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Aug 30, 2001 12:00 AM
From the late 1960s, the high-gloss, instant-drying and excellent resistance properties have made UV the standard for coating products. Early attempts, however, to put UV coatings over conventional inks were disastrous, since the UV and conventional chemistries were very different. Poor compatibility resulted in poor adhesion.
Printers also found that the initial high gloss off the press was not long-lasting. Depending on the coverage of the ink and its formulation, the substrate and press conditions, the gloss could drop as much as 30 points. Known as glossback or dryback, its most significant impact was a lack of gloss uniformity. Dark colors and areas of heavy ink coverage would lose the most gloss, while light coverage and coating over the unprinted substrate would remain very glossy. This variation in gloss, as well as its unpredictability, left the printer in an untenable situation.
Printers found, however, that if they allowed the conventional inks to dry prior to coating, the UV products would perform quite well. The only problem: Conventional inks required spray powder to prevent them from setting off during the drying phase. When the dried products were eventually UV coated, the spray powder would blemish the sheet and appear as small pimples in the otherwise smooth, glossy UV coating. This was not an aesthetically pleasing sight.
To overcome this deficiency, printers found that they could aqueous-coat the sheets in line and minimize the use of spray powder. This led to a desirable result, but required a second pass through the press, or the use of an offline coater to apply the UV coating. The second pass requires more labor, equipment, materials and time. All of this results in higher costs.
In an attempt to overcome some of these drawbacks, several press manufacturers developed presses with double coaters. Basically, two coatings were put over the conventional ink in one pass, via two coating units on one press. First, the water-based coating was applied to seal and prime the conventional ink, and then the UV coating was applied to provide the high gloss.
The concept was sound, but the real-world implementation did not work well. The difficulty was, and still is, adequate drying of the water-based primer. The true drying of the water coating requires the acrylic emulsions in the coating to coalesce. This is not readily sped up on press without causing other problems. Despite the installation of double-coater presses and considerable effort, these machines remain marginal relative to inline coating. There are some excellent examples of jobs printed in this manner; double coaters, however, have not proven successful on the wide variety of job situations that printers must handle.
While equipment changes were being tested, the coating and ink formulators were also trying to develop technologies that would allow the inline UV coating of conventional inks. Several years ago, some success was encountered with inks that were a mixture of conventional materials and UV-curing materials. Results were encouraging, but problems existed: Poor runability on press, instability in the can, gloss inconsistency and roller problems were common. Experiments also found that a lamp placed prior to the UV-coating unit caused the ink to set up and “holdout” the UV coating for high gloss. With this knowledge, ink manufacturers formulated a new generation of hybrid inks.
Hybrid technologies such as Sun Chemical's Hy-Bryte inks are a combination of conventional materials with enough UV-curing materials to set or cure well with the lamp at the end of the press prior to UV coating. When combined with hybrid coatings and cured properly, the gloss obtained off the end of the press will remain uniform and high. Although gloss results depend on the particular UV coating, substrate and type of applicator, gloss levels of high 90s have been attained. Typical results are in the high 80s to mid 90s.
Besides the high uniform gloss of hybrid ink technology, the system provides for other important aspects of printing. Hybrid inks are designed for use on conventional rollers and blankets. This does not mean, however, that every roller or blanket is ideal for use with hybrid inks. Just as when printing with conventional inks, some rollers and blankets are better than others, and have better print qualities and lifetimes. The key to hybrid inks is that they do not require the special rollers and blankets that pure UV inks need. This allows printers to use hybrid inks when needed for inline coating and to use conventional inks when coating isn't needed. Similarly, special fountain solutions are not required.
Hybrid inks also perform on press like conventional ink. UV inks typically have a narrower water balance and are more difficult to control on press. Hybrid inks are as easy to run as conventional materials. The print properties of UV inks are often inferior to conventional inks for dot gain, trapping and print contrast. Hybrid inks are similar to conventional inks and, in most cases, identical. Since the UV portion of the hybrid ink does not “dry” until it is exposed to the intense energy of the UV lamps, it stays open on the press indefinitely and does not skin over like conventional ink.
There are some minimal requirements to run hybrid technology. At least one lamp is required after the last print unit, prior to UV coating. Depending on press speed and “typical” ink coverage, a second lamp may be necessary to prevent glossback. Of course, a coater capable of properly applying a UV coating is necessary, as well as the proper number of UV lamps at the end of the press, to correctly cure the UV coating at average press speeds.
There are several possible press configurations, with the most typical a UV lamp before and after the UV-coating unit. Although the use of interstation lamps is not necessary, several printers have found that there is an advantage to having several UV lamps at other units within the press. At least one printer has installed moveable lamps, so it can place the lamp units where most practical.
Hybrid inks are also used with water-based coatings. Printers have also found that the “set” inks have better holdout for the water-based products, which leads to improved gloss and lay. The gloss is still not as high as with a UV coating, but it is better than water-based coating on conventional inks. An added advantage is that the inks are better set or dry immediately off the press, so they can be processed quicker. Total ink-film properties require the oxidation of conventional materials, but the reduced amount and the assistance from the UV lamps and UV chemistry speed the process.
Tony Bean is marketing manager of energy-curable inks at Sun Chemical (Fort Lee, NJ).