Page 50 - Plastics Engineering

General Properties of Plastics 33

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Fig. 1.1 1 lsrpical thermal propexties of plastics

material to resist the flow of electricity is determined by its volume resistivity,
measured in ohm m. Insulators are defined as having volume resistivities greater
than about 104 ohm m. Plastics are well above this, with values ranging from
about 108 to 10l6 ohm m. These compare with a value of about ohm m for
copper. Although plastics are good insulators, local breakdown may occur due
to tracking. This is the name given to the formation of a conducting path (arc)
across the surface of the polymer. It can be caused by surface contamination
(for example dust and moisture) and is characterised by the development of
carbonised destruction of the surface carrying the arc. Plastics differ greatly
in their propensity to tracking - PTFE, acetal, acrylic and PP/PE copolymers
offer very good resistance.
It is interesting to note that although the electrical insulation properties of
plastics have generally been regarded as one of their major advantages, in recent
years there has been a lot of research into the possibility of conducting plastics.
This has been recognised as an exciting development area for plastics because
electrical conduction if it could be achieved would offer advantages in designing
against the build up of static electricity and in shielding of computers, etc from
electro-magnetic interference (EM). There have been two approaches - coating
or compounding. In the former the surface of the plastic is treated with a
conductive coating (e.g. carbon or metal) whereas in the second, fillers such
as brass, aluminium or steel are incorporated into the plastic. It is important
that the filler has a high aspect ratio (1ength:diameter) and so fibres or flakes of
metal are used. There has also been some work done using glass fibres which
are coated with a metal before being incorporated into the plastic. Since the
fibre aspect ratio is critical in the performance of conductive plastics there can