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PLASTICS IN FOOD PACKAGING
183
give the film a smooth and glossy surface. White pigmented cast sheet material is used in thermoforming pots and dishes for dairy-based products.
Metallising with a very thin layer of aluminium is an alternative way to
achieve opacity by causing a high proportion of incident light to be reflected off
the surface away from the film. This technique has the added benefit of improving barrier properties.
Transparency, the opposite of opacity, depends on the polymer concerned
and on the way the film has been produced. If the film is allowed to cool down
slowly, then large crystals may be formed and this gives the film a hazy
appearance due to the diffraction and scattering of incident light by the crystals. Transparency improves as polymer crystallinity decreases and is also
affected by additives in the film. If the size of the additive particle is too large
or if, as with slip agents, they migrate to the surface, the film becomes hazy.
The surface of a film needs to be as smooth as possible to enhance the
surface for printing. A rough surface will give a matte appearance to the final
printed effect, which is usually considered to be less attractive than a shiny,
mirror smooth appearance. Furthermore, a rough surface may give packaging
machine runnability problems, as it may be difficult to make the film slide
over machine parts without creating static electricity in the film. This is overcome by incorporating food grade additives in the film. Films will also tend to
block and become adhered layer to layer in the reel. Waxes, such as carnauba
wax, are added to minimise the blocking. The action of a slip additive, such as
silica, depends on the particles of silica migrating to the surface of the film
where they act like ball bearings holding the surfaces apart.
For marketing purposes, it may be desirable to create a unique impact on the
shelf at the point of sale, and hence films have been developed which are matte
on one side and have a gloss surface on the other. This is done by casting the
film against the matte surface of a sand-blasted chill roll.
It is possible to combine streams of molten plastic from separate extruders
in the die to make coextrusions. Higher productivity is achieved for a given
thickness of film if the same plastic is extruded in two or more layers and
combined in the die to form a single film. Coextrusion is an area of rapid
development, with extruders capable of combining up to seven layers of differing
plastics to achieve specific properties and characteristics.
7.2.3
Pack types based on use of plastic films, laminates etc.
Single films, coextruded films and coated and laminated films in reel form are
used to make plastic bags, sachets, pouches and overwraps.
Plastic bags are made by folding, cutting and sealing with welded seams
which are also cut in the same operation. Pouches are usually made from laminates. They may be formed on the packing machine either from one reel by
folding, or from two reels and sealing, inside face to inside face on three sides
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