Keeping up Appearances

Learn prepress and on-press practices for ultimate aesthetic appeal using UV/EB and aqueous coatings and adhesives.

by Jessica Millward, Associate Editor

Energy-cured coatings/adhesives

When not matched with the right inks, coatings and adhesives carry the potential for poor visual performance, especially in the case of energy-cured products. Complementary coating/adhesive and ink formulation is therefore an essential first issue in identifying the source of print flaws.

SICPA Eastern Region Technical Manager Dave Steele lists the most common signs of improper coating formulation as deriving from poor wetting or poor adhesion characteristics. If the coating being employed is printed over an ink containing excessive silicone, the coating may exhibit reticulation, pinholing, etc., resulting in poor coverage and poor performance. High levels of silicone within inks also cause inefficient intercoat adhesion between the coating and the ink.

"For coating process printing, it is most effective to 'stage' the level of silicone in the inks," says Steele. This means applying less silicone in the first-down color, and gradually increasing the level in the second-, third-, and fourth-down colors if required. Other formulation possibilities for UV coatings include monomer and oligomer options, as well as specialty additives.

Perhaps the most common aesthetic challenge presented by UV coatings is a low-gloss factor. Rad-Cure's VP/Technical Director Jim Wittig identifies key gloss elements as stock type and coating weight. Porous stocks translate to a reduced degree of gloss, as do thin coat weights. A less obvious culprit, Wittig observes, may be the anilox. Ceramic aniloxes transfer more coating at a given cell count; therefore, a finer anilox (with a high cell count) will deposit a smoother coating, and, in turn, more gloss.

"Another factor could be that the anilox pattern is cured into the product, giving a rough, non-glossy look," he relates. An overly coarse roll, a high-viscosity varnish, or a varnish with poor flow-out might yield such dull results. Maximizing distance between the coating application point and the curing equipment will allow the coating to flow out before it's cured, minimizing the anilox pattern and increasing gloss.

Yellowing is commonly caused by either the coating formulation or over-cure. Wittig recommends keeping an eye on coating selection, because some yellow more than others. Also consider cure speed: "If you cure a high-speed formulation at a low speed, you could potentially over-cure the product, which will give a yellow appearance right off the press," he affirms. Lowering lamp power at slower speeds may reduce yellowing in such cases.

For both UV/EB coatings and adhesives, flow and leveling traits are significant factors in the overall finished product's appearance. As Wikoff Color Corp.'s Paul Robinson, UV/EB section manager, explains, the interval between application and cure determines how well the coating spreads on the printed surface. "A longer distance before cure allows flow and leveling to hide imperfections from film splitting," he points out. Alternatively, a lower viscosity improves flow with a shorter distance before cure.

Northwest Coatings UV/EB Product Manager Kevin Rea states laminating adhesives typically do not experience the "peaks and valleys" phenomenon because of the smoothing effect enacted by the laminating nip. However, too short a pre-cure time period in a laminating adhesive application could cause orange-peel, ropiness, or a grainy texture.

Rea emphasizes the role delivery systems can play in less-than-perfect UV/EB applications. Over-active pumping systems, chambered aniloxes, and high-speed silk screen coaters sometimes generate excessive quantities of air bubbles or "foam." Rea continues, "This can generate reductions in a cured film's smoothness and gloss due to pinholes, voids, and craters, or the unsightly speckling or 'silvering effect' seen in some laminations." While pump adjustments or line speed reductions often alleviate these blemishes, formulation changes may be required in certain instances.

Rea also reminds printers about the printing surface dyne level—it should be relatively high compared to the liquid tension of the UV/EB product to avoid beading up or reticulating off the substrate's surface. One not-so-obvious cause of low-energy surfaces could be migratory materials found in substrates, inks, or base coating layers, which move to the surface after exposure to heat or pressure. Vendor consultation, as always, is important to avoid/resolve such challenges.

Aqueous coatings/adhesives

As Cork Industries Marketing Manager Elmer Griese details, many of the application issues faced with UV/EB coatings are similar to those that crop up in aqueous coating usage. To maximize gloss level, he recommends: 1) Applying enough coating wet weight; 2) Using a smooth stock with high hold-out; 3) Using a high-gloss coating formula; 4) Using a higher-solid coating formula; and 5) Increasing coating viscosity.

While viscosity is a vital factor in the successful application of all liquid coatings, in the aqueous arena, an equally critical element is the evaluation and maintenance of pH. So says SICPA Technical Director Derrick Ayers, who explains, "Most of the resinous materials used in aqueous coatings are not truly water-soluble; thus they are dependent on pH (usually basic) to create a true solution."

An improper pH level leads to build-up in the non-print areas of a flexo plate, ultimately resulting in ghosting, "drying-in" on aniloxes or cylinders, or, in extreme cases, excessive cylinder or plate wear.

Ayers remarks on another potential difficulty leading to mediocre quality: plate swell. "Incorrect defoamer selection, or excessive glycol or organic solvent may cause flexo plates to swell, causing undesirable print quality and possible plate destruction," he elaborates.

When aqueous coatings dry too fast and become brittle, fine lines may appear, ranging in size from 1⁄64˝ to 3⁄4˝. According to Dennis Oscar, Water Base Coatings product manager for Northwest Coatings, "cracking" usually occurs over heavy process colors, and can often be eradicated by putting slightly more coating down and by lowering the stack temperature. He adds, "If these measures do not work, 0.25 percent to 0.50 percent of propylene glycol can be added to the coating to make it more flexible." The addition slows the dry rate of the aqueous coating while increasing its tendency to block.

For water-based adhesives, a near-requirement for ideal aesthetic quality is corona-treated film, in addition to standard good film/ink/adhesive compatability, and proper selection of adhesive products, maintains H.B. Fuller's Tom Rolando, program technical manager, Flexible Packaging Adhesives & Coatings Group. He stresses coating weight control in any adhesive application: "It is best to pre-qualify adhesives with inks and coatings on a lab scale or pilot line prior to any production work to determine the compatibility of chemistry involved."

Northwest Coatings Water Base Technical Manager Charles Wasserman says coldseal/cohesive difficulties arise when coated onto metallized or clear films. He counts three chief appearance problems encountered in such scenarios as streaking; uneven surfaces; and voids, craters, and crawling.

Streaking, Wasserman reveals, is caused by material build-up under the doctor blade, and evinces itself in the land area of an applied pattern. An oscillating doctor blade and micro-etching of the cylinder land areas often corrects the problem. Alternatively, the printer should employ a slower-drying and higher-stability coldseal/cohesive.

An uneven surface results from various factors, including debris from the doctor blade (as above), foam or micro-foam characteristics, and insufficient flow-out after transferring from cylinder cells. Wasserman suggests serious care when selecting an antifoam, and, to correct flow-out issues, he posits adding material which slows drying or improves product wetting on substrate.

To rid future print products using coldseals/cohesives of voids, craters, and crawling, he advises corona-discharge treatment of films. Problems arise when the primary coating surface is improperly treated, or when the surface is contaminated by slip additives such as silicone or wax.