Transmission Efficiency of plastic Films Part 1

TRANSMISSION EFFICIENCY OF PLASTIC FILMS
been stretched during its manufacture will revert to its original dimensions when
heated. The plastics film is loosely sealed round the object to be packed, therefore,
and then heated, when a tight transparent wrap is obtained. The first application for
these techniques was in 1948, using a film bag of vinyl dines chloride/vinyl chloride
copolymer for the packaging of oven-ready turkeys for deep freeze storage. The
turkey was placed in the bag, a vacuum drawn and the bag closed with a wire tie.
The bag and contents were shrunk by plunging in a hot water bath. Shrink wrapping
is now carried out using a heated tunnel. Low density polyethylene film has been
used mainly in the field of transit packaging, where a shrink-wrapped package of,
say, one or two dozen tins or jars, collated on one or two trays, is used instead of a
fiberboard case. The mechanical protection given by the shrink wrap is not as great
as that given by a well designed fiberboard case, but is usually adequate. There is
also the psychological point that a shrink wrapped package can be seen to be fragile
and is usually handled more carefully as a result. Other advantages of shrink
wrapping, this time to the retailer who receives the goods, are:
(1) There is less packaging material to dispose of, after use.
(2) The package takes up progressively less space on the warehouse shelf as the
goods are sold, whereas a fiber board case takes up the same amount of room
whether it contains one can or two dozen.
Low density polyethylene film is also used for the shrink wrapping of whole pallet
loads. The tight, even wrap gives a more stable load than wrapping with paper plus
strapping and, in addition, confers a large measure of weather-proofing on the load.
Other, miscellaneous applications for low density polyethylene shrink wrapping
include rolls of wall paper, cucumbers and other awkwardly shaped objects. Low
density polyethylene is used for all these applications, because it is cheap, tough,
waterproof and has sufficient clarity for its purpose. In cases where a clearer or more
sparkling film is required, then polypropylene or PVC is likely to be used.
Some of low density polyethylene’s shrink wrapping markets are being taken over by
stretch wrapping. Pallet wrapping is one market where there has been appreciable
movement from shrink wrapping and some collation applications are now also using
stretch wrapping techniques.
3. PRODUCE PRE-PACKAGING:
Low density polyethylene is widely used in the pre-packaging of fresh horticultural
produce. A tough film is required for heavy items, such as potatoes, while its use for
green vegetables such as lettuces, Brussels sprouts, etc is due more to its moisture
vapor barrier properties and its low cost. There are complications in the packaging of
fresh produce, however. Respiration of the produce continues, after harvesting, so
that the package must allow ingress of oxygen and egress of carbon dioxide.
Although low density polyethylene is not a particularly good barrier to gases, its
permeability is not great enough to allow the free transfer of oxygen and carbon
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