On Print | Are You Cured? Proper Cure Is Up to You

The 2014 Radtech UV & EB Technical Conference, held in Rosemont, IL, in May, had a focus on printing and packaging. There was substantial discussion in these two areas about how ingredients in formulations for inks and coatings could pass from the packaging into the food.

Both ultraviolet (UV) cure and electron beam (EB) cure use monomers of low molecular weight and low viscosity, which means they are inherently very mobile. UV cure also uses photoinitiators—some of these, or the residues they leave after exposure, also are mobile. These components are not desirable in food, first because there are regulations that limit their presence there, and second because of the damage that can happen to a market or brand if a contamination is detected by, for example, taste or odor, regardless of whether the perceived amount is enough to cause any harm.

Compounds used for radiation-curable packaging ink and coating formulations can pass to food or beverage by three main ways:

  • By migration from the primary package—the inner layer of a plastic pouch or the wall of a plastic bottle
  • By migration from an ink, coating, or adhesive through the layer in contact with the food
  • By offsetting from a printed or coated layer to the side of a film or paper in contact with the food.

With flexible packaging, avoiding compounds that can migrate is the best solution. The next best is curing or binding all components so that they cannot migrate. Another approach is to use barrier layers to separate the ink, adhesive, or coating from the food.

UV and EB formulations include monomers, oligomers, and surfactants, and in the case of UV, photoinitiators. The first line of defense against monomer or oligomer migration or offsetting is to ensure high levels of cure, so essentially all become part of the polymer chain and hence immobile. Any remaining are trapped in the polymer and migrate only slowly if at all.

Still, there are monomers, even from well-cured formulations, that have real migration rates. The good news is that they are no longer so necessary to produce low-migration prints, coatings, and adhesives, as several alternatives for each are, as we learned at Radtech, available.

Photoinitiators are used at several percentage points of the formulation, so residue levels are comparatively high. Some are very mobile and difficult to trap in the polymer or to be contained by the preferred primary packaging films. New classes of photoinitiators without highly mobile residues have been the solution to this problem.

From Tests to Production

Approval for a packaging material to be used with food—whether from a regulatory agency or by a cautious customer—requires demonstration that, after aging tests, the presence of undesirable compounds in the food is below established thresholds. So if you are the packaging manufacturer or printer, to make the samples presented for approval, you probably purchase films and papers free of migratory concerns, and coatings, inks, and adhesives from suppliers who provide low-migration compounds. You will undoubtedly use great care to produce the packaging structures for the direct barrier tests and the specified measurements of migration. If you have manufactured the product properly and the formulations are properly cured, you should be able to get approval and start commercial activity.

Now comes the hard part. Remember your approval is based upon fully cured inks and coatings, so it is your responsibility to always maintain that state. But:

  • How often do you know you have an issue with the cure?
  • How certain can you be that you knew of all the issues?
  • How can you give a 100% guarantee that you will never ship any uncured or under-cured material?

In the typical operation with only end-of-roll testing, a failure from a contamination or short-term coating upset can easily slip by, especially if it does not trigger a fault in a subsequent step in your factory. And the testing is only effective if it responds to cure level.

Perhaps one day you can have in-line monitoring, but in the meantime you are dependent on preventive action—checking the process, lamps, certificates of analysis, formulations, thickness, and so on, routinely and regularly. These are some of the components of the quality management system you should be using—and that perhaps you are required to employ.

Now most incidents are preventable. None of us needs another—not your vendors, not your customers, not the industry, and of course, not you. So please review your systems to ensure that you are never in danger of non-conformance. If you want help with your own formulations, ask your chemical vendors. If you want help with the process, your UV source supplier will help.

Finally if your business is dependent on UV cure, I urge you to participate in the activities available at Radtech, or even better, join. Radtech brings together a remarkable group of experts and has led our industry through some tough challenges, not the least of which has been regulatory compliance.

Printing expert Dene Taylor, PhD, founded Specialty Papers & Films Inc. (SPF-Inc.), New Hope, PA, in 2000 for clients seeking consultation for technical management, new product design, development, commercialization, and distribution, as well as locating/managing outsourced manufacturing. Contact him at 215-862-9434; This email address is being protected from spambots. You need JavaScript enabled to view it.; www.spf-inc.com.

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