Request:
1) Which side of metallised pet film is best suited for Lamination and reverse printing.
2) How the WVTR & OTR of metallised pet can be improved.
3) How the oligomers affect the barrier properties of metallised PET.
Answering your questions in order.
1. Films can be printed and/or laminated in a variety of different
constructions. There are different grades of polymers some of which have print receptive surfaces; others have surfaces prepared for lamination or
metallisation. The preparations are usually aimed at improving adhesion and
are chemically modified to suit the particular materials to be used. Thus
the preparation for aqueous ink is different to that for solvent based ink
and both may well be different to the preparation for lamination. In
addition to this some films are co-extruded with the reverse side being
filled for improved film handling and the front surface is without filler to
have a better surface smoothness. In this case the smooth side is usually
used for metallizing to get the best specular reflectivity.
Thus without the full specification for the material it is impossible to
state categorically which side of a polymer film is best for which purpose.
The best course of action is to talk to your film supplier to confirm what
the two surfaces are designed for.
2. The barrier performance of the metallised film is primarily governed
by the number of pinholes and coating thickness. If there were no pinholes
the barrier performance would be governed by the coating thickness and the
microstructure of the metal thin film. The surface energy of the polymer can
affect the barrier performance as the higher the surface energy the better
the metal will wet the polymer surface. This in turn will produce a
continuous coating at a thinner thickness than for a polymer surface that
has a lower surface energy and where the metal does not wet the surface as
well. The more continuous the coating the fewer the gaps between the metal
crystals which are a further source of oxygen and moisture migration.
Thus the best things that can be done to improve the barrier performance
would be to improve the surface cleanliness of the polymer web to reduce the
number of pinholes. An extreme version of this has been demonstrated by the
process of pre-cleaning the polymer web but then depositing a polymer layer
in the vacuum system immediately prior to the metal deposition to produce
the cleanest, flattest surface possible. This polymer surface needed to be plasma treated to then improve the wetting of the metal and it was also overcoated with another thin polymer layer to protect the deposited metal to
prevent post metallization damage.
Another method of improving the barrier performance that has been used was
to metallise both sides of the polymer web. This works because the pinholes
are unlikely to be directly opposite to each other and so the diffusion path
is so long that there is significant reduction in the oxygen and moisture
transmission.
3. Oligomers can affect the barrier performance because they can be a
cause of poor adhesion allowing for metal pickoff or metal delamination.
They may also be of low melting point and may be vaporised by the depositing
metal causing thin metal deposition where the oligomer is vaporised. These
have the appearance similar to pinholes except that instead of the holes
being completely clean of metal they have a much thinner layer of metal that
the surrounding metal thus when backlit they appear almost the same as
pinholes.
Comments