Page 52 - Plastics Engineering

General Properties of Plastics 35

COI -

Fig. 1.12 Oxygen Index Values for Plastics

Permeability. The low density of plastics is an advantage in many situations
but the relatively loose packing of the molecules means that gases and liquids
can permeate through the plastic. This can be important in many applications
such as packaging or fuel tanks. It is not possible to generalise about the
performance of plastics relative to each other or in respect to the performance
of a specific plastic in contact with different liquids and gases.
Some plastics are poor at offering resistance to the passage of fluids through
them whereas others are excellent. Their relative performance may be quantified
in terms of a permeation constant, k, given by

(1.10)

where Q = volume of fluid passing through the plastic
d = thickness of plastic
A = exposed area
t = time
p = pressure difference across surfaces of plastic.
The main fluids of interest with plastics are oxygen and water vapour (for
packaging applications) and C02 (for carbonated drinks applications). Fig. 1.13
and Fig. 1.14 illustrate the type of behaviour exhibited by a range of plastics.
In some cases it is necessary to use multiple layers of plastics because no single
plastic offers the combination of price, permeation resistance, printability, etc.
required for the application. When multi-layers are used, an overall permeation
constant for the composite wall may be obtained from

(1.11)

1.4.5 Processing
A key decision in designing with plastics is the processing method employed.
The designer must have a thorough knowledge of processing methods because