- January 01, 2002, Dr. Richard M. Podhajny, Ph.D., Contributing Editor
One of the most common problems with printing high-slip films is poor ink adhesion. Such films have low surface energies, and the levels of slip additives make print and post-print operations a challenge. Symptoms of poor ink adhesion are poor tape adhesion, ink ruboff, and lack of ink water-resistance.
The source of these problems is the presence and population of the slip additives. The presence of the slip additives on the substrate surface creates a “boundary layer” that prevents the ink binder from “anchoring” to the substrate.
To improve the ink adhesion, the substrate can be modified to increase the surface tension of the film, or solvents can be added to the ink to dissolve the boundary layer. Whether this can be done effectively depends on the type and concentration of the slip additive. The higher the slip concentration, the more difficult it is to attain ink adhesion to the film.
Primers are used to ensure surface characteristics for some applications. One advantage of using primers is that the surface tension is predictable and not normally subject to change with time. That means it is possible to run at various press speeds and have the same substrate surface tension. On the down side is the possibility of higher solvent retention as well as the cost of applying the primer.
Use of primers for high-slip film is difficult since, like ink adhesion, the primers themselves need to anchor to the film. However, if primers can be used, the results can improve ink adhesion.
Corona treatment is the most common approach to increasing substrate surface tension and assuring good ink adhesion. But, unlike primers, corona treatment loses its effectiveness with time and may cause changes in the film, affecting such properties as heat-sealability.
Corona treatment of high-slip film will increase the surface tension, but the treatment levels fall quickly depending on the type and amount of slip additive. In-line treatment is the preferred method for corona treating of high-slip films. This provides the highest surface tension characteristics and does not allow for treatment decay.
In-line corona treatment of high-slip films has proven particularly useful when running water-based inks. In fact, it would be very difficult to use water-based inks without in-line corona treatment.
Conventional solvent-based inks have better substrate wetting characteristics than water-based inks, because some of the solvents can “dissolve” the boundary layer and allow the ink to anchor to the substrate. Whereas solvent-based inks usually will provide good ink adhesion on moderately treated films, water-based inks are less forgiving and require higher substrate treatment levels.
Water-based ink adhesion to high-slip films has always been a challenge. Earlier formulations used higher levels of VOCs to obtain the necessary substrate wetting and ink adhesion. Levels of 10% VOC or more were not uncommon. Today, demands keep pushing us to the target of 0% VOC.
Continued reduction of VOCs in water-based inks requires innovative formulation approaches to attain good ink wetting and adhesion to high-slip films. Among the formulator's weapons are crosslinking adhesion promoters, high-performance emulsions, and surfactants that can remove the boundary layer. A successful formulation provides not only good ink wetting and adhesion but good water-resistance as well.
Printing on high-slip films requires careful evaluation of the surface tension characteristics of the film, the treatment process, and ink formulation. Successful printing and ink adhesion can be attained on these films through the use of in-line corona treatment and well-formulated inks.