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182
FOOD PACKAGING TECHNOLOGY
Oriented films are brought close to their melting point to anneal or release
stresses in them and to minimise the amount of shrinkage which may take
place when being heated in a post-production process such as printing or heat
sealing. Failure to anneal heat set films will ensure that they have very unstable
thermal characteristics and allow the films to shrink tightly onto cartons or
bottles when heated.
It is difficult to puncture or initiate a tear in an oriented film, but once punctured, the alignment of the molecules allows easy proliferation of the rupture
and tear. This feature is made use of to assist the opening of film sachets by
incorporating a tear-initiating notch mechanically into the sealing area.
Oriented films may have as little as 60% elongation before breaking,
whereas cast polypropylene, for example, may extend by 600% before finally
breaking. This property is exploited to great effect with linear low-density
polyethylene (LLDPE), in the application of stretch wrapping, because the
non-branching polymer chains allow easy movement of the polymer molecules
past each other. By adding special long-chain molecules in the manufacturing
process, it is possible to ensure that the film clings to itself.
The majority of plastic films are transparent and not easily coloured by
dyeing or adding pigments. In order to develop opacity, films can be cavitated
during film manufacture. Cavitation causes internal light scattering, which
gives a white or pearlescent appearance. A simple analogy for the light scattering effect is to consider the example of beating and blending egg white with
sugar to produce a meringue, which has a white appearance due to the bubbles
trapped inside the beaten egg white. With some plastics, such as cast PE,
a chemical compound can be added to the plastic resin, which gives off a gas
such as nitrogen or carbon dioxide, when heated in the film manufacturing
process. The small gas bubbles in the plastic cause light scattering, which gives
the film a pearlescent appearance.
However, because oriented films are thin, there is the possibility of the
bubbles being so large that the film may be ruptured. So instead of using gas
bubbles, a shearing compound or powder is added to the polymer, causing
internal rupturing of the plastic sheet as it is being stressed. This causes voids
in the film and light is scattered across the whole spectrum. Incident white
light is reflected inside the film as a result of the differing refractive index
between the plastic and free air. The process lowers the density of the film and
may give more cost-effective packaging as a result of the increased area yield.
The technique of pigmenting plastics has been developed using white compounds such as calcium carbonate or, more usually, titanium dioxide, to give
a white appearance. The addition of such an inorganic filler, however,
increases the density by up to 50%, lowering the yield and increasing the risk
of mechanically weakening the film. Early attempts to pigment film produced an abrasive surface, and the practice today is to ensure that there is
a skin of pure resin on the outer layers which acts as an encapsulating skin to