By Melissa Donovan
Printing directly to glass with digital printing technologies is possible with more than one type of ink. Proprietary digital ink sets are offered from vendors, including chemistries made up of nano-particles of glass or frit, pigments, and solvent. These are very specific to printing directly to fritted glass. Frit is a mixture of silica and fluxes fused at high temperatures that makes glass. This article looks at UV ink sets used for direct digital printing on all sorts of glass. UV ink’s chemical base allows for proper initial adhesion as well as remaining intact throughout the duration of use.
UV ink is broken down into flexible or rigid configurations. In addition, it can be segmented as free radical—adhering to multiple substrates or specially formulated for printing only to glass. Adhesion promoters or primers are commonly used. Pre- or post-coating treatments are favored depending on the environment and glass type. Manufacturers considering direct digital printing to glass should be aware of all the options.
Above: An example of a piece of glass printed on a swissQprint flatbed using UV ink and handled with a robotic arm.
Deep into Chemistry
UV ink is used for direct printing to glass. The two main reasons for this is, according to Michael Terlizzi, managing partner, ITNH, Inc., first UV ink is designed to cure and bond to the surface instead of penetrating into the glass. Secondly, UV-curable ink sets usually include an opaque white, which maximizes printing opportunities.
Bob Keller, GM, Marabu North America, divides UV ink into two categories—rigid and flexible. Rigid UV inkjet inks offer high reactivity, resistance, and color density, whereas flexible inks offer high adhesion and flexibility. “When selecting raw materials for a UV-curable ink, there is a broad spectrum of monomers from which to choose. Generally speaking, there are tradeoffs associated with these choices. In the case of glass, adhesion and resistance are typically the most important, but are on opposite ends of the spectrum.”
“In general, rigid digital UV inks are preferred for a number of reasons. First of all, you don’t expect glass to flex, so there is no need for added polymers in the ink. This also includes added adhesive properties, though they may still require the addition of a primer,” explains Hugo Gonzalez, senior applications specialist, Mimaki USA, Inc.
Conversely, Stephen Chrisos, marketing manager, Inkcups, argues that flexible UV ink is ideal for printing directly on glass. “In my experience, there is a big difference between a hard and soft ink in terms of adhesion. Flexible ink lasts significantly longer with a higher abrasion resistance. It also passes crosshatch testing opposed to hard ink.”
UV ink—flexible or rigid—that adheres to a range of substrates is referred to as free radical. But, certain UV inks are specifically designed for printing solely to glass. “These contain heat-reactive adhesion chemistry, requiring heating to 200 degrees Celsius for five to ten minutes after printing and UV curing. This provides a higher level of adhesion and scratch resistance,” says Phil Jackman, global product manager – digital, Sun Chemical Corporation.
Diving into ink chemistry even deeper, Mike DeRan, chemist, Kao Collins Inc., further compares free-radical and cationic UV inks involved in glass printing. “Cationic inks exhibit less shrinkage due to the way the polymerization happens during the reaction. Free-radical ink curing usually involves cleaving off a proton from an acrylate, which then starts the chain reaction. Cationic photo initiators instead have a ring opening mechanism that then starts the chain reaction of polymerization.”
Polymerization speeds up when heat is introduced to the post-curing process. Because of how the cationic photo initiators react during this process, they exhibit stronger chemical bonds to surfaces such as glass, increasing ink adhesion. DeRan explains that the improved adhesion is due to the chemical anchoring groups that form during the reaction or are present in the formula.
A Helping Hand
Sometimes ink can’t do it all on its own. In certain cases an adhesion promoter or primer might be required when printing to glass. Application methods include by hand or inline as an ink channel on a printer.
“Glass is notoriously difficult to achieve adequate ink adhesion. For maximum adhesion, it is important the ink dots wet out—or flatten like a pancake—when landing on the substrate as opposed to stating a more dimensional sphere shape. The natural surface tension of some glass does not allow for this to happen on its own,” shares Terlizzi.
Gonzalez explains that adhesion is directly related to a surface’s dyne level—or the measure of surface energy. When a surface’s adhesion ability is measured, it is assigned a dyne level. “Some types of glass have a good enough dyne level to print with good adhesion without the need for primer, but most do require some sort of adhesion promoter,” he continues.
“There are different types of glass, for example, glass bottles for the beverage industry, pharmaceuticals, and glass windows. The surface energy of all these types of glass is different,” explains Mohammed Siddiqui, chief ink architect, InkJet, Inc. “In most cases, an adhesion promoter is required, especially in glass applications. Dry glass requires a silane to anchor the ink to the glass. Similarly, if the ink must adhere to cold filled bottles that may have condensation, a different kind of adhesion promoter is used.”
Free-radical UV inks are more likely to require a primer or adhesion promoter, admits Heather Rockow, business development, Kao Collins. “Most UV and LED inkjet inks are free-radical chemistry, and are easier to formulate, a little less expensive, and adhere to a range of substrates—other than glass and metals,” she notes.
“If the application is subject to extreme environments such as the high heat and humidity associated with commercial and home dishwashers, an adhesion promotor is a requirement,” adds Keller.
Terlizzi suggests applying adhesion promoters by hand immediately before printing. This temporarily alters the surface tension, which allows the ink to wet out properly and achieve better adhesion.
High-adhesion UV inks are often used in conjunction with a wipe-on adhesion promoting primer to enhance adhesion, points out Jackman.
Wipe-on primers are one method, while another is printable primers as part of a standard ink set offered on a printer. These are run in an ink channel and can be selected based on the job. “The big advantage of a printable primer compared to a wipe-on primer is that it is an efficient and clean solution when not printing the whole area of the glass,” says Mike Kyritsi, president, swissQprint America.
“A printed primer is applied in the areas where it is required. This also means there is no wasted primer due to over application. And there is also no additional cleanup after printing to buff out excess areas,” recommends Gonzalez.
Jay Roberts, product manager, UV printers, Roland DGA Corporation, adds that onboard primers are on demand, increasing convenience and often allowing for a more invisible look on the glass.
Additional coatings are required based on glass type. Glass may also need to be pre- or post-treated for reasons including enhanced protection or aesthetics.
“Whenever there is a need to increase durability or adhesion on the glass, employing a pre- or post-treatment helps to achieve the desired results. We often see glass tables post-treated to protect against liquids and chemical cleaners. The clear coat acts as an increased barrier to the glass object,” explains Roberts.
Just as different types of glass require an adhesion promoter or primer, they may also require a pre-treatment. One example is container glass. Jackman says when used with direct digital printing, it is critical this be pre-treated to achieve optimum ink adhesion. “The reason is that glass containers commonly have anti-scuff coatings applied during manufacture. To achieve adhesion, the surface of the glass must be altered,” he advises.
Based on Jackman’s experience, glass surface tension can be changed by two different pre-treatment methods. The first involves spraying an aqueous coating pre-treatment applied as a fine mist and them flame dried. The second option includes applying a flame and a proprietary liquid mix to the surface, which leaves silicon dioxide on the glass and makes it hydrophilic. This technology is available from Applied Surface Technologies, LLC and is referred to as Pyrosil.
The flame involved in Pyrosil removes impurities from the glass. It also applies a fumed silica structure to the surface, which results in a microscopic series of branches that the adhesion promoter and inks can bond, explains Keller.
“We are big fans of pre-treating with flame and chemical inkjet in the flaming process,” agrees Jim Lambert, VP/GM, INX International Ink Co. “It is one of the most reliable ways to gain adhesion on glass. These processes are tried and true, its origins came from screenprinting.”
Free-radical UV inks typically require a pre-treatment to enhance adhesion and durability. “These pre-treatments are commonly liquids wiped on the glass or flood coated on the glass before printing to promote adhesion,” explains Rockow. She likens this type of pre-treatment to a nail polish base coat.
Siddiqui says other pre-treatment coatings used in relation to digital printing to glass include epoxy or polyester.
For post-treatment, coatings like a clear over varnish on the printed area adds durability as well as a desirable look if the finish is gloss, suggests Lambert.
Vendors offer other tips and tricks that are important when it comes to prepping glass prior to and after printing to ensure the graphic remains on the glass and doesn’t flake or chip off, smudge, or fade.
Safety first, says Matt Meany, senior application specialist, North America, Agfa Graphics. “Take the necessary safety precautions before even handling the glass. Glass is a heavy substrate. Wear gloves, protective eyewear, and shoes. We recommend using tempered glass, which is up to five times stronger due to a heat treatment that ‘toughens’ the glass. Car windows and shower doors are all tempered glass.”
Surface preparation cannot be overlooked. “It must be clean of all oils, dust, and dirt to ensure adhesion. One of the most overlooked processes is the way the articles are handled and the overall environment itself. It is crucial to be careful and aware of fingerprints, smudges, or dust on the glass before or during the printing process,” explains Lambert.
Recommended cleaning tools include acetone or isopropyl alcohol, applied with a lint-free cloth while wearing cotton gloves, according to Meany. He believes a second cleaning with demineralized water is helpful.
Prior to printing, check that the “float” or “tin” side of the glass is the side that will be printed on to achieve ideal adhesion, advises Chase Pender, marketing manager, Supply55, Inc. To determine the correct side, Pender says illuminate the glass with a black light—the float or tin side of the glass will glow.
Roberts adds that depending on what appearance the manufacturer is trying to achieve, other tactics such as sandblasting or chemical abasing are common. Not only does this increase durability, it also gives the glass a frosty appearance—creating an artistic look without effecting the project’s design.
Once the printed glass is in place, Gonzalez suggests a few basic steps for taking better care of the application. “It should be kept out of direct sunlight. When cleaning, use a water damp, soft cloth. Also, keep the print away from high-traffic areas where it may be scratched and touched often.”
Lastly, Terlizzi strongly recommends testing before any job. “When it comes to printing glass, it is important to remember that with UV-curable inks, there are typically multiple ink types available. We always offer and recommend to test on several inks to see which will perform best for the particular application.”
UV Prints to Glass
UV ink is a blanket term. It can be broken down much further, to flexible and rigid, free radical, or more specifically created UV ink sets just for surfaces like glass. There are many options. Depending on the glass type and intended application, one or more are a fit. It seems we’ve just scratched the surface of digital printing directly to glass.
Oct2018, Industrial Print Magazine