By Cassandra Balentine
The latest UV digital ink sets are challenged to adhere to an increasing amount of surfaces, including pre-manufactured objects. Proper curing, inks, primers, and adhesion promoters are all essential considerations for ensuring a strong bond to the surface. As adoption of UV ink continues, expectations and demand also grow.
Phil Jackman, global product manager, digital, Sun Chemical, points out that unlike media, which is designed solely to be printed onto, pre-manufactured objects have a primary role to function as that object. “This will mean exhibiting surface properties that are not often designed for printing onto, such as its topography as well as the surface energy and ink receptivity. This is especially true when the item is made of molded plastics, often with additives to enhance slip and flexibility.”
It’s also important to understand that there are more factors at play than adhesion when it comes to creating a strong bond to a surface with UV ink. “Specifically, the surface energy of the substrate must be such that it will support proper inkjet droplet formation or wetting. Some materials with inherently low surface energies will result in a beading effect. Without a method of pretreatment, image quality will be unacceptable. As a rule of thumb, there should be a minimum of 20 units difference between the surface tension of the ink and the surface energy of the substrate. Only after this first hurdle is overcome can adhesion be considered,” explains Bob Keller, president, Marabu North America.
Utilizing a high-adhesion UV ink is the most important consideration to provide the best chance of success on these surfaces, though Jackman admits it may be necessary to employ surface treatment such as plasma or corona to enhance the adhesion in some instances. An alternative approach would be to incorporate a primer to promote adhesion.
Ken Parsley, product manager UV/MP, Mutoh America, Inc., agrees that adhesion on non-traditional substrates can be enhanced by the use of adhesion promoters. He says testing is required to find the best one available for each ink set and substrate combination.
Above: swissQprint offers a universal ink set complemented with a printable primer—the primer is added using one of nine color channels available in every swissQprint system. One of the nine color channels can also hold orange ink.
Driving Demand
Several trends drive demand for more adhesive UV inks or a combination of elements that will create a strong bond to a new surface. Much of this demand comes from direct to object as well as industrial printing and the desire to print to non-traditional substrates like glass, stone, metal, and wood.
“As our world recognizes the value of digital printing, UV inkjet is finding its way into many new segments. We are seeing demand increase in wallcoverings, floor mats, architectural applications, and automotive. These applications are bringing some interesting new materials and some tricky specifications. I think we’re in the infancy of these trends. We’re expecting exponential growth over the next few years,” predicts Keller.
Jay Roberts, product manager, UV printers, Roland DGA Corporation, says the end user’s desire to print to an ever-growing variety of substrates and objects has created the demand for better ink adhesion. “With today’s advanced UV technology the end user can print directly to just about any product or surface. That capability has brought about the need for inks, primers, and digital printing processes that allow for increased durability and better adhesion.”
The wide format digital printing market is continually driven by technological advancements and application requirements. Regulatory changes, faster output, printhead developments and capabilities, higher resolution, smaller drop sizes, and higher frequency droplet firing all demand developments in ink technology. “However, it is the ever-increasing variety of substrates and particularly the desire to use lower cost/quality or more sustainable substrates that continues to push the challenge with regards to adhesion,” offers Jackman.
Print service providers (PSPs) are always looking for ways to differentiate their offering to be more flexible to fluctuating demand. Mike Kyritsi, president, swissQprint America, says the better the ink, meaning the better the properties—like adhesion, post-processing capabilities, and flexibility—the wider the range the PSP can offer. “Therefore, the ink properties—and adhesion in particular—are an essential aspect when evaluating a printer.”
“Customers are always looking for the ‘Swiss Army knife’ of printers. They want versatility and to be able to print on all substrates and media types without extra steps,” shares David Lopez, product manager, Professional Imaging, Epson America. He says customer requests and market trends will always drive the industry forward and encourage ongoing research to meet needs and develop adhesive inks and reliable solutions.
The move from traditional analog to digital systems that support on demand production of shorter runs pushes interest for more adhesive UV inks, adds Pedro J. Martínez, CEO, Afford Inks.
Cost is also a driver. “Production operations are always looking for a way to reduce production cost and increase profits. Even though adhesion promoters can be very effective, they add time and labor cost to the production process. End users would prefer to forgo these added expenses,” suggests Parsley.
Universal UV
For those looking for versatility, a universal UV ink that supports both rigid and flexible substrates is appealing.
Modern presses offer options and must be able to serve multiple end applications and on many media types. The popularity of hybrid printers highlights this trend. “Sun Chemical has leading high adhesion UV inks for graphics within the Crystal product range that adhere to most of the commonly used substrates in graphics today,” notes Jackman.
Lopez says Epson manufactures its printhead and ink technology simultaneously to work together and create a reliable solution to deliver high-quality output for customers. “We chose a semi-flexible ink technology for versatility, as we didn’t want our customers to be limited to one type of output; this specific ink set allows for both rigid and flexible substrates.”
Keller believes that a single ink for all surfaces is very much a reality. “However, that doesn’t mean that we’re at an end with respect to technological advancements. As digital UV inks increase in popularity, so do the performance challenges. Improvements include abrasion resistance, UV light durability or fading, chemical resistance, and flexibility.”
Parsley feels that this goal is difficult to achieve, pointing out there are simply too many different substrate types to make a one-for-all ink set.
“One ink set for all surfaces is absolutely desirable but, in my opinion, it is far from being near due to the different nature of the surfaces of the multiple substrates,” agrees Martínez.
Ink continues to be tweaked, enhanced, and improved based on end user demand. As the printheads and UV lamps change, inks evolve accordingly. “The challenge is not necessarily the ink itself, but rather the combination of printhead viscosity and UV lamp wavelength, which need to work together effectively to cure the ink. When changes are made in the ink formulations, we need to ensure that the new ink flows properly through the printheads and the UV lamps provide optimal curing. This balance of the UV print system must occur to successfully introduce a new and better ink,” explains Roberts.
Ink Chemistry
Advancements in UV chemistry and boding agents allow for heightened adhesion to certain substrates.
“These bonding agents within our specially formulated UV inks act sort of like ‘double-sided tape’ for substrates that may have had poor adhesion previously,” says Roberts.
Juan Kim, CEO, Valloy Incorporation, points out that its specific chemistry is an internal secret, but divulges that it needs to mix three to four kinds of monomers, which has more polarity. “It is not advised to add adhesive additives,” he cautions.
Sun Chemical offers proprietary inkjet chemistry and inks with exceptional adhesive properties within the Crystal product range. Jackman notes that the exact monomer selections and combinations are optimized to provide enhanced physical bonding to the surface of many substrates. “Our low shrinkage chemistry and flexibility make the inks ideal for flatbed printers as well as hybrid printers that can accommodate both rigid and roll-to-roll media.”
Good adhesion is achieved by establishing a chemical—not mechanical—bond at the molecular level. “With a good knowledge of the chemical makeup of the substrate, inks can be designed to contain raw materials that facilitate these kinds of bonds. In particularly challenging cases, primers can be used that contain additional materials that further the chemistry,” comments Keller.
Primer Solutions
Primers and adhesion promoters are often utilized to form a stronger bond between UV ink and its designated surface, particularly if it has low energy.
Primers are a great way to increase the surface energy of a substrate as well as add chemistry to improve the chemical bond between an ink and the surface, according to Keller.
UV adhesive promoters and primers offer an improved method of adhesion with the inks and are popular wherever substrates have sub-par adhesion when printing directly. “Adhesion promoters can be used for appliance, industrial, and automotive parts,” shares Jackman. First apply an adhesion promoter, which can be wiped, sprayed, or printed depending upon the application, and then the UV ink is applied. The result is a graphic image that is better bonded to the item’s surface. “The goal is to avoid the need for primers; their use is the exception.”
Parsley suggests that adhesion promoters can be very useful. “There are quite a few on the market. Most of them specialize in certain types of substrates. Unfortunately the only way to know you have the best one for the substrate being used is to test each one individually.”
Martínez adds that primers will certainly help to provide adhesion or even to achieve higher levels of adhesion. “Also other processes such as corona treatment or plasma will boost the adhesion on plastics or glass.”
Kim believes primers helps UV inks bond to hydrophobic surfaces like glass. Non-toxic acrylic primer is recommended in general.
Although UV inks have improved tremendously in recent years and adhere to a range of rigid and flexible materials, there are still some substrates—like glass or aluminum—that require a primer when using a universal ink set. To avoid this, dedicated ink is an option. “If a PSP mainly prints on glass, for instance, they may opt for an ink set specifically developed for this substrate that does not need a primer for good adhesion,” offers Kyritsi.
However, for those who print on a large variety of different substrates, swissQprint offers a universal ink set that can be complemented with a printable primer for glass and metal applications. “The primer can be added using one of the nine color channels available with every swissQprint system,” adds Kyritsi.
Roberts says Roland considers the primer to be an ink, much like gloss and white. “Our special ‘color inks’ are jettable or printable, which allows for optimum print quality and very clean, controlled ink laydown. The ink basically goes directly to a particular part of the substrate, rather than being applied across the entire area of the substrate, which could cause a disruption to the print area.”
Curing Considerations
Curing is also an essential part of the UV printing process. Once UV inks are cured, a bond is achieved through a chemical reaction. “In a UV ink this reaction starts when the photoinitiators are exposed to a particular wavelength of light—typically 385 to 395 nM. Nearly instantaneously a reaction occurs, which creates polymer chains from the UV monomers in the ink,” explains Keller.
He points out that even though the bulk of the reaction occurs immediately, a 100 percent reaction can take up to 24 hours. “A broad spectrum UV lamp is often used as a post-treatment method to speed the completion of the reaction as well as provide varying wavelengths, which penetrate the final ink to varying depths. This improves curing at all layers.”
Drying or curing is completed at a specific UV wavelength and in addition proper ink flow through the printheads is essential to the overall process. Roberts warns that too much or too little ink viscosity can cause clogging or poor jetting, resulting in lackluster print performance. He explains that the balance between the printheads, inks, and UV lamps is crucial when it comes to ensuring outstanding image quality, increased ink adhesion, and consistent print performance.
Traditionally, conventional mercury UV was perceived to have better overall cure properties than UV LED as it emits a wider range of wavelengths, which is better for activating the photoinitiators in ink to start the curing process, shares Jackman. He says that LED lamps, because they have their output concentrated at higher wavelengths—versus the spread of wavelengths produced by conventional UV lamps—are ideal at penetrating through the ink and providing a good cure at the base, which helps adhesion. “However, LED is not so intrinsically good at surface cure. Therefore, we have optimized the photoinitiator and synergist combinations to provide a good surface cure too. This formulation approach also yields considerable benefits in single-pass applications where LED is starting to be used.”
Parsley suggests that each substrate should be tested for proper lamp power as reflectivity varies greatly with different substrates. If the lamp is not strong enough the ink will not cure completely and adhesion is compromised. “Too much UV power will result in harder, more brittle ink, which will also reduce the ink-to-substrate adhesion,” cautions Jackman.
Martínez also feels that lack of shortness of power may have adverse effects on the adhesion of the substrates. “Therefore, it is important to ensure that the right curing energy is provided for each substrate. In some common materials, such as self-adhesive PVC, an excess of curing will also make the vinyl brittle.”
Kim points out that curing speed needs to be faster and ink layer shrinkage by curing should be minimized. For this purpose, the right monomers and photo catalyst should be used.
Good adhesion is a combination of the right ink set with the correct curing settings. “Hence, when we develop an ink set, we also define the best curing settings for it,” says Kyritsi. swissQprint machines work with UV LED lamps. Every printer has two UV LED lamps with ten LED segments each. Every one of them can be controlled individually, this enables exact adjusting of the curing settings to achieve optimal adhesion on a variety of different media.
Lopez shares that the Epson SureColor V7000 uses a UV LED light to cure the output. “We tested the ink and UV LED light curing intensity to get the perfect drying and ink bonding time. After months of testing, we determined an ideal drying method that works for a variety of materials, including wood, glass, plastic, and metal, so customers don’t need to adjust any settings or worry about drying times—they can just print and focus on creating high-quality, sellable output.”
Continued Advancements
UV ink is continuously advancing and utilized on more surfaces. To ensure a successful application, it is essential that a strong bond is created between the ink and intended surface.
“As the wide format market is very performance driven, UV and particularly LED inks will remain the chemistry of choice for the foreseeable future as they offer wide media capability and inherent adhesion performance,” says Jackman.
We look forward to covering continued developments in ink, primers, adhesion promoters, and curing technologies for UV printing.
Sep2021, Digital Output
