By Melissa Donovan
Part 1 of 2
Coatings serve many purposes in the commercial print space. Commonly, they provide protection for the printed image by protecting printed graphics from wear and tear from the production process out in the world. Secondly, coatings enhance a graphic by adding a texture like a raised element or offering a gloss or matte finish.
Two main types of coatings are used in conjunction with digital printing, aqueous and UV. Aqueous coatings are a mature technology, however UV coatings offer a more energy efficient drying or curing process. The decision to use one or other is determined by the substrate, whether inkjet or toner is used in the process, and the intended reason for the printed piece.
Standing on Preference
Aqueous and UV coatings are used inline on a press or offline, depending on the volume of the printed piece. They are used for all different applications and substrates.
In regards to application, Joe Chiaramonte, VP, Coatings and Adhesives Corporation, says “UV coatings were initially applied offline on two roller coating systems. Advancements in application equipment have revived the focus on the drive to complete against analog printing and it is trending to use inline coating.”
“Coating can be applied either way and is up to the printer how much capital they are willing to invest along with weighing the advantages and disadvantages of both,” advises Matt Apke, coatings product manager, Kustom Group.
Erich Midlik, EVP, Prime UV-IR Systems, Inc., says that high-volume aqueous and UV coating is applied inline and low-volume coatings are applied offline. “The cost of aqueous is less than UV coatings. Consequently, printers may choose aqueous coating to minimize cost. Other times, the chemistry is approved for specific applications and it may require too much time and money to switch to an alternative technology. Or it may be performance based where the aqueous coating performs well for applications at a reduced price point.”
According to Robert Andrews, product manager, publication and commercial and paper-based packaging, ACTEGA North America, Inc., aqueous coatings offer superior physical hold out. This makes them ideal for use with porous substrates, which is a common media option used in conjunction with digital toners. With a porous substrate, the toner is able to print and adhere better compared to a substrate with a good solid surface.
“When coatings are applied over a porous substrate, the lowest molecular weight materials tend to absorb the stock. When this happens, the net effect is a blotchy look,” explains Andrews. Aqueous coating polymers and resins are generally over 3,000 molecular weight, whereas UV coating oligomers and monomers are less than 500 molecular weight.
The chemistry of a UV coating is one of the reasons it is so widely used. “Printers use UV coatings because they’re concerned about the environment. UV coatings offer the lowest emissions, volatile organic compounds, and carbon footprint of all coating types available,” adds Midlik.
Coatings and their Devices
Flood and spot coating methods are two popular ways to apply aqueous and UV coatings. Using one over the other is often dictated by the requirements of the customer.
“Both aqueous and UV coatings are used as flood coats. Aqueous spot coating is typically used for protection to shield an image from wear and tear. Alternatively, UV spot coating is used for aesthetic appeal. Typically UV spot coating provides far greater dramatic effect, making images pop or stand out from the rest of the piece,” says Midlik.
Apke shares that he’s experienced both flood and spot/aqueous and UV and the reason one is used over the other is based on the needs of the print buyer. “Typically, we have requests for UV spot materials because the need is for the best enhancement of some print object. Customers want to highlight something with a gloss image that stands out on a matte background or they want a soft feel coating or glitter coating on a specific image. Our experience has been UV materials provide the highest gloss and ease for applying decorative effects.”
Inkjet vs. Toner
Perhaps the biggest determining factor between aqueous and UV coating is how the finished piece was printed—or simply what ink or toner was used. Inkjet- versus toner-based output does affect the choice of coating.
“When selecting the right coating, the first thing we need to know is—is the digital output inkjet, dry electrophotography (EP), or liquid EP (LEP)? Each one offers its own set of unique characteristics that need to be covered by the coating. Inkjet ink is easier to coat over versus dry EP. Dry EP is generally much more difficult to get adhesion to versus LEP,” suggests Andrews.
There are variations of toner. “Not all toners are made from the same materials and the decorative and functional features of the coating needs to be considered. The gloss level and appearance in combination with the adhesion to the toner surface are just a few considerations. The surface dynamic of what type of surface the coating will be applied over does require different formulations,” explains Chiaramonte.
“The types of toner; wet or dry, dictate the type of coating required to achieve the desired gloss and protection,” adds Apke.
Inkjet also has its own sub-sectors. “Please note inkjet has a range of meanings. It is important to clarify the type of inkjet printing you or your customer will be doing. Dye, pigment, UV, and solvent inkjet systems could all require a different type of coating to be used,” notes Tom Yeager, VP sales and marketing, Strata-Tac.
Aqueous and UV coatings each have a place. Their use truly depends on the customer’s preference in regards to intended cost, volume of print, and even whether they are looking for a more sustainable solution. A number of options are available. The next part in this article series highlights some of the products.
Dec2019, DPS Magazine