By Cassandra Balentine
The quality and performance of digital print technologies improve with the help of pre- and post-printing options in the form of primers and coatings. A variety of primers are available to enable proper adhesion between a substrate and ink or toner. Coatings are applied post print and can be both decorative—high gloss, matte, satin; and functional—low curl, low migration, and can also offer a tactile effect—pearlescent.
Both primers and coatings play an essential role in the digital print production workflow. “Without them, the substrates available to print on would be limited and the protection and functionality of your printed material would be greatly reduced,” shares Thomas Hayes, president, Kompac.
Above: Kompac offers a range of finishing options with a complete line of UV, aqueous, and specialty coatings and primers.
Primers and Coatings
Primers and coatings play different functions in the digital print process. Primers provide a better surface for ink adhesion and drying, and enable print quality. Coatings protect the ink and substrate, as well as add effects post print.
“In the past, priming and coating was relatively simple. However, the shift towards digital print technology and inkjet printing has created new challenges because coatings and primers are printer and substrate dependent. As a result, print producers are always looking for new solutions to everyday problems,” says Hayes.
He explains that this began with the HP Indigo, which required primed substrates for both papers and plastics to achieve the best quality. With the industry shift toward digital technology, companies like Canon, Konica Minolta, Ricoh, Xerox, and Xeikon now have comparable digital presses, each with their own individual toner and ink print systems. The same is true with inkjet. “As a result, coating manufacturers have come up with new coatings that adhere to both pre- and post-finished products to provide customers with excellent quality and consistency,” he shares. The process starts with the printer and is fueled by what the customer wants to produce. In some cases, customers want to reduce total cost by priming their own materials; others want to apply a high-gloss, UV flood coating to protect their printed output; and in other cases, features such as spot coatings are used to enhance the aesthetic value.
Jan Denies, marketing manager, specialty printing, Michelman, points out that together, primers and coatings offer increased resistance to printed material, including water, chemical, scratch, and thermal resistance as well as prevention of color fading. “Typically primers help improve adhesion to the substrate and print quality. However, it really depends on the substrate and the technology you print with if you utilize coatings, primers, or both. For example, if you print with HP Indigo, you will utilize primers for improved adhesion. When printing with UV inkjet on paper, your technology will need to provide both improved adhesion and print quality. The primer improves hold out, reduces bleed and mottle, and makes colors more vibrant.”
“For narrow format, the print house may not realize the heavy lifting done by the primer/coater as the substrate is prepared for digital print prior to entering their facility,” shares Kelly Lawrence, global market manager, digital print enhancers, the Lubrizol Corporation, a Berkshire Hathaway Company.
Digital print is a system of machinery, chemistry, and software that requires collaboration among the parties involved to enable a quality print with a seamless production process. “If further durability is required for the application, an overprint varnish may provide greater durability for spot finishes such as high-gloss areas,” adds Lawrence.
John P. Vogel, president, Chesapeake Digital Consultants and consultant for Alliance Technology Corporation (ATC), says primers are required for some digital printers to provide adhesion to stock, achieve the required sharp printing, seal the sheet to provide a consistent look on the printed piece, and provide proper surface tension for 3D structures.
Topcoats are an integral part of digital printing, regardless of the print method used. Melissa Walsh, product manager, digital media, Acucote, points out that there are materials that will print without a topcoat across a variety of print technologies. Specifically for labels, she suggests running topcoated materials designed for the specific print system to enhance the image quality.
On the other end, some coatings are applied post printing. They play an essential role in delivering functionality, protection, workflow, and aesthetics to digital production printing—particularly in the digital package printing market, says Heather Buchholz, marketing program manager, Sun Chemical.
Coatings add protection to printed materials. “Digital inks may lack good rub, scratch, and water resistance as well as other environmental issues required by the finished product,” says Vogel. Coatings are also used to add features like high gloss, matte, and semi-gloss. Special effects like soft touch, strike through, and pearlescent are provided with UV coatings.
Application of Primers and Coatings
Depending on the specific environment, the application of primers and coatings vary. Primers are applied prior to printing—either by the print provider, mill, or inline—a variety of coating units, including blanket, gravure, flexography, screen, inkjet, anilox roll, and roller coating systems.
“These coater types may be applied in a typical offset press, a standalone unit, or part of an entire finishing system,” says Matt Apke, product manager, coatings, Kustom Group. He explains that aqueous coatings are dried via hot air and infrared (IR), while energy curable materials use exposure to UV or LED light and electron beam technology to cure.
Denies says Michelman’s water-based coatings are applied with gravure or flexography and need to be dried before printing.
Primers are applied inline or offline, and press owners can choose to do the priming themselves. “Typically, printers apply primers inline while paper mills, substrate vendors, manufacturers, and converters apply primers offline,” offers Denies.
Several factors impact curing and drying—is the coating water-, solvent-, or UV-based? What is the coat weight? What is the substrate? “A longer dryer system with lower temperatures may be used to cure a coating on a BOPP film versus an absorbent substrate that can withstand higher temperatures such as corrugate board,” states Lawrence.
The application of coatings requires a skilled operator with knowledge of coating equipment. “Like any equipment, regular maintenance is required to yield good product performance and production rates. The manufacturing footprint should also be assessed. Total cost of ownership is a consideration prior to any capital investment,” says Lawrence.
Advantages and Challenges
Primers provide more flexibility in terms of the substrates that are able to run through a digital press.
“Printers with offset presses have a large stock of offset papers. When they purchase digital equipment, they utilize offset papers and other substrates because of access to inline priming,” says Denies.
Apke agrees, noting that primers provide a better print surface for improved graphic quality and adhesion. “Primers can also be used to improve drying capabilities of certain inks,” he adds.
Both primers and coatings are used to improve functionality and quality, which allows for printing higher end applications. “Priming and coating technology allow for higher levels of water, thermal, mechanical, and chemical resistance that allow printers to enter into markets they would not otherwise be able to,” shares Denies.
These solutions enhance overall print performance with improved color pop and durability by enabling ink to anchor onto the substrate. “The higher performance required the more critical the coating/primer,” says Lawrence.
Coatings help deliver the required surface. “We are starting to see smart OEM initiatives that rely on coating strategies designed to expand printing applications by enhancing interaction between primers, inks, and overprints. These strategies should deliver better matched systems, contributing to better performance and higher productivity,” says Buchholz.
Coatings can also control the finish—gloss versus matte or clear versus opaque, offers Lawrence.
One challenge is finding a product to satisfy all OEMs within a print category. There is no one size fits all. “The media that passes and is approved by one OEM may need adjustments by another OEM—even though both presses are of the same print technology,” offers Walsh.
The challenges around primers revolve around adhesion to multiple substrates, use of multiple formulations and chemistries, and limited success in providing a solution for aqueous inkjet printing. Also, paper primers may not be the same for plastic.
Apke explains that HP Indigo ElectroInks require a primer that is only available in aqueous chemistries, limiting drying and application capabilities. “Aqueous inkjet relies heavily on absorption and air flow for drying and print quality, making its use on top of non-porous substrates or energy-curable coatings extremely challenging.”
Walsh adds that when a product services multiple print methods, it is not always economical. “As a supplier, the number of topcoat vendors remains limited. Working with these vendors to formulate new topcoats takes time. It is also a developmental process to ensure the proper coat weights are applied and efficient drying takes place,” says Walsh.
Apke says coatings must overcome surface tension dynamics created by surfactants in inkjet inks, carrier solvents in wet toner inks, and drum release fluids of dry toner inks. “These materials create low surface energies that are difficult to wet out and adhere to. Unique chemistries and higher cost materials are used to supply effective coatings. Certain substrates are challenging—especially extremely porous materials that require coatings with higher viscosities and other unique properties to effectively protect and dry.”
Additionally, some digital primers have a limited shelf life for the stock. “Care must be taken to rotate your stock and use it in a timely fashion. That is why some companies prefer inline priming or priming offline, in house, as needed. Others have chosen to purchase pre-primed stock from a paper converter and order as needed, but stock selection could be limited,” says Vogel.
He adds that not all UV coatings lay well over every digital ink. “Some digital inks require a primer over the print to accept the UV top coating. Others could require an IR unit or corona treatment unit to change the surface tension of the digital ink to accept the UV top coatings.”
He adds that there is a difference between top coating inline versus offline.
As digital printing technology continues to gain acceptance in the overall industry, the bar is heightened for quality and performance expectations.
Walsh says as OEMs continue to advance presses and inks, it is important to be at the forefront of product launches to ensure the current solution is still viable.
The role of primers and coatings changes with the evolution of inks, toners, media, and equipment. “It’s based primarily on the outcome of requalification. With this being a Labelexpo year, I suspect media suppliers will be working diligently to qualify substrates on the latest press innovations and new ink technologies,” she says.
As end user driven performance expectations continue to rise, primers and coatings are asked to bridge the gap between ink and substrate performance, comments Lawrence.
The increasing functionalities of primers, coatings, and inks expand the applications of the printing and packaging industry. “Technology allows for inserting some primer functionality into ink, but it will make ink more expensive with potentially higher maintenance costs. This brings the balance of cost and functionality to the forefront. Primers play a more important role as presses are utilized for more creative and demanding applications such as labels for personal care products, edible oils, and household chemicals. There will always be a need for primers because they allow the ink and substrate to work together for higher quality output, especially as the industry demands more from its presses and moves from paper substrates to synthetic substrates,” says Denies.
Vogel believes the role of primers and coatings is changing as printing companies seek to transition more print from analog to digital. “The challenge for paper, coating manufacturers, and coater manufacturers is that the digital equipment manufacturer offers solutions that differ from one another—digital inks, printheads, and software vary greatly. Some companies sell their inkjet technology to more than one digital equipment manufacturer. For the printer, choosing the correct digital printer for today’s work and tomorrow’s can be confusing,” he admits.
Primed and Coated
The need for a primer is dependent on a variety of factors within a digital print setting. Primers and top coats are used to optimize compatibility between certain media and equipment, improving adhesion for better quality and durability.
After a print is imposed, coatings add protection, vibrancy, and special effects, adding value to the application.
Finding the right combination of primers, coatings, and media requires a knowledgeable staff and clear communications with the press manufacturer and media supplier. dps
Sep2018, DPS Magazine