Transmission Efficiency of plastic Films Part 1

5.1 PERMEABILITY
FILM TESTING
TRANSMISSION EFFICIENCY OF PLASTIC FILMS
In essence there are two mechanisms by which a gas or vapor can pass from one
side of a plastics film to another. If the film is porous then the gas or vapor can flow
through the holes. This effect is not usually present except in the case of very thin
films.
Suffice it to say that where permanent gases are concerned, and conditions of
constant temperature and a constant partial pressure differential, a steady state will
be achieved after a certain time has elapsed.
1. WATER VAPOR PERMEABILITY
Tests for water vapour permeability are described in ASTM E-96 & BS.2782: Part 5:
1970. Methods 513A and 51313 are carried out by sealing test specimens with wax
over the mouth of metal dishes containing a desiccant.
The dishes are weighed initially and then placed in a temperature and humidity
controlled cabinet. Weightings are carried out at regular intervals and the gains in
weight are measured. In Method 513A, the temperature of the cabinet is maintained
at 25°C ± 0.5°C and the relative humidity at 75 %±2 %. These conditions are taken
as representative of ‘temperate’ climates. For ‘tropical’ conditions the tests are
performed at 38°C ± 0.5°C and 90% ± 2% relative humidity.
It may often be found preferable to carry out water vapour permeability tests on
completed packages rather than on dishes. Test conditions are the same as above
but the desiccant is contained in a heat sealed sachet instead of a metal dish.
Weighing are carried out as before. In either case, the water vapour permeability is
reported as g/m where a quick indication of the potential barrier properties of a film is
required; the film can be clamped in an instrument where one side of the film is
exposed to a high humidity atmosphere while the other side is in contact with dry air.
As water vapor diffuses in, it is detected by an infra-red absorption cell or by a
resistor, the value of which is affected by changes in relative humidity. The
instrument then measures the time required for a particular change in relative
humidity as detected by the cell or resistor.
It should be noted, however, that difficulties can arise where hygroscopic materials
are used (e.g. laminates based on regenerated cellulose or a cellulose-base film
coated on both sides with a moisture barrier). Taking the latter as an example, the
basic cellulose contributes little to the water vapour barrier properties, which are
almost entirely dependent on the coatings (taken to be identical). The specimen is
initially in equilibrium at 35% relative humidity and is then placed in an enclosure with
saturated air (100% relative humidity) on one side and dry air (10% relative humidity)
on the other. Under these conditions the initial rate of transmission of water vapour is
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