Although the primary objective in the development of filled PTFE has generally been to produce materials with a range of mechanical properties, the addition of fillers does have a marked effect on the electrical properties as well. In particular, graphite- and carbon-filled compounds may have a relatively high conductivity, which can assist the dissipation of static charges in applications where these are a problem.
Filled PTFE is a mixture of materials, and voids — no matter how small — are always present: as with all porous materials, the properties are dependent on the nature of the environment and of any inadvertent contamination.
The electrical properties are no exception and are markedly dependent on the environmental humidity. The spillage of conducting liquids, electrolytes and greases on filled 'PTFE' can affect the properties of the material even in otherwise dry conditions.
To some extent porosity (and therefore the effects of humidity and spillage) is dependent upon the method of fabrication, but even in a relatively non-porous part, the surface is liable to be absorbent.
Under dry conditions, the glass-filled grades are excellent electrical insulating materials over a wide range of temperature. Their insulating properties deteriorate will increasing humidity, but even at 95% r.h. the properties are comparable with those of plasticised PVC and of some thermosetting compounds commonly used for insulation.
The effect of temperature on the loss tangent of some glass-filled grades is shown in the graph
( Loss tangent Vs. temperature at 1 KHz glass filled PTFE )
The level of dielectric loss tends to increase as glass content increases. However, the mechanisms giving rise to dielectric behaviour of filled materials are complicated, and in general it must be assumed that these materials will exhibit Maxwell-Wagner loss processes of relatively large amplitude at low frequencies (log10 frequency < 0) in the dry state, moving progressively to higher frequency with increasing moisture content.
No grade of filled PTFE is a good conductor when compared with for example copper, aluminium etc. However some have sufficiently low volume and surface resistivity to be considered for use in antistatic applications.
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