Transmission Efficiency of plastic Films Part 1

TRANSMISSION EFFICIENCY OF PLASTIC FILMS
A novel use for polyester film is for the packaging of processed cheese. Molten
cheese is pumped into a continuous polyester film which is flattened and heat sealed
at intervals. The continuous ribbon, containing the hot cheese, is cooled with water
and cut to give an individual pack for each slice. Machines are available to produce
100—550 slices per minute. The film has good oxygen and water vapor barrier
properties and is generally acceptable from the food contact point of view.
Oriented polyester films are frequently used as components in laminations (in
conjunction with a sealing layer) in order to achieve a balance between cost and
function. Examples include retort pouches, boil-in-bag and lidding for “oven able’
packs. Coextruded, heat sealable films based on oriented polyester are often
metalized. They are then used in applications such as the packaging of snack foods
where barrier properties are important and where opacity is also necessary to
prevent the ingress of UV light with its consequent influence on fat rancidity.
1. ACRYLIC MULTI-POLYMER
This is still a relatively new material and in the U.K. its high price has impeded its
acceptance as a packaging material. In the U.S.A. the uses for thermoformed
containers have been confined to those markets where its resistance to oils and
greases and its good impact strength are particularly relevant. Such applications
include tubs for margarine and peanut butter. In the case of peanut butter the product
is particularly sensitive to oxygen pick-up, leading to rancidity, so that an acrylic-multi
polymer near to the ground oxygen permeability is also valuable.
3.2 CO EXTRUSIONS:
There are already many applications for coextrusions based on well-established
polymers such as PP, EVA, Surlyn (ionomer), LDPE and PS. In addition, much work
has been carried out, and will continue, on the production of high performance
coextrusions based on PVDC, EVOH and polycarbonate (PC).
In blown or cast film form, uses include breakfast cereal carton liners, pillow packs for
sweets and certain shrink wrap applications. One large market for films and
laminates generally is that of food snacks. This is a very competitive market so that
consumer appeal is a key requirement. Sandwich printing has a lot to offer in this
respect and this has often held back the use of coextrusions. One way round this
limitation is co extrusion coating. The substrate (which can be film, foil or paper) is
printed and then coated by a multi-ply extrudate. It can be seen that one other
advantage of co extrusion coating is that it allows the fabrication of a composite
material containing several polymer layers and a non-plastic substrate in a single
processing step.
Thermoforming, too, has benefited greatly from the availability of coextruded
materials, particularly the high barrier ones based on PVDC or EVOH. Two products
that require high barrier packaging are fats and long shelf life milk products.
Incorporating PC in the co extrusion gives the added possibilities of hot fill and retort
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