Transmission Efficiency of plastic Films Part 1

TRANSMISSION EFFICIENCY OF PLASTIC FILMS
packaging. Autoclavable medical trays and medical blister packs are two other
applications which could benefit from PC’s clarity and heat resistance.
One popular coextruded structure is PP/tie layer/PVDC/tie layer/PP, and it has been
used to construct a number of different restorable containers. The containers are
thermoformed and are able to withstand autoclaving and subsequent reheating in
boiling water, microwave ovens or conventional ovens, up to 110°C. Gas barrier
properties are excellent and are said to be retained for at least one year under
unrefrigerated storage conditions. Coextrusions are also used on thermoform/fill/seal
machines. In Europe, structures such as PS/tie layer/PVDC/tie layer/PS have been
used for thermoform/ fill/seal yoghurt pots. The high barrier (low oxygen
transmission) properties of this structure confer a long shell life on the product (six
months or more). Other TFFS applications include the unit dose blister packaging of
pills, tablets and capsules. One coextruded structure that has been used successfully
in the U.S.A. for the blister packaging of tablets is PP/tie layer/PS/tie layer/PP.
One co extrusion with a particularly good combination of properties is that made from
low density polyethylene and nylon. It is a good barrier to both water vapor and
gases, the nylon confers particularly good grease resistance and odour
impermeability and the co extrusion is useful over a wide temperature range. In the
U.K It has been used for sachets to hold putty, quick frozen cream and smoked
salmon. In Germany it has been used, or suggested, for such diverse applications as
vacuum packaging of bacon, cheese, sausages and fish, the packaging of greases
and ointments and the packaging of liquids such as fruit juices, essences and milk.
An interesting variation is blow-up packaging where the package is pressurized with
air or inert gases. This not only provides the right sort of atmosphere but also acts as
a cushioning pack, giving protection to the contents. The nylon/polyethylene co
extrusion gives the gas barrier properties and strength required for this particular type
of pack.
Foamed plastics can also be utilized in co extrusion techniques and some have
already been developed. One example already produced in the U.S.A. is a 3-layer
film with a core of foamed low density polyethylene sandwiched between two layers
of unformed LDPE. It should also be noted that it is possible to extrude multi-layer
coatings on to substrates such as paper or aluminum foil.
More straightforward coextruded laminates (but ones that have good potential in the
field of packaging) have also been developed.
These are polyester and biaxial oriented PP with heat sealable layers on the outside.
The co extrusion based on oriented PP, in particular, is being used in the packaging
of moisture sensitive products such as potato crisps. The coextruded heat sealable
surface consists of a copolymer of propylene with a small amount of ethylene. The
copolymer has a lower melting point than that of the homopolymer and with careful
temperature control a completely satisfactory heat seal can be obtained.
39