A Sticky Situation
What good would print be if the ink didn’t stick to what it’s supposed to? Strong ink adhesion ensures a printed item maintains its graphical integrity, which is essential when it comes to packaging where appearance is everything.
However, with the rise of inkjet printing, not all substrates are fully compatible with this new technology. Fortunately, there are preemptive procedures that can help ink stay where printers and brand owners want it.
As with conventional printing processes, some of the more difficult substrates for inkjet ink to adhere to are some of the most common materials used in packaging. Among these challenging substrates are plastics like polyester and polypropylene, plus metal and glass.
According to Paul Edwards, vice president of inkjet technology for EFI Inkjet Solutions, the best way for an ink to stick to a substrate is if it actually dissolves directly into that material.
“What happens in that case is that when something is dried and cured, molecules of the two materials intertwine and really become a hybrid material.”
One of the more difficult surfaces to make that happen with is polyester, Edwards details. However, there are ways to improve adhesive properties that the substrate manufacturer can implement.
When dealing with polyester, Edwards says a coating can be placed onto the surface of the material that is attracted to both the polyester and the ink. This coating, called an intercoat, provides a buffer between the ink and substrate.
“It transitions from one chemistry to another chemistry because the intercoat has characteristics of both the ink and the substrate,” he explains. “Often, it’s used as a prime layer to improve adhesion of all sorts of different materials to a polyester.”
In some cases, rather than adding material to a substrate’s surface, adhesion can be increased by altering the properties of the substrate. Frequently, this is done with corona treatments.
Heather Rockow, UV business development manager at Collins Inkjet, says printers often prefer corona treatment because it can be done inline with the printing, unlike adding a coating, which isn’t always an inline process.
“There’s some resistance to putting on a coating on something as a separate process, because you’ve probably gone to inkjet because you didn’t want separate processes,” Rockow says. “Then you’re back to corona treating or flame treating and building that into your printer or printing process.”
Edwards explains that the act of corona treating actually increases the surface energy of the substrate. By allowing it to have a higher surface energy than the ink, Edwards says the ink will then spread across the substrate and grip better to the surface, from both a chemical and physical perspective.
One drawback of corona treatment though, is that it is not permanent. For example, Edwards says if a corona treated item is exposed to air for an extended period of time, the treatment will wear off.
“If you do a roll of substrate, and you roll it up tightly, once you get past the outer layer it’s going to be pretty good for a long time,” he says. “But if you do the corona treatment and left that substrate sitting around in the air for a few weeks and then try and print it, all that surface energy will dissipate and you’ll have a problem with adhesion.”