#1426
Author: Charles A. Bishop, C.A.Bishop Consulting Ltd.
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Subject: Poor bonding of paperboard and metallized PET
If I understand the process correctly you are coating the paperboard and laminating it to the metallized PET using an UV adhesive. What I am not certain of is at what point the adhesive is UV-cured. If it is cured after it has been laminated to the PET then I would suspect that the UV is being absorbed by the metal layer, and this is preventing the adhesive from being cured to the correct extent. Since UV is attenuated with depth and aluminum is opaque to UV, a very high UV intensity would be needed to cure through the aluminized PET.
If the adhesive is cured before the lamination stage, making it a contact adhesive, I would then suggest that you check the adhesion strength against a standard material. UV lamps are notorious for degrading in performance due to a degradation in the lamp. The dopant within the lamp can condense out around the electrodes, and the UV output declines rapidly. The lamp appears the same. It lights up, and the color seems unchanged, but this is true at 100% output and 10% UV output. This is not true for the microwave-powered electrode-less UV lamps. This type of lamp is much more expensive but has a much more consistent output.
If you do not have a standard material to use to check the adhesion level, it is possible to measure the UV output with a radiometer, which can check the UV output when the lamp is new and as it ages.
If the adhesive is OK, then the surface it is adhering to needs to be checked. It is possible that a slip coating has been applied to the PET to make it handle better. This could act as a weak boundary layer and cause poor adhesion. This sort of information may be available from the PET supplier. Alternatively it is worth checking the surface energy of the polymer. Also check the polymer surface energy where the adhesive has been separated. The adhesive may have taken off the contamination and this will result in the surface energy at the area of separation being higher than it was before the lamination was attempted.
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