It depends on what technique the converting operation employs, its age and performance criteria. The issues around ultraviolet (UV) coating were explored at length in the April issue of Converting. Most of our paper contacts are finding that UV coating systems are being developed in-line, since off-line application is fraught with both environmental challenges (ventilation) and maintenance issues (lights, etc.).

Aqueous coatings, off-line or in-line with offset printing, seem to be a more sustained trend, particularly in Europe. In this column, we'll assess both systems.

From a paperboard perspective, UV coatings can be applied universally, i.e., onto everything from untreated corrugating to coated cover stock. UV coatings are easily applied, but not often easily dried, depending on the stock used and UV system employed. The stock and system will also have a great affect on the final print gloss. But in all systems on all stocks, UV coatings produce "orange peel"—a surface quality that can affect the gloss and rub resistance of the coated substrate.

Orange peel refers to the surface of a coated substrate that has small pits in no certain order (like the skin of an orange). It is produced in nearly every coating system, from paper machines to direct coatings.

The thickest films will have little orange peel, but the thin coatings, like UV, have pronounced orange peel.

UV coating is often compared to varnishing—a thin layer of a synthetic material dried by lamps. Unfortunately, this is the perfect machine for producing orange peel. When the coating is applied to the paperboard, depending on the quality, the coating will fill and sit on the board surface, since it, too, will have an orange peel-like surface, or, if uncoated, a very rough surface.

The UV-coated board travels through a series of lights, which apply BTUs fairly uniformly across the sheet. However, whatever deep pits and high peaks are already fixed in the coating process become more pronounced in the drying process.

If the process does not produce a hard varnish or quick cure, the result can be mottling or variable gloss at best, poor reproduction, galvanizing and delamination at worst.

If you operate a UV coating operation, ask your vendor about orange peel and its predictability on their stock. Most boards will be sold with a smoothness quality (PPS or Parker Print Surf, Gurley Density or air permeability) and perhaps a reading on ink rub resistance or abrasion. The paper mill produces data and trends on all of their products, and these should be reviewed and available, especially if you are using roll stock. Paper mills also have R&D arms that can research your specific situation before you buy their products.

Because of the environmental challenges facing UV systems, aqueous coating systems seem to be an attractive alternative, both for in-line and off-machine applications. The application of various films and drying technology are both advancing very fast, so that aqueous systems are competing well in print/gloss performance with UV and other coating systems.

Aqueous systems also work with offset printing to produce excellent quality image and text reproduction for the high-end markets like cosmetics, apparel, jewelry and even DVD and CD packaging.

The same quality considerations that are an issue with UV paperboard can affect aqueous coatings, such as smoothness, coating brightness and strength, and orange peel effect. But more importantly, the tendency for the paper surface to absorb, or its porosity, becomes a factor.

Since the aqueous coating is likely to have a film thickness greater than UV, the orange peel effect is not likely to dominate. The coating simply fills in all the peaks and valleys at the same caliper, and drying is more uniform. Of course if the surface is too rough, then big problems with mottle and gloss can result. But most quality products are not likely to have those problems.

Only uncoated and brown grades will have high roughness values, which are likely to be intolerable in high-end converting applications.

The combination of a very high-end coated paperboard and a modern aqueous coating system can produce the same high-quality gloss result as the best UV system today. This is true for two-sided products and single-pass runs.

The key is the absorption property of the board surface. A surface that is too porous, and allows the aqueous coating to sink in, will reduce print gloss and produce mottle.

However, a coated board with strong water resistance and hard "sizing" keeps the aqueous coating and ink on the surface where it can be dried more uniformly. This produces an ink density of color and text acceptable to the most critical designers.

Ask your mill rep to provide you with some insight into their coating process. How do they develop surface strength? What absorption values correspond with their brightness targets? Buying on brightness alone can be a flawed approach when applying board in an aqueous coating system.

Again, the best vendors will provide sample services and trial runs at their R&D facilities to help you make the right choice of stock when running your converting system's coating line.

Michael J. Ducey

Consulting Technical Editor

415/928-7297

paperfo@earthlink.net