By Digital Output Staff
Advancements in ink sets address considerations like color gamut, durability, and adhesion.
Over the years, adhesion has become a top feature as more digital print devices are touted as “printing to anything.” Unfortunately, while the machine may print to anything—it doesn’t mean the ink will remain on that surface or look aesthetically pleasing for that matter.
To address ink adhesion, pretreatments and primers are an ink set’s best friend. “Primers are used for all ink sets and it really comes down to the substrate or media needing adhesion to find the right one,” explains Syd Northup, VP of sales – digital, Marabu North America.
Above: Several clients of Marabu utilize inline spray booths to pretreat glass. Others flame treat and post spray with a printer to enhance ink adhesion.
Evolving Adhesion Needs
Today’s ink sets—for this article’s purposes we look at solvent, latex, and UV—are able to be used without a pretreatment. However, a pretreatment is required in many scenarios.
“Ink sets evolve on a continuous basis with regard to their interaction with various substrates. Ink designers aim to reduce pretreatment requirements on substrates to achieve good adhesion,” shares Nitin Goswamy, president, A.T. Inks.
Over the last decade, engineers have worked on ink adhesion and how to better adhesion to different substrates and products. “There have been many improvements in the types of substrates and ink application techniques to better adhesion for our customers,” shares David Lopez, product manager, Professional Imaging, Epson America.
According to Paul Edwards, VP of the digital division, INX International Ink Co., “over the last decade, there’s been a transition from solvent through UV and most recently, water-based/latex technology. These ink types do have different challenges with regards to pretreatments.”
However, “adhesion promoters will always be needed for some substrates,” admits Ken Parsley, product manager, Mutoh America, Inc.
“It depends on the application and ink set. Each ink technology can work fine without a pretreatment on certain materials. But, this is very limiting. A pretreatment is needed to print to a range of materials,” says Adam Tourville, channel manager, Fluid Color LLC.
Substrates direct printed with UV digital inks may need or even require primers/pretreatments for adhesion. “There is an increase in sales with our primers in the UV market for both flatbed and direct to object printing,” shares Northup.
“We see this more often in applications focused on customization and personalization that involve direct printing on materials such as glass, metal, as well as some plastics. As the popularity of product customization grows, we are often faced with adhesion challenges,” agrees Brian Brooks, manager of product management, Roland DGA Corporation.
Thinking Ink
Pretreatment is used with solvent, latex or resin, and UV ink. Most of the time a primer or pretreatment is chosen based on recommendations from the ink manufacturer and—if applicable—the hardware manufacturer.
Solvent inks are limited to PVC and polyester fabrics, where pretreatment is generally not needed. However, some of the new substrates used might need pretreatment to use solvent inks, shares Goswamy.
“There isn’t an ink-based pretreatment for solvent ink, however the material needs to be coated specifically for solvent ink. The coating applied allows solvent ink to adhere to the media properly. This is a required step for media providers manufacturing substrates for solvent printing,” explains Lopez.
Latex or resin-based ink is dependent on the manufacturer. For example, Epson leverages an optimizer built into the SureColor R-Series printers. Epson’s UltraChrome RS ink includes CMYK+Optimizer, and the optimizer acts as a pretreatment. “The optimizer is printed first, then the color droplets adhere to that in a method called ‘pinning,’ which ensures each dot is applied in the exact spot required for the intended image,” notes Lopez.
Similar to latex/resin, pretreatment for a UV ink is dependent on the manufacturer and the UV ink’s unique chemistry. “Epson developed UltraChrome UV ink as a semi-flexible UV ink that has great scratch resistance and can adhere to a variety of materials, however there is still a need for pretreatment on certain substrates. Often untreated metals or items that will have a lot of friction, such as a cell phone cover, do require a pretreatment to help promote ink adhesion,” says Lopez.
UV printing opens up a wide assortment of substrates to be printed on. “Each substrate comes in many different configurations, with different properties and special coatings. Testing a substrate’s adhesion and durability is often the only way to determine if an adhesion promoter is necessary,” recommends Emilio Rangel, UV desktop and Vertelith product manager, Mutoh America, Inc.
“Of the three types of ink, UV ink is more tolerant towards a wider range of substrates with strong adhesion without pretreatment,” adds Goswamy.
Surface Energy
In terms of substrate or surface, a pretreatment or primer is used when the surface energy of the printable item is too low for the ink to adhere properly.
“The primer is used to create surface tension, so that the ink will bond to the substrate/media,” explains Northup.
There is a second reason why pretreatment might be used. “This is for print quality purposes. In order to maximize print quality, drops need to be positioned correctly and spread to a given size. They also need to have limited interaction with each other to avoid coalescence, which has an impact on changing the shapes and position of the ink drops,” explains Edwards.
Employing a Dyne Pen is helpful to determine the surface energy of a substrate. “A Dyne Pen contains fluid with a known surface energy. When the pen draws a line on the substrate, the line of fluid should not flow out and it should also not retract and pull in on itself. Pens are tested until a fluid ‘works’ and that is the surface energy of the material,” explains Tourville.
Once the surface energy is determined, the method of pretreatment can be chosen. This is decided not only based on the surface energy but also the surface type.
Plastic presents a few options, according to Tourville. “The easiest but most expensive is to order the substrate with a digital-grade primer already applied. The second option is to have the plastic corona treated. Corona treatment applies an electric charge to the surface of the substrate causing the surface energy to increase. The third option is using a propane flame treatment to increase the surface energy and then apply a wipe-on adhesion promoter.”
Polyethylene (PE) and polypropylene (PP) substrates need corona/plasma treatment to achieve strong bonding with the ink—regardless of the type. “PE and PP have low surface energies. Corona/plasma treatment increases the surface energy and interfacial adhesion increases. Some inks may not adhere even after surface treatment. In such cases a primer is applied after the corona/plasma treatment,” explains Goswamy.
Stainless steel as well as glass benefit from both Pyrosil and a wipe-on adhesion promoter. Propane works well to increase the surface energy of acrylic surfaces, in addition to using a wipe-on adhesion promoter for good adhesion, shares Tourville.
“Materials with highly glossy, smooth, or irregular surfaces might require a pretreatment to optimize ink adhesion. Transparent or translucent materials, typically used for backlit applications, can also benefit from pretreatments to enhance color depth and uniformity. Additionally, when printing on materials that will be subjected to demanding environmental conditions, such as outdoor applications exposed to UV rays, rain, or fluctuating temperatures, a pretreatment can act as an additional protective layer,” says Brooks.
Another substrate increasingly printed to using digital technologies is corrugated. “This application requires the ink to adhere well, have very good print quality, and an excellent color gamut. However, a large variety of substrate types exist under the term ‘corrugated,’ and vary quite a bit in terms of absorption,” admits Edwards.
In the scenario involving corrugated, the pretreatment is in the form of a coating that is either jetted or applied via an analog technique. If printing with latex ink, “you want to control water absorption into the substrate to maximize the color and to control the drop-to-drop interactions. A pretreatment formulated to ‘crash’ the pigments is commonly used to achieve this. For UV, you similarly want to reduce absorption and control drop size. In this instance it is more common to use a polymer-based coating,” shares Edwards.
Apply It
Pretreatment can be applied inline, near line, or offline.
“Offline equipment pretreats glassware or creates custom pieces. Wipe-on primers can be used in spray guns to apply primer to an item. Corona, flame, and plasma treatments can be applied inline or as a standalone unit,” explains Tourville.
Goswamy agrees that corona and plasma pretreatment can be easily integrated into a printing line. In addition, pretreatment using a primer can also be added. “However, the use of primer in the printing line might increase the equipment footprint and may impact the printing speed. Primer may be applied manually or digitally. It is most common to apply the primer manually through rollers or spray coating.”
Marabu has several customers that utilize inline spray booths to pretreat glass. Others flame treat and post spray with a printer to enhance adhesion.
“When looking at different printing methods it is usually easier to provide an inline pretreatment on a single-pass printer than a scanning printer. This is both due to the design challenges and the fact that print quality is easier to achieve consistently on a scanning unit. Latex-type technology often does have pretreatment inline on a scanning printer due to greater challenges with coalescence. However, it is not uncommon to hand apply pretreatments to the substrate before printing on a scanning printer,” notes Edwards.
For example, Epson products do not apply pretreatment inline, but application is conducted manually. “In house we use a wipe-on adhesion promoter and spread the pretreatment on the surface of the substrates we are working with. We let it dry a few minutes before printing. For print providers looking to streamline the process, there is auxiliary equipment that does the pretreatment application,” shares Lopez.
Adhesion promoters can be applied inline if the printing device is setup to do that, admits Parsley. They can also be applied near line with a separate coating device or manually with minimal investment. Mutoh’s XpertJet 1462UF UV LED printer features an optional plasma bar.
Several of Roland’s UV printers include primer in the ink configuration. “Inline pretreatment involves applying the treatment directly during the printing process with the use of primer inks, offering a seamless integration and time savings,” shares Brooks.
Learn More
Raising awareness of pretreatment and primer options is important, especially as more substrates and surfaces are introduced and expected to be digitally printed to.
“Many customers who are new to UV printing say they want to print on glass without a primer, which will inevitably lead to bonding failure. Customers are not aware of the range of surface properties of polyurethane (PU) substrates. When the customer experiences good adhesion on one type of PU, they expect that the ink will/should work on all PU substrates. Typically, issues come up when print service providers (PSPs) move from solvent or other types of printing into UV,” shares Goswamy.
While Edwards believes print providers are generally aware of pretreatment options, this is a truer statement if their printer is already equipped with some form of pretreatment. But this is not always the case, as often service providers are told “this printer/ink will print on anything.”
“OEM printer trainers cannot train an owner/operator for every substrate so they focus on the most popular. Another issue is that new operators are taught how to use the printer, but do not receive training on chemistry, adhesion, and surface energy. Owners should provide operators with more training to help eliminate downtime and returned signs due to adhesion issues,” suggests Tourville.
Northup also stresses education—especially to those selling the printers and telling their customers that all products “just work”. Instead, “they will need to test with primers on substrates that ink will not stick to like glass, plastic, acrylic, and metal.”
“It’s not uncommon for a customer to come to us to troubleshoot an application, and when we suggest pretreatment, they are unaware of the process. So, I would say there’s not enough awareness in the market on the benefits of pretreatment and how it helps to improve ink adhesion,” suggests Lopez.
He notes that some media and other products are already pretreated and ready for ink. These are often marketed as “print ready” or “digital print ready.”
“The occasional oversight or lack of awareness among some print providers can sometimes be attributed to rapid technological advancements, where newer inks and printing technologies often reduce the need for pretreatments. In some cases, the number of substrates and applications can also make it challenging for PSPs to keep up with every best practice,” adds Brooks.
Not a Need
Much of the digitally printed output discussed today focuses on durability and longevity. This is made possible thanks to the ink set as well as a pretreatment and/or primer that allows that ink set to adhere to the surface or object in question. While ink sets like UV, solvent, or latex/resin do not require pretreatment or primers, many applications and environments benefit from their use.
Dec2023, Digital Output
