Vacuum Web Coating: Problem re. Ink adhesion to metallized film

In fact we first laminate the metal side of METPET in to 1.5 mil Polyethylene film using solventless adhesive of Liofol 7980/7275.
Once it is cured, in second process we surface print on PET side of the structure.
Following is the final structure from outside to inside:
                                    OPV (OVER PRINT VARNISH)
                                    INK
                                    PET SIDE (OF METPET)
                                    METAL SIDE (OF METPET)
                                    ADHESIVE (SOLVENTLESS)
                                    POLY SEALANT FILM
DEFECT:
      The ink adhesion is good but with slight twist of the film the ink is released from the PET showing Missout and a kind of pinholing showing metal.
      This happens mostly in shipping and in handling the package at customer plant.

      If we try to separate the layers, the metal is released from the PET but the do not delaminate with Poly sealant layer.

   How to have 100% ink adhesion on PET of METPET?

Forgot to mention one important information, sorry.
The metal on METPET is on the corona treated side.

Answer

It sounds as if the possible problem is the same for the ink as for the metal.

Corona treatment is helpful to a certain extent but is a variable treatment. When PET is manufactured there is always some residual unpolymerised material left in the polymer. This oligomer is low molecular weight and is easily able to migrate through the polymer and it will appear on the surface. The amount on the surface will depend on the age and storage conditions of the polymer roll.

If untreated and left on the surface both the ink and the metal will bond to this oligomer, which forms a weak boundary layer, and will have a poor bond strength.

Corona treatment will improve this bond but has limitations. The corona treatment needs to be optimised and also it needs to be done immediately before coatings are applied. The corona treatment varies with humidity and so what might be optimised on a dry day with low humidity may not be optimised for a very humid day. The treatment is also not a permanent treatment. The oligomer will still be present within the bulk of the polymer and with time and/or temperature it will migrate back out to recontaminate the surface.

Thus if the corona treatment is done before the metallization on both sides of the PET the metal adhesion may be at one level but as the film will have been heated during the metallization process the back surface treatment may already be recontaminated by the time the film leaves the metallizer. Similarly if the lamination is done using heat there will be a further migration of material to the PET surface.

The indication you have given is that the ink is printed onto a surface that has not been corona treated. Thus the ink adhesion could be improved using a corona treatment before the printing.

I personally would not rely on the corona treatment for the metallization but would consider using a plasma treatment within the metallizer, if at all possible, so that the treatment is immediately before the metallization and there is less chance of re-contamination. I would also use an oxygen containing plasma to make sure that I removed as much organic contamination as possible. An argon plasma has no mechanism for removing organic material. It just knocks material off the surface which can re-deposit on the surface. Oxygen will convert organics into volatile species that can be pumped away by the vacuum pumps. Also the oxygen will form bonds on the surface that will also form bonds with the aluminium and increase the bond strength.

If you cannot carry out a plasma treatment but only have corona treatment as an option I would make sure the process has been optimised and would check that this has been optimised at different humidity levels. I would also make sure that the time the roll is stored between corona treatment and metallization is minimised again to minimise the time for recontamination.

Re-contamination occurs by two processes one is the migration of material from the bulk as mentioned above and the other is by transfer of material from one surface to another. Bear in mind that, as the back surface has not been treated, the low molecular weight, and often low surface energy material on the back surface will be brought in contact with the high surface energy front surface as the film is re-wound. As nature tries to equilibrate energies it is preferable for some of this low energy material to transfer from the back surface to the front surface.

This is the reason for minimising the time the film is standing between corona treatment and metallizing.

Hopefully this explains a possible source of variable adhesion and some options to correct the problem.