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PLASTICS IN FOOD PACKAGING
193
biaxially in two directions results in a film with a high strength. This amount
of biaxial orientation increases the area compared with the area extruded by
a factor of 25, and the film is, proportionately, reduced in thickness. It is
possible to produce oriented film with a consistent thickness of 14 μm for
packaging.
PP and PE have the lowest surface tension values of the main packaging
plastics and require additional treatment to make them suitable for printing,
coating and laminating. This is achieved with a high-voltage electrical (corona)
discharge, ozone treatment or by gas jets (Grieg, 2000). These treatments lightly
oxidise the surface by providing aldehydes and ketones which increase the
surface energy and therefore improve the adhesion, or keying, of coatings,
printing inks and adhesives.
OPP film is produced in widths of up to 10 m or more to achieve cost-effective
production. The limiting factors in production are either extrusion capacity for
the thicker films or winding speed for the very thin films.
Most extrusion units today have more than one extruder, thereby enabling
production to run at higher speeds and the use of different polymers feeding
into one common die slot. Typically, a film will be made up of three or five
layers of resin. The centre layer may be a thick core, either opaque or transparent, secondary polymer layers may have special barrier properties or pigmentation, and the outer layers may be pure PP resin to give gloss to the surface
and/or protect the inner resins should they be moisture sensitive, as is the case
with EVOH. In addition, thin layers of special adhesion-promoting resins,
known as tie layers, may also be extruded.
The range of food products packed in PP films include biscuits, crisps
(chips) and snack foods, chocolate and sugar confectionery, ice cream and frozen
food, tea and coffee. Metallised PP film can be used for snacks and crisps
(chips) where either a higher barrier or longer shelf life is required. PP white
opaque PP films and films with twist wrapping properties are available. There
are several types of heat seal coating, and in addition, it is possible for converters
to apply cold seal coatings on the non-printing side, in register with the print,
for wrapping heat-sensitive food products, such as those involving chocolate.
Paperboard can be extrusion coated with PP for use as frozen/chilled food
trays which can be heated by the consumer in microwave and steam-heated
ovens. Major food applications of PP are for injection-moulded pots and tubs
for yoghurt, ice cream, butter and margarine. It is also blow-moulded for
bottles and wide mouth jars. PP is widely used for the injection moulding of
closures for bottles and jars.
PP can provide a durable living hinge which is used for flip top injectionmoulded lids which remain attached to the container when opened, e.g. sauce
dispensing closure and lid.
It is used in thermoforming from PP sheet, as a monolayer, for many food
products such as snacks, biscuits, cheese and sauces. In coextrusions with PS,