Coating Control, Quality, & Paperboard Packaging

It is often forgotten the vast majority of folding cartons are coated with either a water-based or ultraviolet (UV)-cured coating. In fact, most new presses sold today include a coating unit. UV coatings are chosen for their gloss and chemical resistance. However, they are significantly more costly and difficult to use than water-based counterparts. Water-based coatings are one of the smallest components of a printer's cost, usually much less than 3% of the total cost of the package. At typical coating weights, one might apply much less than 1 cent of coating per sheet (assuming 40×25-in. sheet; 0.5 lb/1,000 sq ft dry coating wt; $1/web lb coating cost; and 40% solids coating).

Even so, a poorly chosen or applied coating can result in rejected packages or customer claims. Although the coatings are relatively inexpensive, they can affect other components of cost dramatically, including excessive press downtime and press operator intervention.

This article will review why packages are coated, what is meant by consistency, the importance of coating weight, and controlling coating viscosity.

Why Coat Packages?
The reasons for coating a package depend somewhat on your perspective. The consumer associates the quality of the product with the quality of the package. The package buyer is looking for performance: a specific gloss level (high, satin, or matte) or a particular coefficient of friction (COF). These are critical surface properties. Of course, since the printer sells to the package buyer, the needs of the package buyer become those of the printer as well. The printer also must deal with production concerns such as ease of use and dry speed.

Since the offset ink can take several hours to cure, water-based coatings are used to provide a quick-drying, protective seal. The packages then can be handled without damaging the ink, reducing the printer's processing time and work-in-process. Printers can use larger lifts, reducing handling costs. Work-and-turn coatings are used so the second side can be printed soon after the first side.

The coating helps reduce the need for offset powder that can give the packages a dusty or sandy feel. Coatings also are used in flexographic- and gravure-printed cartons, mostly to impart critical surface properties.

Critical Surface and Barrier Properties
The coating is the last “layer” on the package. The rough surface of the substrate's clay coating is covered with the relatively smooth water-based coating. Therefore, the water-based coating will influence the surface properties of the package greatly. These coatings are formulated to provide specific surface properties, including gloss, rub resistance, COF, chemical resistance, and barrier properties.

Gloss is one property that is strongly influenced by the coating. Higher gloss often is preferred for greater customer appeal; gloss is associated with a “premium product.”

Alternatively, some designers specify a lower gloss or matte finish to differentiate their packages better. However, packages often require properties other than gloss, such as COF and scuff resistance.

The COF or slide angle is important for smooth processing through downstream operations, including die-cutting, gluing, and high-speed filling lines. Cartons with incorrect COF will not travel up inclined ramps or pass smoothly through the filling and folding operations.

Safe handling of the filled product in warehouses and at the retailer is important, especially for beverage cartons. Filled cases should not slide off of stacked piles, yet they should not stick together (block) — even under hot, humid conditions. Coatings can provide resistance to scratching and scuffing during the forming, filling, shipping, and display of the package. Coatings can provide chemical and water resistance as well. This is important if the package contains a liquid.

Regulatory compliance can be another property of interest, especially for food packaging. Food and Drug Administration (FDA)-compliant coatings can provide a barrier to cover the noncompliant ink. The coating prevents the ink from coming into contact with the food product. FDA-compliant coatings can be formulated using resins and emulsions having direct food contact for all types of paper under all use conditions except extended shelf storage after cooking in the package.

A note of caution: The coating does not control the surface properties completely. The substrate and ink also can play a role. Table 1 shows gloss and slide angle measurements for the same coating, printed over three different substrates. The choice of paperboard strongly impacts these surface properties.

Table 1
Some Measured Surface Properties for the Same Coating Over Different Substrates
Substrate 60 deg Gloss Slide Angle (deg)
CCNB-Clay-coated news back 66 44
SBS-Solid bleached sulfate 58 28
SUS-Solid unbleached sulfate 73 34

Reasons for Coating Consistency
Coating consistency is important in two major areas. One is the critical surface properties we have discussed. The other is operational consistency. That is, does the coating continue to run in a consistent, reproducible manner on press?

The physical properties of the coating, like the viscosity, can change with time, leading to operational problems on press. The press operator will need to stop and adjust the press settings or the viscosity of the coating to maintain optimal press speeds and avoid “slinging” and “blocking” problems. These two facets of consistency should be considered from several of points of view:

  • Within a print job: Does the coating change during the print job?
  • Shift to shift: Do different press crews use different coating application conditions?
  • Print job to print job: Does the printer get the same coating properties and performance every time he runs the same print job?
  • Season to season: Does the coating perform in the same manner in the winter as it does in the summer? Does your coating supplier need to sell you summer and winter formulae to compensate for seasonal changes?
  • Press to press: Do different presses require different coating formulae for the same print job due to coater design differences? Does the appearance of the print job depend on the press used? Can the package buyer expect consistent-looking packages on the shelf? Do the packages produced in one plant look the same as those produced in another plant?

Controlled Coating Weight
Coatings are formulated to give a specific balance of properties. However, for the coating to work as designed, it must be applied at the correct rate.

Coating weight is the amount of coating applied to the substrate per unit area usually reported in either in g/m2 or lb/1,000 sq ft. If coating weight is inconsistent, other properties will be inconsistent as well.

There are operational issues as well. Coating weight also influences the drying rate of the coating on press. If the coating weight is too high, the sheets cannot be dried adequately before they are stacked, and they will stick together (blocking). One can sometimes observe an “orange peel” pattern.

At the other extreme, if not enough coating is applied to cover the ink, the printer will encounter offsetting and blocking as the wet ink is not properly covered.

How Are Coatings Applied?
Water-based coatings are applied in-line on the printing press. Offset, flexo, or gravure presses all can have coating stations. The coating station type depends on the press type. Flexo and gravure presses use these same techniques to apply the coating as they do to apply the ink. On offset presses, the coater usually falls into one of two categories; roll coater or anilox (flexo) coater.

There are many different configurations of roller coaters. Each type has a roller that picks up the coating from a coating pan. The coating is metered by another roller, transferred to a blanket or plate cylinder, and applied to the substrate. The press settings, including relative roller speeds and the pressure between the rolls, are used to adjust coating weight.

Coating viscosity has a strong influence, since higher viscosity coatings tend to transfer more to the substrate, leading to higher coating weights. The speed of the press also will influence the coating weight, with faster speeds tending to transfer more coating.

Anilox cylinders have small, engraved cells that meter the coating onto the plate. The volume of the cells controls the amount of coating transferred. Cell volumes are reported in cm3/m2 or in BCM (billion cubic microns/sq in. of surface area). Approximately 19%-25% of the cell volume is transferred from the cells to the substrate.

On a roll coater the coating weight is influenced by the press settings, press speed, and coating viscosity, whereas in the anilox configuration, the cell volume mostly controls the coating weight. The printer simply chooses the cylinder with the right volume to provide the desired coating weight. The dry coating weight can be estimated using Equation 1.

Equation 1

CW = TE/100 • CV • ρ • S/100

Where
CW Coating weight (g/m2). Note that 1 lb/1,000 sq ft = 4.8823 g/2
TE Transfer efficiency (as a percentage)
CV Anilox cell volume (cm3/m2). Note that 1 BCM = 1.55 cm3/m2
ρ Coating density or specific gravity (g/cm3). Note this is usually about 1.0-1.05 g/cm3. ρ= 1 can be used with very little loss of accuracy
S Percent solids of the coating

This calculation is done most easily using the metric system. Imperial units also can be used, but the various volume and area conversions required to convert microns, square inches, pounds, gallons, and square feet make the calculations much more cumbersome. If desired, BCM can be converted to cm3/m2, and g/m2 can be converted back to lb/1,000 sq ft easily.

Consider this example. The cell volume is 22 cm3/m2 (14 BCM), the transfer efficiency is 25%, and the coating is 40% solids, the applied dry coating weight is approximately that of Equation 2.

Equation 2

Dry coating weight = 0.25 • 22 cm3/m2 • 1g/cm3 • 0.40 = 2.2 g/m2 (0.45 lb/1,000 sq ft)

The anilox cell volume controls the wet coating weight; therefore maintaining constant coating solids is important to maintaining dry coating weight. Higher coating solids lead to higher dry coating weights.

An identical coating weight analysis can be computed for gravure coating as well. In this case, the transfer efficiency is usually double that of a flexo coating, around 40%-50%. The transfer efficiency can be somewhat influenced by the cell geometry and coating viscosity, but the major determining factor of coating weight is the cylinder cell volume.

Importance of Coating Viscosity
Coatings must be used at the correct viscosity for the particular printing press. When the viscosity is too high, the rollers pick up too much coating, and the coating sprays off the rollers onto the press (slinging) — especially at higher press speeds. Moreover, too much coating is transferred to the substrate, causing coating drying and blocking problems.

To correct for high viscosity, press operators add water to the coating. On the other hand, if the viscosity is too low, not enough can be transferred to the substrate to get the desired coating weight. Coating viscosity decreases as the solids decrease. This is why adding water to the coating decreases the viscosity; it decreases the percent solids. If the coatings are adjusted incorrectly, you may encounter problems with the coating weight and critical surface properties.

All coatings lose water to evaporation as they are used on press. The percentage of solids increases, causing an increase in viscosity. Many press systems circulate coating between a drum and the press. The viscosity of the coating continually rises in the drum as water evaporates. It is most severe on roller coaters, where a thin film of coating is continuously exposed to the air. Printers report they find “sludge” in the bottom of the drum of coating. This is the high-viscosity coating that resulted from 8 hr or more of water evaporation.

Temperature also affects coating viscosity. As the temperature increases, the coating viscosity decreases.

What Viscosity Can Do
Transfer on roll coaters is influenced by viscosity. Low viscosity will result in low coating weight. High viscosity leads to “slinging,” drying, and blocking problems. Uncontrolled dilution reduces the coating solids and dry coating weight in an erratic manner, resulting in uncontrolled changes in the surface properties of the package:

  • Within a print job: Coatings can increase in viscosity over time due to the evaporation of water, leading to operational issues. The careless addition of water will lead to uncontrolled changes in coating weight.
  • Shift to shift and print job to print job: Coatings can be supplied at higher-than-press viscosity. Each press operator then adds water to reduce the viscosity. Since each press operator might prefer a different viscosity, or is less careful in water addition, the applied coating solids — and probably coating weight — will be different for each operator.
  • Season to season: Coating viscosity will change as the temperature changes. Press operators might adjust the solids of the coating to maintain the desired viscosity.
  • Press to press: Each printing press might require a different coating viscosity due to differences in coater design. Each coating will be diluted by a different amount to achieve different coating viscosity. This means the solids of the coating will be different on each press. Therefore, even if each press operator applies the same wet coating weight, the dry coating weight will be different for each press.

In summary, when a printer allows inconsistencies in coating viscosity, he or she can expect to find inconsistencies in press operation, package appearance, and performance.

Dr. Paul Gloor is senior engineering associate in the New Business Development group of Johnson Polymer, Sturtevant WI, a producer of polymer styrene acrylic resins for use in the printing and packaging market. He has a Ph.D. in chemical engineering from McMaster Univ. His experience includes technology manager for Claris Technologies (a div. of Johnson Polymer) and research and engineering positions within the product and process development groups of Johnson Polymer. He has been an industry consultant dealing with polymer reaction engineering and computer simulation modeling. He is a US patent holder and the author of several published papers. Contact him at 262/631-4967; Paul.Gloor@jwp.com.

The views and opinions expressed in Technical Reports are those of the author(s), not those of the editors of PFFC. Please address comments to author(s).


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