Page 49 - Plastics Engineering

32 General Properties of Plastics
fibrous reinforcement, but the development of new polymer matrices is the key
to further escalation of the useful temperature range.
Table 1.8 indicates the service temperatures which can be used with a range
of plastics. It may be seen that there are now commercial grades of unreinforced
plastics rated for continuous use at temperatures in excess of 200°C. When glass
or carbon fibres are used the service temperatures can approach 300°C.
The other principal thermal properties of plastics which are relevant to design
are thermal conductivity and coefficient of thermal expansion. Compared with
most materials, plastics offer very low values of thermal conductivity, partic-
ularly if they are foamed. Fig. 1.10 shows comparisons between the thermal
conductivity of a selection of metals, plastics and building materials. In contrast
to their low conductivity, plastics have high coefficients of expansion when
compared with metals. This is illustrated in Fig. 1.1 1 and Table 1.8 gives fuller
information on the thermal properties of plastics and metals.

25mm Wyurelhanc
4Omm Pdystyrrw
LSmn Hlreral d
50mm Cork
65mn Fibnbmd
1LOm softrrmd
3Barm Concrpl~ blocks

Equivalent thickness of common
building and insulatm materials required
to achieve the same degree of insulaiion
Fig. 1.10 Comparative Thermal conductivities for a range of materials

Electrical Properties Traditionally plastics have established themselves in
applications which require electrical insulation. PTFE and polyethylene are
among the best insulating materials available. The material properties which
are particularly relevant to electrical insulation are dielectric strength, resistance
and tracking.
The insulating property of any insulator will break down in a sufficiently
strong electric field. The dielectric strength is defined as the electric strength
(V/m) which an insulating material can withstand. For plastics the dielectric
strength can vary from 1 to loo0 MV/m. Materials may be compared on the
basis of their relative permittivity (or dielectric constant). This is the ratio of
the permittivity of the material to the permittivity of a vacuum. The ability of a