The Root of the Problem

A troubleshooting guide to coatings and adhesives.

by Kate Sharon, Associate Editor

COATINGS AND ADHESIVES are the unsung heroes of the packaging world. They protect a package and its goods, while making it look and perform exceedingly better than it would otherwise.

But when a problem arises in the application or formulation of a coating or adhesive, those protective, enhancing, and functional substances can be the sole reason for a job's rejection.

To get to the root of such costly issues and for ways to prevent them, packagePRINTING recently asked industry suppliers for their advice.

Coatings

Coatings play a very critical role in the cosmetic and performance makeup of a package. Aesthetically, coatings add shelf appeal with the ability to add a glossy, matte, or tactile finish to the package, explained John Signet, marketing manager of Water Ink Technologies. Performance-wise, coatings are a package's shield against scuffing, wetness, and chemicals, and they enhance the substrate's printability.

However, it doesn't take much for a coating to become the root of a problem. In fact, it's as easy as changing substrates or application methods. "In an effort to control costs and provide increased value for their customers, packaging manufacturers often look at changing the packaging substrates," explained Don Matthiesen, director of marketing and communication at Environmental Inks & Coatings. "Increased absorbency, material flexibility, reduced surface tension, and various slip packages may alter the immediate outcome and worse yet, cause failure days or weeks later." And, Matthiesen continued, "Changing the application method can cause a great product to yield poor results. In particular, the application rate will effect the flexibility, adhesion, COF (coefficient of friction), and rub resistance of a coating."

There are many factors to consider when trying to pinpoint and get rid of a coating problem, of which one of the most common is blocking.

Problem: Blocking

According to Don Duncan, director of research at Wikoff Color Corp., the most common press-side problem connected to coatings is blocking. Blocking occurs when the coating hasn't been completely dried or cured in the application process.

When a water-based coating hasn't been fully dried, sheets block in the load, Duncan said. And if a UV coating isn't agitated during printing, wax can be distributed unevenly also causing blocking.

Ambient conditions can also have an effect on coatings when using a thermal dryer on water-based coatings, said Stephen Lapin, vice president, technical director of Northwest Coatings. "It will be more difficult to dry the coating on a humid summer day compared to a dry winter day," he said. "Air movement in the dryer is very important for drying water-based coatings and is much more effective than IR heaters with little air movement. Adding more heat to the substrate is not always a good thing, and can actually make blocking worse if the heat remains upon rewinding or stacking of the material after drying."

In addition, Lapin said, under curing a UV coating is most often the result of poor lamp maintenance. The varying lifetime of a lamp—which can depend on the process, and cleanliness of the environment inside and proper temperature control in the lamp housing—can be countered with proper lamp upkeep, Lapin explained.

The resultant effect of all of the above is blocking. "In these cases, the coating will not have been fully dried/cured causing wet coating or adhesive to trap beneath the surface of the next layer of substrate above," explained President of Newchem Corp. David Newell. "The end result is a roll or stack of product that is adhered to itself resulting usually in the complete loss of that job."

Solution: Getting rid of blocking issues is simple. Lapin recommends first making sure the right coating is being used. "Converters will often stock several different coatings for different substrates and different applications," he said. "It is important to retest and requalify the coating anytime there is a significant change in the application."

If the coating is not the problem, Lapin said, "The next thing to check is the drying or curing process. Of course, the faster the line speed, the more drying or curing capacity is needed."

Newell suggests that the press or web operator work with the coating or adhesive supplier to establish the proper drying parameters for the product and substrate before a specific job is run. "The run must then be practiced through a period of trial and error until the optimum conditions are established," he said. "Then production can move forward."

Problem: Yellowing

Ultraviolet light is the culprit behind yellowing of a coating. "Yellowing is a problem that may occur when a finished product is exposed to unusual amounts of ultraviolet light such as close exposure to fluorescent light, or storage near a large window where sunlight passes through, or in an exterior application where there is an exposure to sunlight," Newell said. "In these instances, the ultraviolet light advances the aging of the coating and results in a yellowing appearance to the printed substrate that cannot be reversed."

Solution: The best way to prevent yellowing is a good working relationship between the operating personnel and the supplier, who can help plan ahead for the type of environment a finished product will be exposed to, Newell said. How the print job will be stored, its expected shelf life, and whether it's an exterior or interior application are all aspects that need to be determined and considered before a job is run.

If that doesn't work, Newell said, "There are non-yellowing additives that can be incorporated into an adhesive or coating to help prevent this occurrence."

Problem: Brittleness

Seen with UV coatings, brittleness results in cracking during scoring or poor scratch resistance—a symptom often confused with poor adhesion and cure. But really, Duncan said, "The issue is that the coating formulation is not matched to the UV lamp energy being used, and the coating is being cured beyond it's design criteria. This causes excessive crosslinking and results in brittleness."

Solution: Lamps, an aspect unique to UV coatings, are an important part of eliminating brittleness. A converter's coating supplier needs to know the lamp configuration being used, and if more lamps will be added to the current system, Duncan said. When a supplier knows the details of the operation, the right coating can be matched to the right UV lamp wavelengths and wattage for optimum curing.

Adhesives

Adhesives are literally the glue that keeps the whole package together—in more ways than one. "Adhesive applications range from laminating adhesives used to combine two substrates together to produce one finished printed product, to pressure-sensitive adhesives applied to the reverse side of labels for final adherence to products for identification or sale," Newell said.

While adhesives are used across the board in all kinds of package printing, the rigid demands of flexible packaging are driving advances in the substances. "While adhesive development has usually kept pace with market need, the fast growth in flexible packaging has spurred packaging scientists to develop new films and packages that satisfy the need for higher performance at a lower unit cost," said George Papsin, flexible packaging lab manager at H.B. Fuller. "Flexible packaging adhesives must provide excellent adhesion appearance, strength, and finished goods resistance properties. They must also be extremely user friendly, allowing for ease in coating and laminating, and allowing for a wider range of processibility."

Today's adhesives are versatile and—more than not—live up to their requirements. But when an adhesive doesn't deliver the desired properties, it can mean failure of the entire package.

Problem: Lamination Defects

According to Papsin and Newell, there are many possible causes of delamination and appearance defects. Compatibility issues between the adhesive and the ink or substrate choice; poor curing; improper or lack of surface treatment; and poor formulation of adhesives are just a few examples.

Solution: In order to prevent lamination defects, the converter must work closely with the adhesive, the film, and the ink vendors in the design and planning stages so that adequate testing can be done.

For example: UV adhesives need to be formulated with a clear understanding of how to overcome the possible inhibition reactions when used over water-based inks, said Zubair Khan, EIC's Energy Curable Products manager. "If adhesives are not properly formulated, they could result in undercuring and lower bond strength," Khan said. "This undercuring may result in pigment bleed. As a result, the lamination bonds will be lower in those areas where there is presence of uncured adhesive. Working closely with both the adhesive supplier and a knowledgeable ink supplier can help avoid this problem."

Furthermore, other solutions to correct appearance defects include changing the film, modifying the laminating process, or changing the adhesive. "Film selection must be made with the expectation that film clarity and performance usually have a positive correlation with price," Papsin said. "A hazy film will not get better after it is laminated." Papsin also suggests watching process conditions like corona treatment level, line speed, and roller condition, as well as a good maintenance program to keep appearance issues at bay. Also, he said, "Make sure the adhesive has the proper rheology and leveling properties for the application equipment and film choice.

Problem: Adhesion

Poor adhesion may be a result of more than one factor. According to Papsin, there are three main causes of poor adhesion: film and print surface issues, low adhesive coat weights, and improper adhesive selection. The underlying diagnosis is usually insufficient bond strength, resulting from the wrong choice of adhesive, Newell said.

Another possible cause of poor adhesion, especially when printing a non-porous package surface, is improper wet-out, said André DiMino, ADM Tronics marketing manager. Wet-out and adhesion are completely intertwined, DiMino said, "due to the fact that without proper wet-out, it is impossible to achieve good adhesion."

Solution: Low surface tension can be corrected with in-line or pre-treated substrates, and will make the substrate better able to hold inks and adhesives.

Low coating weights can be fixed at the coating stations or by ensuring that the correct amount of adhesive is specified for a given application. This is essential, since coating weights control the rate of application, and in turn "the application rate will effect the flexibility, adhesion, COF (coefficient of friction), and rub resistance of a coating," said Matthiesen.

The application rate can be tricky, however—it can change inadvertently due to switching coating roller manufacturers even when the roller specifications are the same. "In addition, coating rollers may become plugged or prematurely worn, causing a change in coating weight if not closely monitored," Matthiesen said. "Frequent coating weigh checks will control this process."

But first and foremost, Papsin said, "It is imperative that an adhesive be adequately tested for every laminate structure." This testing includes ink compatibility, visual appearance, peel adhesion, and desired resistance properties.

Problem: Undercuring

When a UV or EB adhesive isn't fully cured, bond strength after application may fail standard bond testing, creating a situation where the curing environment of the adhesive lacks strength, Newell said. And, Kahn added, with undercuring comes the potential for migration of adhesive components, pigment bleed, lower bond strength, and—ultimately—product failure.

Solution: From the start, it's necessary to establish the correct amount of UV light and EB needed to cure the adhesive. This isn't something a printer can often do alone. "Usually, the adhesive supplier must determine the available amounts of UV and EB curing capacity and provide an adhesive that best fits within those parameters," Newell said.

Proper maintenance of UV curing equipment including cleaning the reflectors and bulbs is very critical, Kahn said. Dirt on the reflectors of a UV lamp "alters the UVC band of the UV radiation," and the adhesive most likely won't be properly cured.