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FLEXO Magazine : September 2011
(photoinitiators), or create small molecules during curing that can cause odor or migration issues (photoinitiators). The most important point in providing the proper UV flexo ink is COMMUNICATION. The printer MUST tell the ink for- mulator what the application is, or the formulator will provide the most cost-effective product that meets the needs as the formulator understands them. This will guarantee a failure and an ugly claim in food packaging. There are monomers, slip aids, photoinitiators and other in- gredients that will perform great in food packaging, but these nearly always make a more expensive ink and the inkmaker will not propose this approach unless the need is clear. Given this clear communication and a properly formulated ink, UV flexo is most commonly used on food and pharmaceu- tical labels. These can be paper or plastic, shrink or static. I've also seen UV flexo used on microwavable packaging, like microwave popcorn boxes. The right ink can give no offensive odors, even in the microwave. You don't see much UV flexo in flexible packaging because that is typically wider web and the heat from the UV curing lamps can cause substrate distor- tion and registration issues. White inks to be printed by the EB flexo process are, like conventional UV flexo inks, very low in VOC content and are essentially 100 percent active products with no diluent or carrier. Since they are cured by EB, they must be the last thing printed before entering the EB chamber at the end of the press. They are typically very opaque and usually somewhat lower in viscosity than UV flexo ink colors. They are used as a last-down opaque white, applied in a flexo unit (or a flexo coating station), at the end of a flexo press, a gravure press, or on a litho press, immediately before the EB unit. Because these are EB-cured products, they are photoiniti- ator-free and can easily be formulated with fast-curing, low- migration raw materials that makes them suitable for food packaging. These last-down EB flexo whites are used mostly in flexible packaging. Full-color EB flexo has been an area of research among ink formulators and press manufacturers for at least a decade. The problem is that the EB process requires curing at the end of the press, and thus requires wet-trapping of inks, while flexo printing requires the inks to be fully "dry " between print- ing units to avoid tracking and other undesirable problems. Sounds like a real conundrum. An approach was developed that involves adding a volatile material to the ink that will evaporate between the printing units, leaving the ink 'dry ' (or dry enough to trap on without tracking) and still allowing curing to take place at the end of the press. There are two basic implementations of this technology: one is where the volatile material is water, and the second is where the volatile material is an organic solvent. This is no place to get into patent claims and counterclaims, but both processes are running, at least a little. There are less than 10 presses worldwide running both technologies com- bined, and both technologies have had their growing pains. The biggest issue with the water process involves stability of the inks. The water must evaporate extremely fast, but no changes should occur until the ink hits the substrate. Water, not being very smart, will try to evaporate in the ink sump, on the anilox rolls, on the plates and anywhere else it sees the atmosphere. This makes a closed system and on-press temperature control critical. The solvent process has the disadvantage of containing solvent. The solvent must evaporate, and VOCs are emitted, and the solvent flammability hazard remains. When things are good, print quality from both technolo- gies can be outstanding--very low dot gains, extremely good graphic reproduction and very good chemical resistance. The target market for this technology is flexible food packaging. The low-residual, photoinitiator-free EB process coupled with the strength of flexo printing on wide-web presses running film is a perfect fit. When all the remaining technical, business and supply chain issues are worked out, this might be THE thing for flexible and folding carton food packaging in the next decade or two. About the Author: Don P. Duncan is director of research, Wikoff Color Corp. He will sere as a two-time presenter at CPP Expo 2011 in Las Vegas, NV. Both of Duncan's talks are set for Tuesday, Sept, 27. The first is entiled "Choosing UV/EB, What Printers Say;" and the second is "UV/EB For Food Packaging." 64 FLEXO SEPTEMBER 2011 www.flexography.org