Transmission Efficiency of plastic Films Part 1

TRANSMISSION EFFICIENCY OF PLASTIC FILMS
the use of polypropylene can be justified. One general field is in the shrink wrapping
of collations of cosmetic or toiletry items. Polypropylene film is also used for the over
wrapping of single cartons in order to give protection against pick up of moisture
vapor.
This is not shrinking wrapping although it is sometimes the case that the cartons are
quickly passed through a heated oven in order to tighten the wrap and remove
wrinkles. One of the advantages of polypropylene film in such applications (compared
with regenerated cellulose film, which it replaces) is that the moisture vapor barrier
properties are intrinsic to the polypropylene and not conferred only by the coating.
With moisture proof grades of regenerated cellulose the barrier properties of the
package are often reduced because of damage to the coating at edges and corners
of the carton.
POLYVINYL CHLORIDE:
Thin plasticized PVC film is widely used in supermarkets for the Shrink wrapping or
stretch wrapping of trays containing cuts of fresh meat. Requirements for such a film
are quite stringent. It must have a high enough oxygen permeability to allow the
formation of oxymyoglobin which gives the desired purple ‘bloom’ of fresh red meat. It
must also be tough, able to withstand low temperatures, be shrinkable and have
good clarity and gloss. The low oxygen permeability of rigid PVC is increased by
plasticization and the toughness of PVC allows thin gauges to be used, thus
increasing the gas permeability still further. A similar application is the shrink
wrapping of supermarket trays of fresh produce such as tomatoes and apples. The
relatively high moisture vapor transmission rate of PVC is useful in preventing
condensation on the inside of the film.
In thicker gauges, plasticized PVC film is used for the manufacture of sachets for
individual shampoo packs. Transparency, toughness and resistance to detergents
are all relevant properties in this application. In addition, it is possible to seal the
sachets, through the liquid, by high frequency methods. PVC sachets have also been
used for the packaging of anti-freeze and automotive lubricating oils.
In some countries, notably Italy, PVC has been used for the manufacture of heavy
duty sacks for fertilizers. But, as already mentioned, this market has been taken by
low density polyethylene in the U.K. One difficulty with early bags made from PVC
was loss of plasticizer, when the sacks were stored in the open air, and this led to
embrittlement, particularly at low temperatures. Formulations which overcome this
problem are more expensive. Difficulties were also experienced with hot filling of
fertilizers in the United Kingdom, where PVC sacks collapsed.
Unplasticised homopolymer and copolymer grades of film are widely used in the field
of thermoforming because of their rigidity, toughness and excellent reproduction of
mould detail. Copolymer grades are frequently used for blister packs while both types
34