Vighnesha K M
Executive Officer (QC Offset), Toyo Ink India PVT Ltd, Dahej, Gujarat, India
UV-curing is a method where ultraviolet radiation is employed to cure inks and coatings. The phase change from liquid (wet) to solid (dry) is almost rapid as UV curable material is being passed through the UV curing system. The process occurs without releasing volatile organic compounds (VOC’s) into the environment compared to conventional curing methods. This technology is therefore considered “green” as it confines to environmental restrictions. Rapid curing enables leading production rate and facilitates for immediate handling of the cured goods and savings by low energy consumption and reduced UV unit space requirements since no drying ovens are required. Major UV curable inks/printing technologies are:
Low energy UV curing
UV curing methods are varying from time to time. The majority of ultraviolet drying is still carried out using mercury vapour quartz lamps. But it takes time for these lamps to warm up besides there are potential health and environmental hazards at end-of-life. These limitations are amplifying the advantages of low-energy curing. This curing system is meant to dry the print using much less energy (with one mercury doped lamp) than the normal mercury vapour lamps. Low energy UV system delivers increased print run security at a full production speed using only 90 W/cm and 42% less energy. By the low energy lamp technology, ecological advantages are derived. Here the lamps filter out or eliminate wavelengths below 290 nm. Consequently, there is no ozone created and hence the need for exhaust is removed. The inks are formulated using a selection of photoinitiators (highly reactive) which absorb at the higher end of the spectrum and are improved for high-speed curing. The need for multiple UV lamps is unnecessary and thus reduces operating costs for the printer.
UV LED curing
One of the intensifying areas in UV coatings and printing is the development of UV LED curing. UV LED curing uses light-emitting diodes that emit a narrow band of UV also delivering a peak of UV energy. This gives a broad distribution of output across the spectrum including visible and infrared. The applications of UV LED curing has been continuously increasing in existing and new markets due to less heat release onto the substrate. This means heat-sensitive materials or parts can now be UV cured without damage. Different wavelengths ensure safe and fast curing or drying. Besides, UV LED’s do not need any warm-up time. It can be switched on and off as often as required and are immediately ready for operation at any time. This significantly reduces production downtime.
Electron beam curing
The electron beam (EB) curing technology has rapidly advanced in recent years. Here the high-energy electrons in controlled doses are used to polymerize and cross-link polymer materials. Generally, acrylate monomers, oligomers and prepolymers are used in many commercial applications. The high energy of this curing method guarantees a high degree of the oligomer to polymer conversion. The use of this actinic ray occurs under an inert atmosphere like nitrogen to scale back ozone and other species generated under radiation. The non-oxidative atmosphere promotes a high level of cross-linking between the ingredients in the formulation. Increased crosslinking reduces free monomer and oligomer fragments to a level where the cross-linked product is acceptable for food packaging applications.
Quite a few ink manufacturers are developing novel methods of formulating inks to broaden the scope of UV and electron beam inks to improve their performance on the press. There is limited space for interdeck drying in flexible packaging on traditional flexo impression presses and ink manufacturers are creating UV and EB curing inks using water or other solvents. These act as diluents and there is a degree of evaporation at the print stage that permits improved wet-on-wet trapping. A significant advantage of such hybrid inks is that they are not classified as hazardous materials but traditional 100% solid UV inks and coatings are. This means manufacturers do not need to apply a hazardous chemical label onto the pack and they are often transported in bulk. In the case of UV ink, it is categorised as hazardous and cannot be transported in a container larger than 25 litres.
Low migration inks and coatings
Low migration ink is designed to use on food packaging. It is formulated through selected components to ensure the migration from the resulting ink film is intrinsically within the official migration limits for the intended application. Radiation curing is widely utilized in food, pharmaceutical and tobacco packaging. No components must migrate from the print into the product and cause any organoleptic effects. This is particularly crucial in tobacco packaging where the standards for odour-free and migration-proof packaging are driven by the very fact that tobacco is hygroscopic. Ink formulators may choose components from the existing range of accepted ingredients or select new classes of available materials that effectively result in lower contamination.
1. Patrick Glockner et al, Radiation Curing: Coatings and Printing Inks, pp 117-144, 2009
2. R.H. Peach et al, The Printing Ink Manual, Fifth Edition, 2007
3. Anthony J. Bean, Radiation Curing of Printing Inks, pp 301-332, 1992
About the author
Mr Vighnesha K M has experience as Executive officer in R&D, (UV Flexo inks and coatings) Manipal Technologies Limited, Manipal, India (Feb 2018 to Feb 2020).