Migrating to Safety: Progress in Inks for Sensitive Packaging Applications
Ink stays down where it’s laid down — it’s a fundamental assumption in every kind of printing. But in some label and packaging applications, parts of the ink may have other ideas.
This is called ink migration, and when it occurs, it compromises product safety and threatens consumer well-being. Brand owners fear it, and regulations bar it. As manufacturers, label and package printers find themselves carrying a heavy share of the responsibility for preventing it.
They’re getting help from inks and coatings that are formulated to hold migration to a minimum while printing with the level of quality that producers of packaged goods expect. Ink isn’t the only key to dealing with migration — many other factors inside and outside the pressroom contribute both to creating the phenomenon and to holding it in check.
But, together with tighter process controls and better manufacturing practices, advances in ink chemistry are stiffening resistance against footloose contaminants and the headaches they can cause.
Migration takes place when ink components penetrate from the printed to the unprinted (product facing) side of the packaging substrate. It also can result from that perennial problem of wet-ink printing, set-off: incompletely dried ink on the top side of the sheet contacting the bottom of the sheet above it in a stack or a roll. Evaporation and condensation may trigger migration in packs in which foods can be boiled or baked. Heat-sterilizing a package may do the same harm.
For obvious reasons, labels and packages for foods and beverages lead the list of applications for low migration inks.
Tyler Newsom, product manager for sheetfed inks for Flint Group North America, says that when paperboard and other primary packaging materials don’t provide adequate barrier protection against migration, greater safety expectations are placed upon the ink. He cites “lightweighting” beverage bottles (i.e., molding them with less plastic) as an example of a packaging trend driving demand for low migration fluids.
Dr. Ronny Borms, product manager for Toyo Ink, says that it isn’t just the packaging — napkins, placemats, and other printed items that could have direct or indirect contact with food must minimize ink migration as well. When speaking about migration, he avoids drawing distinctions among “primary,” “secondary,” and “tertiary” packaging since the risk can be present in all three types.
The degree of risk varies from package to package depending on, among other things, how much ink is being used, notes Tom Molamphy, Agfa’s industrial ink business development manager, North America. Printing full information on a label increases concern, but “when there is only a batch number or an expiration date, the ink volume is very low and typically there is no need for low migration inks,” he says.
A category of special sensitivity is the subset of food packaging that consists of bags and other containers for pet products. A dog can detect the taint of migratory contaminants in concentrations too small for a human to notice, says David Biro, director of paste ink, screen and industrial products for Sun Chemical, North America.
He ranks tobacco packaging as the No. 2 application for low migration inks after food, noting that producers of packaged cannabis products are starting to show similar interest in printing with them. Cosmetics and pharmaceutical packaging are also high on Biro’s application list. He adds that since traces of contaminants can affect the performance of electronic items, the packaging for these products needs the protection of low migration inks as well.
Techniques for detecting ink migration are growing more powerful — contaminant quantities that would have been too small to identify five years ago are bright red flags today. Fortunately, ink recipes have changed in ways that can prevent the incidence of even these minute amounts.
Today’s products are “cleaner,” says Newsom, with less potential for migration than their predecessors. They also perform better on press than low migration inks used to, especially those formulated for energy curing.
“UV was always frowned upon from a migration perspective,” he says.
Now, the inks’ improved photoinitiators — chemicals that react with UV radiation to stimulate the molecular bonding of monomers into polymers — make them safer to use and more predictable to print with. Good news for the increasing adoption of production inkjet presses, says Molamphy, is the arrival of low migration UV inkjet inks, including those formulated for high-efficiency LED-UV curing systems.
With ink, things that happen at the molecular level determine the degree of resistance to migration. According to Borms, ink components with large molecular structures don’t pass as readily through substrates as molecules with smaller structures and thus are less likely to migrate. Biro says that the few post-cure chemical fragments left behind by today’s UV and EB inks tend to be “functionally reactive,” which makes them non-migrating as a result of becoming part of the printed material.
Curing efficiency is everything in package printing with inks made for UV and EB exposure. Curing, says Biro, is not a binary process in which all of the wet ink suddenly becomes hard and dry. It progresses along a scale of exposure until the cure is complete and the printed product is “fit for use.”
Only with a 100% cure, says Newsom, will ink components be fully cross-linked within a layer from which they cannot migrate. Maintenance and proper operation of curing systems are crucial to achieving this outcome.
When migration does occur, the ink shouldn’t automatically be rounded up as the usual suspect. “All parts of the packaging are important,” Molamphy says. “There are cases known where migration issues resulted from substrate impurities as well as from adhesives.” Ink is only one of a number of sources from which migratable materials can come. Substrate behavior, environmental factors, and printing conditions also need to be addressed if migration is to be controlled.
An example is the composition of recycled paperboard for folding carton applications. Some grades, says Borms, include pulp from discarded magazines and newspapers — pulp that may not have been completely de-inked and thus may contain residues that can migrate. Such stocks should be tested for barrier properties before they are selected for food packaging. As Newsom says, printing with low migration ink does not guarantee that migration will not happen — all elements of the package’s manufacture have to be evaluated for safety.
That includes printing conditions and production routines. A dirty press, for example, introduces potentially migratable materials; so do the ingredients of solvent-based press washes and other kinds of pressroom chemistry. Borms says that manufacturing impurities like these, classed as non-intentionally added substances (NIAS), are coming under regulatory scrutiny in Europe for the role that they play in migration.
Printers like to operate their presses at high speeds, but a press running near the top end of its range may be sending incompletely dried sheets into the delivery — sheets that could be prone to ink migration from set-off. Because heavy ink coverage on packaging material poses a greater risk of migration than lighter coverage, decisions about adjusting the percentage of ink coverage may have to be made upstream of the printing.
With low migration inks now available for most printing processes, no method is far behind any of the others in terms of migration prevention. Flexography gets a plus because its evaporative inks leave little migratable residue behind. Borms calls EB curable ink “one of the best options” for packaging in view of the fact that it doesn’t need photoinitiators — which are potentially migratable in elements in UV inks — to achieve cure.
Inkjet printing, however, may have some room for improvement. Its disadvantage, according to Molamphy, is that it uses low viscosity inks containing low molecular weight compounds that can pass through substrates more easily than the heavyweight ingredients of offset and flexo inks. “Every low migration ink needs conceptual design to make it safe,” he says.
Because product integrity and public health are at issue, ink migration in package printing has attracted a good deal of regulatory attention. The most influential rule is the so-called “Swiss Ordinance” — legislation passed by the government of Switzerland to identify components deemed safe to use in inks for packages containing food and other sensitive products.
The ordinance also sets “acceptable” migratory limits for these components in parts per billion. Nestlé and other leading brands have made it the basis of their own stringent requirements for packaging safety. In the U.S., the Food and Drug Administration regulates ink components that may come into direct or indirect contact with food, but it does not certify inks as “low migration” or “food safe.”
Some printers and converters have set up their own migration analysis facilities to be certain of providing the protection their customers expect from them. Others rely on trade groups such as NAPIM (the National Association of Printing Ink Manufacturers) for guidance.
Ink makers understand their obligations on this front to printers and converters. Inks of all types “are inherently very safe,” says Newsom, adding that the industry keeps them that way by acting as a self-regulating “voluntary watchdog.”
Education provided by the ink industry is “the most pragmatic way” to ensure that the entire packaging supply chain has the resources it needs to control ink migration, agrees Borms.
After all, he says, “We are all consumers, even as industry people. We have to take care of every human being.”