Primers & Pretreatments: Critical Factors for Adhesion and Quality

Expanding knowledge in this area can expand possibilities for your business.

The gap between simply being able to print on something – for instance, a sheet of metal – and printing it in a way that meets its use expectations, such as exposure to the elements over a period of ten years, is indeed profound. Beyond durability, successful print quality, including color control, requires ink and substrate to work hand-in-hand. For certain substrates, what joins these hands, so to speak, is often a pretreatment or primer.

Defining the Challenge

According to Ray Weiss, vice president, of e-learning and certifications with PRINTING United Alliance, there are many reasons why primers and pretreatments would be needed for wide-format and other printing applications. The primary reason, he says, is that it addresses anything, be it dust, oils, and even surface energy, that can get between in and substrate, thus inhibiting expected adhesion. He says that with the rise of UV inkjet printing, the need for surface preparation or pre-treatment has become increasingly important because UV inks must sit on and adhere to the surface, whereas solvent inks would bite into the surface, enabling adhesion.

As already alluded to, surface preparation and/or pre-treatment can begin with simple shop cleanliness, such as working to create a dust-free environment, and, if needed, wiping down certain substrates prior to printing.

Travis Barcelona, Inkjet Product Development manager for Nazdar Digital Inks, echoes this sentiment. He explains that if you don’t properly store your substrate, for example, a plastic one like polystyrene or polypropylene, then you might run into the issue of plasticizer migration.

“If you don't take care of your substrate, meaning keeping it in a controlled environment, dust-free, et cetera, then instead of printing on the substrate, you're now printing on plasticizers. And it's difficult for some inks to eat through this plasticizer and then attach to the substrate. Essentially, it's like you're driving in the rain, and you start hydroplaning,” Barcelona says.

Another common pre-treatment scenario is with plastics, where expected ink adhesion – and by extension, print durability – is dependent on the surface energy of the plastic substrate used. For plastic substrates to appropriately accept inks, their surface energy must be between 40 and 50 on the dyne scale, which can be achieved through either flame or corona treatment. While certain plastic substrates can be ordered pre-treated for dyne level, he notes that dyne level is not permanent. He advises testing dyne level – which can be done with special reactive pens – when shipments arrive and being aware of how long they sit in inventory. In some cases, he says, dyne levels can be regulated through quick treatment with a torch or even wiping with isopropyl alcohol.

Surface preparation activities are dependent on both the type of substrate and how it will be used. For instance, Weiss says. Pre-treatment activities for printed glass are different, he says, if the glass is for display purposes (such as a mirror printed with a beer logo) than if that same logo is printed on a glass chalice that will repeatedly go into a dishwasher.

Shop Strategies

Weiss says primers can often be acquired from the OEM that provides the inks. He adds that while third party primers and adhesion promoters are available, graphics producers may find themselves in a quandary when adhesion failure occurs. “Who do you blame” he poses, the ink manufacturer or the third party?

Asked whether pre-treatment steps could be considered as optional, Weiss replies simply, “It depends.” Substrates designed to be used for print applications, and thus already treated with receptive coatings, are ready to go. Other products – he mentions, from personal experience, wrestling with adhesion issues on golf balls and glass flasks – may certainly need treatment. He advises that knowing what products you need to use, understanding what works, and maintaining that consistency, is a good strategy for success and a way to keep from wasting money. That said, manufacturers sometimes make slight changes in formulations, which can throw a wrench into a carefully set process. Hence, a quick test before printing can prevent headaches and limit spoilage.

However, if you ask Paul Edwards, vice president of the Digital Division for INX international, he believes that for wide-format printing specifically, it is possible to bypass a pretreatment on most materials if you’re using UV-curable inks.

“With most wide-format printing, you're relatively lucky because, essentially, you don't go very long between printing the drop and curing the drop. So, the time between that drop hitting the substrate and being sort of cured is pretty short, you know, fractions of a second,” Edwards says. Essentially, even though some materials like plastics and acrylic may be harder for ink to adhere to, Edwards believes with the right ink formulations and curing, pretreatments might not always be necessary.

“Generally, for most substrates, you can get adhesion as long as the ink spreads. And for those substrates with a low surface energy, there can be a struggle, but it is still possible you know with the correct formulation and curing to do that,” Edwards says.

For lower surface energy substrates like acrylic and plastics, Barcelona explains there is a low-cost solution to adhesion, known as adhesion promoters. Essentially, you can wipe it on your substrate, let it dry, and then print. However, warns that these promoters can affect color quality – so for jobs where that is a concern, your best option is an optically clear primer.

With primers, Barcelona says you can apply them in one of two ways: You can screenprint them onto your substrate, or you can roller coat them on. Ultimately, the best method should be determined by the type of substrate you’re printing on.

Doing It Yourself

What about DIY approaches to pre-treatment? Weiss says, in some cases, for instance, a torch can be used to flame-treat plastics for short runs. This approach, however, does not “scale,” and flame treatment with a torch can prove less exact than, say, corona treatment performed within a manufacturing facility.

And if you are pre-treating your substrates yourself, specifically corona, make sure you don’t overdo it. “You can over corona treat stuff and get some real problems when you start to do that. What happens is that you create molecules on the surface that can react with the inks and the print heads if you do too much. So, there's a sweet spot for corona treating and that's going to be dependent upon the substrate you choose,” Edwards says.

Ultimately though, when it comes to acceptable ink adhesion, Barcelona breaks it down like this, “You need cure. You need to control your surface contaminants on the substrate. You need either cure for UV or you need the right dryers and airflow to evacuate, and you need to make sure you follow a standard of adhesion methodology,” he says.

And for those who maybe don’t deem it a necessary step, Barcelona has two words “Good luck. Especially if you’re printing on cups or stuff that goes through dishwashers, if the ink comes off after the first wash, that isn’t a good look for you. So, it's really about, how serious you are about your business and, and how serious you are about the quality of your work.”