The Blog recently received the following two-part question: Why do inks applied to a corona treated polyester film pass tape adhesion test but completely transferred to the polyethylene film side after lamination with solvent less two component adhesive? Is there any test other than a tape test by which we can check adhesion of ink to the PET base substrate?
Answer: In regards to the apparent change in adhesion in the lamination relative to the simple tape test there are several potential reasons why the adhesion appears to change. First, the two part adhesive sticks to the ink better than the tape does so the failure plane changes from the ink/tape interface to the ink/PET interface. The exact cause has to be determined by a chemical analysis of the two failure surfaces. So to a certain degree the adhesion of the ink to the PET has not changed, but when you change the structure of the test lamination you change the interfacial forces in various portions of the lamination and the interfacial failure plane can change as you experience.
In this case the best you can say without the chemical analysis is that the ink sticks better to the adhesive than the adhesive sticks to the polyethylene (no adhesive ink delamination) and the adhesive sticks to the polyethylene better than the ink sticks to the PET (no adhesive/PE delamination).
Part of solving the delamination “problem” is determining where you want the structure to delaminate. Ideally you might want the delamination to be a cohesive failure in the adhesive, or an adhesive failure of the adhesive to the 1) PE film or 2) the ink surface then you can change the adhesive to fail in that manner. The location of the “ideal failure” is usually dependent on which part of the lamination you supply, the printed PET or the PE film or will depend on the customer’s specification for the lamination. In reality the failure levels should be set by the lamination packaging machine and use requirements.
In regards o the question about a new tape, ultimately the best test will always be the lamination itself. So to replace the lamination with a new “tape test” you will have to find the “right” tape and test structure. In this case the “right tape” seems to be a tape which mimics the adhesion to the lamination adhesive. So, if you want to make changes to the ink/PET interface and you want to see the impact of process or ink selection on the lamination failure, then you need to find a new tape which has the same bond strength and peeling behavior as the lamination. This has to be determined experimentally, in essence you have to develop the off line or laboratory test for your specific structure/process in order to study the process without making the lamination it self.
No one can tell you which tape will work best with your inks and process, you will have to find the best tape to mimic the lamination adhesive. You will have to do that by screening tapes by measuring peel force to the ink and comparing to the adhesive. As an alternative, you can make your own tape by coating the PE with the lamination adhesive for use in lab testing. However, this may not be possible as the adhesive cures with time.