Transmission Efficiency of plastic Films Part 1

TRANSMISSION EFFICIENCY OF PLASTIC FILMS
dependent on the equilibrium relative humidity of the base film (35%), the relative
humidity of the dry side (10%) and the water vapour transmission rate of the barrier
layer separating them. After some time, the base film reaches equilibrium and the
water vapour moving from the 100% relative humidity side equals that arriving on the
dry side. As the two barrier materials are identical and have a much lower
transmission rate than the base film, this will reach equilibrium at a relative humidity
about midway between the wet and dry conditions, i.e. 55%. Under these conditions
the transmission of water vapour will have increased above that in the initial
conditions because the vapour pressure difference is due to a relative humidity
differential of 45% (= 55% — 10%), compared with the initial value of 25% (= 35% —
10%). The transmission rate will, therefore, rise by a factor of about 1.8.
Similarly, a specimen where the base film is initially in equilibrium with a relative
humidity greater than the final value will have a high apparent water transmission rate
initially, and this will fall as the specimen approaches equilibrium. Hence, although
rapid test instruments can measure transmission rates in a short time, the results
may differ significantly from those obtained after dynamic equilibrium has been
reached within the test cell. With non hygroscopic arrivers results agreeing closely
with the dish method can be obtained.
2. GAS PERMEABILITY
Measurement of gas permeability is carried out under controlled conditions of
pressure, as well as of temperature and relative humidity. In essence the usual tests
consist in making a partition between a test cell and an evacuated manometer.)The
pressure across the film is usually one atmosphere. As the gas passes through the
film sample the mercury in the capillary leg of the manometer is depressed. After a
constant transmission rate is achieved, a plot of mercury height against time gives a
straight line. The slope of this line can be used to calculate the gas transmission. Gas
permeability measurements are described in ASTM D. 1434 and BS.2782: Part 5:
1970.
The foregoing method is known as the pressure increase method. Another common
method is that of concentration increase. Two gases are used, a reference gas and
the test gas. A partial pressure difference across the barrier material with respect to
the test gas is created, without a difference in total pressure. Because the method of
measuring the concentration of the test gas can be specific 10 that gas even in the
presence of other gases or vapors, equipment can be developed in which the relative
humidity of both the test and the reference gases can be controlled. This is not
possible where equipment has to be evacuated and is very important when
Determinations are carried out on barrier materials having a hygroscopic base.
Various methods of measuring the concentration change are used, including
chemical analysis, gas chromatography and radioactive tracer techniques.
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