Transmission Efficiency of plastic Films Part 1

TRANSMISSION EFFICIENCY OF PLASTIC FILMS
In the first case, the aqueous or solvent-based laminating adhesive is applied in
precise amounts to one web by means of direct gravure or reverse kiss coaters as
already described under the heading of ‘Coating’. The coated web is then passed
through an oven, to remove all water or solvent, and then combined with the other ply
in a pressure nip, which may sometimes be heated. The set-up is shown
schematically in Figure. Unlike wet bonding, this process is very suitable for the
laminating of plastics films to other substrates.
Factors affecting the success of this type of laminating when used for plastics films
include tension control, accurate adhesive application and accurate control of drying.
Film tension should normally be kept to a minimum and will depend on the distance
the film has to be pulled through the laminating equipment and on the sharpness of
any change in direction as it passes over the various rolls. Effective control of tension
is easiest in those machines where both the unwind station and the rollers in the
drying tunnel are in direct line. Where a reverse roll coating system is used the
reverse roll should not run too fast otherwise drag will occur, thus introducing
increased film tension. Smooth and accurate application of the adhesive is extremely
important and failure in this respect will probably lead to delaminating. Both gravure
coaters and reverse roll systems should give adequate control of adhesive
application but adhesive viscosity is another factor and this should be kept as
constant as possible.
Drying oven performance is particularly important when laminating plastics film where
solvents cannot be absorbed into the film. Excess solvent remaining in the adhesive
at the nip stage is a major cause of delaminating. Films vary in respect of solvent
absorption and cellulose acetate, for instance, is able to absorb a slight amount of
solvent without serious effects during lamination, whereas oriented polypropylene is
much more critical in this respect. The temperature stability of oriented polypropylene
is lowered in the presence of certain adhesive solvents and shrinkage can occur at
temperatures of around 100°C whereas in the absence of solvent it would withstand
130—140°C. For films such as polypropylene where drying is essential, the oven
should have a maximum flow of air.
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