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Technology Focus

Guidelines for UV Coating Applications

by Kenneth Corman, Westlam

February-March, 2008
As the use of ultraviolet (UV) cured coatings increases in popularity within the printing and packaging industries, so does the need to understand and monitor the potential for problems. As the extremely high gloss of UV coating exceeds all other coatings (i.e. varnish, aqueous coatings, and even in-line UV coating on press equipment), off-line UV coating will enhance the surface of whatever it is applied to. In addition, UV coating does not create environmental problems (as do solvent-based coatings) and when cured properly, the UV coating will polymerize to become a hard layer, which in turn provides an excellent protective surface for many applications.

In order for printers to properly prepare the printed sheets for UV coating, there are certain guidelines that should be followed. The following information will help ensure that the finished product meets the expectations of the both the printer and the final customer.

Preparation
To guarantee that the project gets off on the right foot, a layout sheet should be provided to the UV coater, indicating guide and gripper sides with final trim marks. Areas to be left uncoated (such as glue flaps, etc.) should be marked. Allow a gripper of ?" and a minimum of ¼" from the image to the edge of sheet on the remaining three sides. 

When spot coating, a film positive (right reading, emulsion down) with registration marks should be supplied. Be sure to tape the film in position on a press sheet pulled from the center of the load, and strip it into position. Larger sheets such as 28" x 40" and book-weight stocks have a tendency to grow when absorbing moisture on rainy or humid days and shrink on warm dry days, which affects film fit.   

Inks
UV-compatible inks must be used in conjunction with UV coatings. In the past, all that was required was for the inks to be wax free. Considering the chemical make-up of inks and the solvents used in them today, UV coating has three requirements.

First, the inks must indeed be free of paraffin waxes, silicones, and teflon additives. Although these additives are used in inks to resist scuffing, they greatly reduce the adhesion of the UV coating to the inks.

Second, the pigments used also must be UV-compatible. Today, all UV coatings contain optical brighteners which, in conjunction with the alkalines in pigments, can cause certain colors to bleach to a lighter shade. The colors most prone to bleaching are Reflex Blue, Rubine Red and Rhodamine Red, and any PMS color containing these colors, such as purples.

The third requirement is that the inks are hard-drying, so as to resist the heat generated by the UV curing lamps. This is to avoid softening the inks under heat, and offsetting at the delivery and when stacking in skids. Whatever inks are used, ample drying time should be planned in the production process, usually 24-48 hours depending on the coverage.

Varnish and Aqueous Coating
Although varnishes and aqueous coatings can be UV coated, caution must be used as to which coatings are uv-compatible. Varnishes and aqueous coatings will seal in wet inks, slowing the drying process and when UV coated, will result in unacceptable adhesion. Press coatings (varnish / aqueous) fall into two categories:

Solvent Type: Solvent should be avoided as it uses extremely hard resins or wax additives, which make adhesion to the substrate virtually impossible.

Water-based Topcoats: Water-based topcoats are usually an acrylic type and many of these will not allow UV coating adhesion. While water-based primers are recommended and usually work well, they should be tested for compatibility with the UV coating used prior to print production. 

Additives
To reduce Volatile Organic Compounds (VOCs) and improve air quality, printers have had to eliminate the use of Isopropyl Alcohol in dampening solutions. Alcohol was used to improve the rheological properties of the ink and promote faster ink setting and drying. With the introduction of alcohol substitutes replacing Isopropyl Alcohol, specific problems have resulted in the printing / UV coating process.

After the sheets are printed and the aqueous coating is applied, wet ink becomes sealed between the sheet and the aqueous coating. The emulsified ink and alcohol substitute (which has a high vapor pressure) is then trapped, which significantly slows the drying of the ink and ultimately requires two to three times the normal drying time. The glycols and glycol ethers used in alcohol substitutes act as plasticizers, softening the ink which results in poor UV coating adhesion. To reduce adhesion problems, pressmen should run the lowest water setting possible and use fountain solutions, which contain the lowest levels of glycol and glycol ethers. The correct aqueous primer coating will greatly affect adhesion with UV coating.

There is a word of caution in regards to offset spray powder. Although some UV coaters have in-line sheet cleaning systems, these units are not a cure-all. Spray powders should be kept to a minimum and all sheets should be properly dusted on both sides prior to UV coating. Spray powder on printed sheets will be intensified when UV coating is applied, resulting in an unacceptable sand paper effect.  

Scoring, Folding, and Gluing
Any scoring required on a job should be done after the sheets have been UV coated. Normal grain direction will help ensure that a coated sheet scores and folds correctly. When coated paper stock is to be printed with a heavy coverage of ink, UV coated, and then scored and folded, cracking can occur at the fold. This is an indication that the stock may not be suitable due to the clay coating on the stock. This can be caused by stocks that have been stored for long periods of time or by slight changes in the chemistry of the clay coating at the mill. The clay coating should be one that has sealed the sheet well so the UV coating stays on top during the curing process, achieving the deep mirror gloss or ‘wet’ look.

Other Considerations
Some stocks have a clay coating that is porous and can absorb UV coating before it is cured, causing a streaking effect. Cast-coated sheets, such as Kromekote and King James, should not be used in conjunction with UV coating.

Please be aware that when UV coating darker solid colors, fingerprints will be more likely to show than on lighter solid colors or process colors. Uncoated stocks also should be avoided with UV coating. When choosing a stock to be UV coated, always be sure to use an acid-free stock. Acids in stock can cause the UV coating to ‘yellow’ and also can create adhesion problems.

It also is best to avoid UV coating glue tabs whenever possible. The coating will deter glues from adhering during the folding/gluing process. Although there are hot glues now on the market to work with difficult coatings, the best solution is to keep coatings off the glue tabs altogether.

As most finishers know, UV coatings are nearly impossible to foil stamp, therefore the foiling process should be done prior to UV coating. Foils that are compatible to UV coating should always be used. Embossing should be done after the UV coating process.  

Metallic inks should be used with caution in conjunction with UV coating, as these are actually tinted varnishes with metallic particles. As the varnish dries, the metallic particles rise to the surface and rub off easily, creating an unstable surface. This makes a very weak bond for the UV coating adherence. In the majority of cases, the UV coating will flake off unless the metallic is sealed for protection. If an aqueous coating is used, it must be sealed after the metallic ink dries thoroughly or the wet metallic ink will be sealed under the coating and will not dry sufficiently to allow the UV coating to bond. A water-based primer may be used if ample time is allowed for drying - usually a minimum of 48 hours prior to the UV coating process. 

A Final Word
For best adhesion results, inks, varnishes, and aqueous coatings must be free of waxes, silicones, and Teflon additives. Also, special attention should be given to press chemistry since alcohol substitutes and one-step fountain solutions contain glycol, an oil-based product. UV coating will not adhere to oil. Please consult your supplier to be sure of product compatibility.

In print production, prior planning is essential. Please check with your UV coating company regarding equipment capabilities, overs required for set-up, makeready, and run waste before printing your job.

Kenneth Corman is president of Westlam, a UV coating and laminating company in Santa Ana, Calif. He can be reached at (714) 641-1400 or by e-mail at ken@westlam.net.