by Cassandra Balentine
Part one of two
The digital print process is maturing at a rapid pace, enabling its use on more mediums. However, some surfaces require help when it comes to ink adhesion, and this is where primers come into play.
For companies looking to utilize digital printing technology to print directly onto surfaces like ceramic, glass, and metal, primers are an essential consideration. Applied to the surface of the material being printed, primers prepare the surface so the ink can bond appropriately. In addition to ensuring adequate ink adhesion, these solutions may also bring durability benefits including scratch, tape, wash, mark, and humidity resistance.
Improving Ink Adhesion
Primers are applied to surfaces that don’t easily accept digital ink.
Glass and metal surfaces tend to have a low surface energy, which makes it difficult—if not impossible—for UV digital inks to permanently bond to them. “Primers provide several benefits for the product quality. They also solve printing and manufacturing productivity by altering the surface for UV inkjet ink adhesion,” says Jayson French, operations, Boston Industrial Solutions.
In many cases, primers are needed to ensure proper adhesion, uniform application, and/or both instances. “Primers prepare a substrate to receive the inks that are applied to the surface,” offers Sofia Machain, product manager, digital ink division, INX Digital.
While primers are often viewed as the part of the process needed for adhesion, it also has a critical role in the visual appearance and finished product quality of the printed piece, according to Jim Lambert, VP, digital division, INX Digital. “Controlling the surface tension of the printed substrate helps control the way the ink droplet spreads on the surface, which is often called dot gain. Primers help control the drop size to ensure the appropriate color and visual appearance,” he offers.
Pedro J. Martínez, CEO, Afford Inks, agrees, pointing out that typically, primers generate an intermediate layer to provide good adhesion to a substrate, making it easier for ink to stick to this immediate layer. “Ceramic, glass, and metal are very difficult surfaces that need an additional system to be printed,” he attests.
“Primers, or what some people call“adhesion promoters, are vitally important to producing a long-lasting print or one that will stand up to wear and tear,” says Michael Perrelli, marketing director, Innovative Digital Systems.
“Ceramic, glass, and metal fall into the hard-to-print substrate category with UV ink adhesion frequently impacting quality of the finished printed product,” shares Chase Pender, marketing manager, Supply 55. Utilizing an adhesion promoter improves UV ink adhesion to most substrates.
French suggests using a dyne test to determine if inkjet ink will adhere to a specific substrate. “Dynes are a good way to gauge if the substrate has low surface energy which will in turn cause problems such as halos, dot gains, fill-ins, and beading. These are basically what we call wettability challenges,” explains French.
Dr. Arnd Schimanski, managing director, SURA Instruments GmbH, says digital printing on ceramics, glass, and metals are not stable without the application of primers, especially when the printings are stressed.
Selecting the right primer for the surface and use is essential. In addition to ensuring proper adhesion, these solutions may offer additional benefits. However, it is important to note that this is generally a secondary result and not the main focus of a primer.
Since primers provide better adhesion Martínez says as a result, other properties may be enhanced. “The final result will be variable and depending on the chemistry of the primer used,” he notes.
The key benefit of a primer is to solve printing challenges by making the ink adhere to the substrate, improve manufacturing and decorating processes by enabling printers to print on products that could not be printed to before, offers French. It is important to create a permanent bond between the substrate and the ink to improve the surface finish as well as the look of the final print and enable the product to pass scratch, tape, industrial wash, chemicals, and abrasion resistance.
“One thing to keep in mind, primers allow the ink to stick to the material and that is an important part of ensuring a durable print, but it isn’t the only thing. You’ll want an ink formulation that won’t degrade and an ink and can’t be easily scratched or gouged. Most UV printers have the ability to add a varnish—clear ink—base or top coat that can help overall adhesion, scratch resistance, and durability,” adds Perrelli.
Machain agrees, noting that the primer is usually applied to the area that will be printed on. The ink is then applied directly on top, making it adhere to the surface. “With this improved adhesion there can be scratch, tape, wash, or mark resistance. However, this is not a given. The ink itself must be formulated with these properties in mind,” she cautions.
“Yes, adhesion is just one of the benefits,” confirms Lambert. “Some performance benefits of protection are scratch, tape, and wash resistant. I like to term those as performance characteristics. Often overlooked are the visual characteristics, things such as color gamut, drop clarity and printed edge definition.” Lambert.
Schimanski adds that some studies observed its SURALink primers increased scratch resistance and wash-ability. “We interpreted this effect by additionally cross-linking reactions that will take place during UV hardening with the functional primer groups. This cross linking is related to the recipe of this products,” he explains.
The main benefits of solutions like IRIS’ Glass Primer is to improve the adhesion of UV inkjet inks on glass and to optimize the waterfastness of that bond. Fiedler says it is important to note that that UV inkjet inks in general tend to absorb water, which makes them swell and become prone to scratching. “This has nothing to do with our primer but it is a physical property of the UV inks. When they dry they become hard and scratch resistant again.”
One or More
It is important to pair the right primer for the right surface for the job at hand. There are specific solutions tailored to particular surfaces as well as “all-in-one” type options.
Perrelli argues that while there isn’t a product that can be viewed as the solution for every single product and material, there are some primers that are widely versatile. “In some instances, that one primer can handle treating many of a company’s products. Printer owners tend to have a couple of options on hand for specific materials,” he suggests.
While versatile options do exist, different types of substrates are manufactured from different components, so separate primers are often required. “For example, the most widely used glasses are silicate glasses, formed from silica, SiO2. Silica consists of a three-dimensional network of tetrahedra where every corner oxygen atom is shared with the adjacent tetrahedron. This formation is different that plastics. Plastics, also called polymers, are produced by the conversion of natural products or by the synthesis from primary chemicals generally coming from oil, natural gas, or coal. In general, plastics are composed of various elements such as carbon, hydrogen, oxygen, nitrogen, chlorine, and sulfur. Metals are made of atoms are arranged in layers, etc. As a result, each type of substrate requires a different primer, there is no all-in-one option,” explains French.
“We prefer specific primers of each different surfaces although we also offer some all-in-one primers, but the best results on a specific substrate are obtained by specific primers,” agrees Martínez.
“Different primers are needed for different applications,” indicates Machain. For example, INX has different formulas for metal, plastics, glass, and other surfaces. “Each one needs to work well with the makeup of each substrate in order to create the optimal bonding environment for the inks to adhere and stay secure.”
“We have recently sent out first trial samples of a new IRIS Universal Primer, which has been specially designed for certain plastics, metals, melamine laminates, and other non-glass surfaces. Depending on testing results we will consider introducing it to the market later this year,” shares Stefan Fiedler, owner, IRIS GLASS PRIMER.
By Hand or Machine
Primers are applied by hand, manually or automatically by machine. They are also applied inline or offline.
“Depending on the primer being used, these products may be hand applied, used with a flame treatment system, or jetted through the equipment’s printhead,” says Perrelli. “I prefer to treat the products offline as I want to maximize my printer’s output. Many of the hand applied primers will allow you a little time before printing so you can batch them as needed.”
“Different primers may be designed for hand application, mostly for the sign and graphics industries. However, there are also some primers that are designed to be applied by analogical processes, both for inline or offline applications,” admits Martínez.
“Primers are applied online or offline,” agrees Lambert. “They can be applied by hand, roller, or digitally jetted onto the surface depending on the volume and workflow.”
The application method depends on the formulation and the intended use, notes French. “Primers applied by machine are more efficient and reduce wastage. They are also suitable for long production runs and are sustainable.”
Schimanski points out that some systems are only usable in offline modus due to their drying time. Other products are used inline.
Pender adds that however they are applied, for the best results it is recommended to apply the adhesion promoter immediately before printing.
It is always important to consider safety. “These chemicals are easy to apply, but operators should always follow the necessary safety precautions when handling them,” offers Perrelli.
Primers, or adhesion promoters, serve the primary purpose of helping ink adhere to low energy surfaces. They are often required when digitally printing with UV inks to glass, ceramics, and metals. In addition to ensuring adhesion, these solutions often provide added benefits in terms of durability.
Stay tuned, in part two we discuss primers on the market and pricing considerations.
Sep2021, Industrial Print Magazine