Transmission Efficiency of plastic Films Part 1

TRANSMISSION EFFICIENCY OF PLASTIC FILMS
An important factor is that of dispersion, to develop colour strength and maximum
optical properties. Dyes are not normally a problem because of their soluble nature,
but pigments need to be properly wetted by the plastic or by suitable vehicles. Poor
dispersion can lead to processing difficulties with thin films. Improvements have been
made through surface treatment of pigments or through better grinding techniques.
7. HEAT STABILISERS
The high temperatures encountered during film extrusion can cause thermal
decomposition of certain plastics. This follows a similar path to the decomposition
reactions caused by oxidation. There is a close relationship between the two
reactions, thermal decomposition being catalysed by oxygen and oxidation being
catalysed by heat. Some polymers such as low density polyethylene and the nylons
are stable enough not to require heat stabilisers, but PVC, high density polyethylene
and polypropylene may require significant amounts.
A wide range of heat stabilisers is available, the most common being
barium/cadmium, organotin and lead compounds. The choice in any particular
instance is governed by the temperatures likely to be encountered, the duration of
protection required, the amount of oxygen present and the presence or absence of
antioxidants. The resin Slip additives act as external lubricants and the materials
normally used are fatty acid amides. In addition to solving problems in film handling
equipment, slip additives are also useful in applications where sheaves of film are to
be dispensed.
8. UV SCREENING AGENTS
UV radiation has a harmful effect on many products, particularly on certain foodstuffs.
Where these effects are likely to be significant they can be greatly reduced by the
addition of UV screening agents to the film wrapper or sachet. These screening
agents act by absorbing or reflecting the UV radiation. The reflected radiation is
dispersed harmlessly but absorbed UV radiation is converted to radiation of a
different wavelength and this might have an effect on the plastic. If the converted
wavelength is in the visible or infra-red spectrum then it is relatively harmless but
shorter wavelengths, such as different areas of the UV spectrum, could have a
deleterious effect on the film itself In most food packaging applications, however, the
time of exposure to UV is likely to be small and the effect on the plastic should be
negligible.
UV screening is achieved either by pigments or by colorless organic compounds that
absorb the radiation. One example of a bag incorporating a UV screen is the one
used for pre-packed potatoes. This is often tinted a light brown color to prevent
‘greening’ of the potatoes. The most effective pigment is carbon black which gives
optimum protection at concentrations of about 2—3%. Films intended for outdoor
use, particularly in the tropics, are often protected in this way.
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