Material Matters

The trinity of inks, substrates and adhesives form the core of nearly every completed package. But with so many variations among these materials, an understanding of how they relate to one another is important for printers and converters to grasp before going to press.

There’s some pretty advanced chemistry that goes into the formulation of these packaging materials, but by maintaining a steady line of communication with their suppliers, printers and converters can be confident in their material combinations.

ACTEGA North America offers varying inks and coatings for several markets.

Three Substrates, Several Combinations

Though there are numerous styles of packaging, printers will generally encounter just three base-categories of substrates — paper, film and foil.

Because of the differing surfaces of substrates, ink is accepted by the substrate through varying methods. When printing with water-based inks on uncoated paper, media porosity is used to absorb ink. But when printing with UV inks, surface roughness provides anchorage for the ink. It is important to note however, that uncoated paper can present print quality challenges, as colors can appear washed out unless special chemicals are added to prevent this.

“Paper gives you anchorage because the surface is rough and the ink can go into the grooves and nooks and crannies of the paper and get locked into it,” says Radha Sen, senior research associate for Avery Dennison Label and Packaging Materials. “That’s one of the reasons why you typically don’t have issues with anchorage and paper.”

When dealing with printing on smooth, nonporous films, foils and some coated paper, there is no porosity or roughness to rely on. Such coated papers and films, Sen says, do not provide absorption opportunities for water-based ink and no physical interlocking for UV inks. If the application is being printed with a water-based ink, Sen explains that a microporous coating can be implemented, which allows the water, pigment and dye components of the ink to enter into this layer. UV ink however, will sit on the surface of the material and will need to be cured in place as the ink anchorage is being provided via the material compatibility or reactive chemistry.

Print quality is determined by many parameters, such as ink and substrate interactions and the application method. If these parameters are not properly matched, quality issues such as grain, washed out colors, streaks and pinholes can appear. Therefore, selecting the correct ink/media combination and application method is key to successful printing.

Sen explains that a coated paper will provide stronger visuals. Printing directly on paper, she says, will result in the media wicking the water-based ink. With a coated stock however, a controlling agent helps keep colorant close to the surface providing an improved image. If there is a dye involved, Sen explains, the water will penetrate the media, but the dye will remain close to the surface, provided the substrate has the controlling agent.

Dealing with Durability

Despite all the factors that go into whether an ink will run well on press, converters need to be aware of the performance characteristics ink needs to maintain when the package is placed on shelf and purchased by a consumer.

For example, a product like a jug of milk will not be on a grocery store shelf or in a consumer’s home for more than a couple weeks. Therefore, Keith Nagle, senior applications specialist at ACTEGA North America, says it is not necessary to waste resources on fade-resistant or lightfast ink.

“If that label fades, it’s not that big of a deal because it’s usually in a recycling container before the life of the label expires,” Nagle says. “So you don’t want to waste your money on ultra lightfast.”

On the opposite end of the spectrum however, Nagle says the ink that is used on a windshield sticker for a car needs to be able to withstand light, moisture, extreme temperatures and the test of time. In this case, an ultra lightfast ink with automotive grade pigments is needed to ensure this ink does not fade.

Mark Hill, VP of research and development for INX International, explains that with the rise of flexible packaging in the food space, many inks are now being put through the wringer before they even reach the shelf. This is because many pouches containing food products are often subjected to processes designed to increase the shelf life of the product.

For example, the difference between an item like a soup can and a pouch going through the retort process is that a label is typically applied to a can after retort is completed. With a pouch that does not require a label, the inks used on the pouch must be able to resist the extreme heat and moisture inflicted on it during the process.

“The ink has to pass the sterilization process where it’s subjected to heat and moisture for a certain period of time to sterilize the product that they put in that pouch,” Hill says.

Adhesive Implications

The key to finding the right adhesive for a product, says Mike Witte, director of national accounts, polymer, at Franklin Adhesives & Polymer, is understanding the required performance of that adhesive.

Witte explains that the three primary adhesive performance properties the company will look into are the peel strength, tack strength and the shear strength. When the requirements for those three qualities are assessed, Witte says it can then be determined whether Franklin already has an adhesive available for the customer, or if it needs to formulate one.

“The most important thing to understand is what the product has to stick to and under what environmental conditions must the label stick,” he says. “What do the performance properties have to be? And do we have anything that can deliver on all those fronts?”

Witte says another key component when developing label adhesive is ensuring it consistently anchors to the label stock. For example, whether a label is printed on paper, film or foil, making sure the adhesive stays on the label — even if it is pulled off — allows it to not leave messy adhesive residue behind.

“We want the adhesive never to be confused about what surface it needs to stay on,” Witte says. “We don’t ever want to have a situation where we can peel the label off the adhesive and have all the adhesive remain on whatever that label was stuck on.”

Critical Communication

Because there are so many details to be aware of when developing an ink, substrate and adhesive combination, it is imperative for converters to maintain a consistent line of communication with their supplier partners.

One potential pitfall that converters may run into, Hill says, is not specifying how slippery they need the packaging to be once it is completed. This coefficient of friction, he explains, is essential because not only does the substrate material play a role in how slippery the final package is, the ink impacts it too.

Hill explains that if a package is going to be sent down a high-speed filling line, for example, it will need to be slippery so it doesn’t get stuck on the machinery during the filling process. Other packages should not be slippery, he says, such as large bags of grass seed or fertilizer that get stacked on top of each other.

“They manufacture [slipperiness] into the substrates in a lot of cases,” Hill says. “But if there’s ink going on top of that substrate, the ink has to have a certain slip property as well.”

Communication is also essential when ink migration factors could be present.

Nagle explains that converters should be up to date on the ink materials that are listed in regulations such as the Swiss Food Ordinance and the Nestlé Guidance Note on Packaging Inks. If printers do not have these documents readily available, he says, they should be easily accessible through ink suppliers.

Franklin Adhesives & Polymers runs its adhesives through a peel test.

Beyond just using low-migration inks, Nagle says there are other steps that should be taken to ensure a compliant package. For example, after running non low-migration inks for five days, he says just simply switching out the inks for a low-migration job on the sixth day is not enough.

“In theory, you need to sterilize the press of all migrants,” Nagle says. “You have to clean out all those idler rollers and clean out the press because all those labels you ran for the last several days have migratable components in them that could offset onto your idler rollers and other parts of the press that the web touches.”

While low-migration inks receive a great deal of attention in the food packaging world, Sen explains that substrates can also play an essential role in the migration equation. For example, with film substrates, Sen says that the film itself will likely not be enough to eliminate migration, though thicker films can reduce migration speed. Implementing a barrier layer into the substrate can help decrease migration more effectively.

There are many forms that migration can take, one of which is set-off migration. For example, if printed coffee cups are stacked inside each other, the ink could theoretically migrate from the outside of one cup onto the inside of the cup it is stacked in. In this situation, Sen explains that placing a coating or varnish over the ink will help to keep it in place and avoid this type of migration.

“In that case you probably want a laminate on top of the ink,” she says. “The ink needs to be sandwiched between two functional barriers to be perfectly sure nothing goes through.”

Witte explains that adhesives, even if in a dried state, can start to move when put under stress, particularly in situations with elevated temperatures. For example, Witte says it is important for adhesive manufacturers to understand ahead of time how an adhesive is going to behave during diecut or rewind at the end of the converting process.

If an adhesive is placed in a temperature that it is not meant to withstand, he says, adhesive movement can occur through a process referred to as cold flow. Likewise, another unfortunate adhesive movement scenario called edge ooze can occur if a roll of labels is wound too tight, causing adhesive to leak out of the edges of the roll.

A shear test is also administered to determine the quality of an adhesive.

Because an adhesive manufacturer is typically at the very beginning of the supply chain, Witte says communication becomes that much more important. Once an adhesive is developed, it then goes to a coater who applies it to the substrate. Then that coated material is sent to the converter.

“There’s a lot of opportunity for confusion,” Witte says. “We try to eliminate as much confusion as we possibly can up front so all parties have the same understanding of what the final properties of that label, package or laminated construction must be.”

No matter what the packaging application, Hill says strong communication is essential, and no detail should be overlooked. It may not seem significant at the time, but it’s better to be safe than sorry.

“It’s a cradle to grave philosophy,” he says. “The ink company needs to know what occurs to a package throughout its entire lifecycle.”