- April 01, 2004, Dr. Richard M. Podhajny, Ph.D. Contributing Editor
When it comes to the subject of “wetting,” everyone has some degree of experience. However, the wetting of a film by a printing ink is a complicated physical/chemical process.
To begin with, printing is a dynamic process that involves wetting surfaces at high speeds. Whether it is flexo, rotogravure, or lithography, the key element of good printing is wetting the substrate.
Much of my career has been spent in efforts to improve or attain the desired wettability on a substrate for a specific printing apparatus. Ink formulators improve ink wetting through creative formulation of ink ingredients, while the printer uses press adjustments and surface treatment techniques to achieve the same end. The successful transition to printing films with water-based inks was due in large part to the use of in-line corona treatment.
The ink formulator has several variables he can juggle to get good ink wetting. The first and foremost is to see that the ink has a low surface tension. This is achieved by the choice of solvents, resins, and selected surfactants. These ink components must possess not only low surface tension, but they must possess low dynamic surface tension to meet the wetting demands of high press speeds.
If you can't improve the ink wettability through ink formulation, you might be able to improve it on press. Corona treatment of films improves ink wettability as well as ink adhesion. This complex physical/chemical process produces surface polar groups that raise the surface tension and provide the basis for better substrate adhesion characteristics. As a result of corona treatment, not only does the ink wet the film better, it wets out much faster. This is important for several reasons. The faster the press speed, the greater is the need for the ink to wet out and form a smooth ink surface. The higher surface polarity provides increased bond energy for adhesion.
Higher surface tension provides better wettability and faster surface flow. That means the ink will reach an even thickness and take out the ripples caused during the ink splitting process to apply the ink to the film surface. The benefit here is that the ink will dry faster through the press. Higher surface treatment levels can provide 100-200 fpm of additional press speed and maintain the same level of solvent retention. These results were obtained with solvent-based inks when energy costs drove everyone to optimize their press drying performance.
Besides better ink wettability, ink adhesion, and faster press drying performance on corona treated films, another unusual effect has been verified: Films that go through a corona become “surface sterilized.” This effect is not necessarily permanent, but film samples that are corona treated show greater resistance to microbe growth. It is suspected the presence of ozone and high surface oxidation conditions of the corona contribute to this effect.
Continuing pressure to improve print quality is increasing continued focus on improving film wettability. In-line corona treating offers better print quality for water-based as well as solvent-based inks. In addition, in-line corona treatment offers productivity enhancement as a result of higher press speeds while maintaining good printability and ink adhesion. Higher film surface tension provides faster ink flowout, reducing the amount of energy necessary to dry the thinner ink film.
The effectiveness of corona treating depends on the film itself and its composition. Treatment of high slip films always is more problematic, as rapid migration and orientation will lower the treatment level with time. In-line treatment is better suited for improved adhesion and film wettability.
On the down side, excessive corona treatment of films can lead to lower water resistance, since the surfaces are more polar and hydrophilic. More is not always better.
Corona treatment of films was commercialized to meet the challenge of film wettability and ink adhesion demands. It is still the primary surface treatment used to meet the stringent demands for ink wetting and adhesion.
Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 267/695-7717; firstname.lastname@example.org