Transmission Efficiency of plastic Films Part 1

TRANSMISSION EFFICIENCY OF PLASTIC FILMS
Tests are usually carried out by exposing samples of the film to light of a specified
wavelength, or combination of wavelengths, for a given time and noting the effect on
various properties such as tensile strength or the various optical properties. The
intended end use is of importance when assessing the suitability of the film as the
type of radiation may be very different. In addition to the effect of sunlight it is
important in many applications to know the effect of shop or display cabinet lighting.
The screening effect of glass is also important when packs are to be on display in
shop windows. In the case of colored films, exposure, daylight or artificial light may
cause undesirable color changes.
EFFECT OF TEMPERATURE
HIGH TEMPERATURES:
One consequence of a high temperature on a thermoplastic film may be to soften it -
and - if high temperatures are likely to be encountered during use then a plastic with
an appropriately high softening point must be chosen. For boil in the bag packs, for
instance, films with a softening point above the boiling point of water are required.
High temperatures may also lead to other
Undesirable changes, especially in the case of plasticized film, where loss of
plasticizer can be accelerated leading to brittleness of the film.
LOW TEMPERATURE
Low temperature behavior of film is also important in many applications, including the
packaging of frozen foods. Some films, such as polypropylene and plasticized PVC,
suffer a loss in impact strength at temperatures around 0°C although oriented
polypropylene is very much better in this respect. Where low temperature use is
envisaged, it is preferable to carry out tests on the actual package since any stresses
and strains in the film may influence the behavior.
5.5 IDENTIFICATION OF FILM MATERIAL
The most reliable method of identifying a plastics film is by infra-red spectroscopy but
the equipment is expensive and requires skilled operation. There are many simple
tests, however, and if they are applied in combination surprisingly good results can
be obtained, with experience. If the material to be identified is a laminate then it is
necessary to separate it into its component parts before applying any tests.
SEPARATION OF LAMINATE PLIES
This is best carried out by refluxing the sample with a suitable solvent. The choice of
solvent is dictated by the adhesive used for laminating but if this information is not
available the best solvent must be found by experiment.
Table shows some of the commonest laminating adhesives together with possible
solvents.
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