Find out how FIM functions in a variety of high-end applications and determine whether it’s a fit for your business.
By Neil Bolding
Unfortunately, the inability of the solvent to penetrate PET means that getting good adhesion is more difficult than it is on more solvent sensitive films. Many people believe that surface energy is a key measure of adhesion. A proper scientific understanding of adhesion reveals that this is not true. Surface energy is important only to provide good wetting. Poor wetting is self-evident, and where it is not achieved, the ink will visibly repel from the surface.
With PET, the film surface is relatively chemically inert and only very aggressive (two-pack curing) systems are able to create chemical bonds. These tend to have limitations in use, such as poor pot life or poor formability, and must be used with care. With more tractable inks we must rely on the interpenetration mechanism. On PET interpenetration can only be achieved using a primer or pre-treat. These are coatings applied during manufacture of the film with complex chemistry and process technology to ensure that the coating is bonded to the PET surface.
Pre-treats are applied during the primary manufacture of the PET film itself. Primers are applied as a separate subsequent coating operation. The primer or pre-treat itself is more open to penetration by the ink solvent than the PET itself so interpenetration of the ink molecules into the pre-treat can occur giving excellent adhesion.
It should be noted that the forces used in adhesion testing of FIM components can be very high. In general, some failure is seen, and it is important to define exactly where the failure is happening. Common failure modes and other problems include:
Pure adhesion failure True adhesion failure where the film surface is left smooth and clean. Pure adhesive failure is normally a symptom of incorrect ink selection, incorrect or missing pre-treat, or very poor drying.
Cohesive failure of the ink The ink splits down the center, leaving a residue on the film. Cohesive failure is normally symptomatic of a high pigment ratio in the ink (especially metallic flake pigments) or poor ink drying.
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