cure rate, without changes to the ink or varnish and curing speeds can be increased between two and four times. Recent tests carried out with a food processing and packaging equipment manufacturer in Europe demonstrated how cross-linking on a coating was improved by passing it through the GEW nitrogen curing unit. Here cross-linking was critical to product performance for thermal transfer printing on a varnished label where incomplete curing of the coating leads to clogging of the print heads in the ongoing application process. The graph clearly shows the effect on cure of increasing lamp power and reducing oxygen concentrations, with the lower values indicating better cure.
Application of the technology
Inert curing is not a new process. Silicone and food packaging applications have been in existence for many years. Hitherto the cost of inert gas UV equipment has been high and this together with the cost of producing the gas itself has limited installations in the field. Equipment for high-speed web applications must be precisely manufactured and installed to achieve low levels of oxygen with modest levels of gas consumption. GEW has introduced a range of precision equipment that when used in conjunction with a nitrogen generator makes inert gas curing a more cost-effective option. Applications for inert gas are manifold. We are already achieving four times the speed of conventional curing on CI presses for curing base white inks at over 400m/min and curing at lower power levels to achieve full cure on thermo-sensitive materials without the use of water-cooled chill rollers. The reduction of photoinitiators to address the issues of initiator migration and odour essential for food industry compatibility also offers benefits in reduced ink costs. Curing of screen and gravure inks under inert atmosphere significantly improves consistency and cross-linking qualities.
We see the uptake of inert atmosphere UV curing as the next step in the evolution of the technology, with limitations steadily being eroded by new do-it-all systems that can adapt to almost any printing and packaging application or end user demand.