Page 51 - Plastics Engineering

34 General Properties of Plastics

be problems due to breaking up of fibres during processing. In this regard ther-
mosetting plastics have an advantage because their simpler processing methods
cause less damage to the fibres. Conductive grades of DMC are now available
with resistivities as low as 7 x ohm m.
Optical Properties. The optical properties of a plastic which are important
are refraction, transparency, gloss and light transfer. The reader is referred to BS
4618:1972 for precise details on these terms. Table 1.9 gives data on the optical
properties of a selection of plastics. Some plastics may be optically clear (e.g.
acrylic, cellulosics and ionomers) whereas others may be made transparent.
These include epoxy, polycarbonate, polyethylene, polypropylene, polystyrene,
polysulphone and PVC.

Table 1.9
Typical properties of plastics
Refractive Light Dispersive
Material index transmission power
Acrylic 1.49 92 58
Polycarbonate 1.59 89 30-35
Polystyrene 1.59 88 31
CAB 1.49 85 -
SAN 1.57 - 36
Nylon 66 1.54 0 -

Flammability. The fire hazard associated with plastics has always been
difficult to assess and numerous tests have been devised which attempt to
grade materials as regards flammability by standard small scale methods under
controlled but necessarily artificial conditions. Descriptions of plastics as self-
extinguishing, slow burning, $re retardant etc. have been employed to describe
their behaviour under such standard test conditions, but could never be regarded
as predictions of the performance of the material in real fire situations, the
nature and scale of which can vary so much.
Currently there is a move away from descriptions such as jre-retardant
or self-extinguishing because these could imply to uninformed users that the
material would not bum. The most common terminology for describing the
flammability characteristics of plastics is currently the Critical Oxygen Index
(COI). This is defined as the minimum concentration of oxygen, expressed as
volume per cent, in a mixture of oxygen and nitrogen that will just support
combustion under the conditions of test. Since air contains 21% oxygen, plastics
having a COI of greater than 0.21 are regarded as self-extinguishing. In practice
a higher threshold (say 0.27) is advisable to allow for unforeseen factors in a
particular fire hazard situation. Fig. 1.12 shows the typical COI values for a
range of plastics.