Electrical Properties of Filled PTFE

Although the primary objective in the development of Fluon® grades 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 Fluon® grades 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.

INSULATING filled PTFE MATERIALS :

Under dry conditions, the glass-filled grades are excellent electrical insulating materials over a wide range of temperature. Their insulating properties deteriorate with 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. Tables 5, 6 and 7 give values of loss tangent, permittivity and volume and surface resistivities, measured at two levels of humidity, for Glass Filled PTFE grades. The effect of temperature on the loss tangent of some glass-filled grades is shown in Figure 29. 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 frequencies with increasing moisture content. Rudner reports on, but does not interpret, the properties of PTFE filled with titanium dioxide using samples that had been kept at a constant low humidity with silica gel crystals present to absorb moisture. It is suspected that these losses are due to a similar mechanism.

CONDUCTING filled PTFE MATERIALS :

No grade of filled PTFE is a good conductor when compared with, for example, copper or aluminium. However, some have sufficiently low volume and surface resistivities to be considered for use in antistatic applications: see Table 8.

Table 5. Loss tangent at room temperature

‘Dry’†			After 9 weeks at 95% r.h.
	103 Hz	104 Hz	103 Hz	104 Hz	105 Hz	106 Hz
Unfilled PTFE	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001
5% Glass Filled PTFE	0.00033	0.00032	0.14	0.122	0.058	0.0126
15% Glass Filled PTFE	0.00065	0.00063	0.32	0.36	0.275	0.068
25% Glass Filled PTFE	0 00107	0 00097	0.39	0.28	0.225	0.062

† Dried for 150h at 0.1mm Hg at 116°C (241°F).
Note: Samples were skived from tape 1.0mm thick veneered (0.04 inch) from blocks preformed at 700 kgf/cm²; 10 000 Ibf / in² freely sintered for 1 ½ hours at 380°C (716°F) and cooled at approximately 30°C per hour (54°F per hour).

Table 6. Permittivity at 10⁵ - 10⁷ Hz

	Ambient humidity	95% r.h.
Unfilled PTFE	2.02-2.09††	2.02-2.09††
15% Glass Filled PTFE	2.2-2.5	2.2-2.5
25% Glass Filled PTFE	2.2-2.5	2.2-2.5

†† Depending on density

Table 7. Volume and surface resistivity

	Volume resistivity(1) (ohm cm)	Surface resistivity(2) (ohm )
	Dry(3)	Ambient humidity	95% r.h.
Unfilled PTFE	>1 x 1018	1017	-
15% Glass Filled PTFE	>2 x 1015	1015	1015
25% Glass Filled PTFE	>2 x 1015	1015	1015

Notes :
(1) Measured on discs of skived tape approx. 50.8mm (2 inch) in diameter and 10mm (0.04 inch) thick, using evaporated aluminium electrodes. 1 min. value: 120 V / mm (3 V / 0.001 inch).
(2) 1 min. value at equilibrium with the environment.
(3) Dried for 150 h at 0.1mm Hg at 116°C (241°F).

Table 8. Volume and surface resistivity of Fluon® grades

	Filler(1) by weight %	Volume resistivity(2) (ohm cm)	Surface resistivity(3) (ohm)
33% Coke Filled PTFE	33%	102	104
25% Coke Filled PTFE	25%	104	107
63% Bronze+Graphite Filled PTFE	63%	104	104
15% Graphite Filled PTFE	15%	106	1014
60% Bronze Filled PTFE	60%	107	109

Notes :
(1 ) Filler % by weight, the remainder being PTFE.
(2) Volume resistivity measured at 23°C (73°F) and 50% r.h. on tape specimens 0.25 to 1.0mm (0.01 to 0.04 inch) thick clamped between disc brass electrodes. The values given are an indication only and may vary widely with fabrication methods.
(3) Surface resistivity measured at 23°C (73°) and 50% r.h. on tape specimens 0.25 to 1.0mm (0.01 to 0.04 inch) thick using disc and ring brass electrodes applied by pressure only. No backing guard electrode used. With some materials the measured surface resistivity is very high even when the volume resistivity is low. This is probably because the veneering method used to prepare tape specimens may have tended to smear a fine PTFE layer on the surface.