Among the myriad properties of PTFE that make it such a sought-after material, electrical resistance is one of the more popular. Because of its extraordinary dielectric strength and high breakdown voltage, PTFE is an invaluable addition in heavy electrical applications. As such, the following products are used across industries:
1. Skived PTFE tapes for heavy insulation
2. Expanded PTFE (ePTFE) Tapes for cable wrapping
3. PTFE Radomes
4. PTFE transducer covers
5. PTFE insulation blocks
These are but a few products that are commonly used. We often find that applications where electrical discharge is likely to be high benefit from a component made from PTFE to ensure that the equipment remains safe and does not leak current, causing harm.
Anti-static PTFE
The downside to the extreme insulative properties of PTFE lie in the build-up of static charge. Most applications do not find the build-up of static electricity to affect their process. However, certain assemblies – especially those where the equipment is being used in environments where flammability is high – require the charge to dissipate through the insulation so that sparks or static bursts do not occur. In such a situation, pure PTFE can cause problems, as it is such a strong insulator, that it does not allow the static charge to run through it.
To mitigate this problem, anti-static PTFE materials can be made, which employ conductive materials such as carbon to give mild conductive properties to the PTFE and allow it to discharge static build-ups through the carbon mixed into it.
Anti-static PTFE Tapes
Increasingly, applications that require PTFE tapes have started using anti-static tapes in areas where static build up can be an issue. However, most applications are very specific about the resistivity of the tape. Too much resistivity and you risk static build up; too little and you end up with a material that is too conductive to insulate effectively.
The base property of PTFE gives a surface resistivity of 10^14. For most conductive applications, this value needs to be reduced to 10^4. In order to achieve this, the base filler of carbon needs to be adjusted. A lot of this final property depends on both the base property of the virgin PTFE (different grades will vary from the base value mentioned above) and the conductive properties of the carbon additive itself. Various types of carbon will offer different levels of reduction in surface resistivity, implying the percentage of filler needs to be adjusted accordingly.
Another complication that arises due to the addition of carbon is that there can be a physical discharge of materials from the surface of the PTFE. Because PTFE and carbon are merely combined as a mixture, there is the likelihood that fine particles can come loose from the surface of the PTFE. For many high-purity applications, this can be a showstopper. Hence the quantity of the conductive filler needs to be minimized, while also allowing the conductive properties to be met. One material that has emerged as effective in this regard is Vulcan. While standard grades of conductive carbon need to be mixed to the extent of 10-15% into the PTFE (85% PTFE, 15% Carbon), with Vulcan, the same level of conductivity can be achieved with as little as 1-2%. However, as Vulcan is expensive, its incorporation is restricted to applications where there is a stringent need for no particle discharge to happen.
Electrical properties of pure virgin PTFE
|
General properties |
|||
|
Density |
ISO 1183 |
2,16 |
g/cm³ |
|
Transparency |
|
Opaque |
|
|
Mechanical properties |
|||
|
Stress at yield |
ISO 527 |
10 |
MPa |
|
Tensile strength |
ISO 527 |
20-25 |
MPa |
|
Elongation at break |
ISO 527 |
350 |
% |
|
Tensile modulus (Flexural Modulus) |
ISO 527 |
420 |
MPa |
|
Flexural strength @ 3.5% deflection |
ISO 178 |
14 |
MPa |
|
Ball pressure hardness |
ISO 2039-1 |
28 |
MPa |
|
The standard for ball pressure hardness |
|
H358 / 30 |
|
|
Hardness Shore (A/D) or Rockwell (R/L/M) |
ISO 868, ISO 2039-2 |
D55 |
- |
|
Izod notched impact strength 23 °C |
ISO 180/4A |
185 |
J/m |
|
Friction against steel without lubrication |
|
<0.1 |
- |
|
Abrasion relative to the pressure |
|
420 |
(µm/km)/MPa |
|
Electrical properties |
|||
|
Dielectric constant 50 Hz |
IEC 60250 |
2,1 |
- |
|
Dielectric constant 1 MHz |
IEC 60250 |
2,1 |
- |
|
Dissipation factor 50 Hz |
IEC 60250 |
0,5 |
10-Apr |
|
Dissipation factor 1 MHz |
IEC 60250 |
0,7 |
10-Apr |
|
Dielectric strength |
IEC 60243-1 |
7.5-24 |
kV/mm |
|
Thickness for electric strength |
|
3 |
mm |
|
Volume Resistivity |
IEC 60093 |
1.00E+14 |
? · m |
|
Surface resistivity |
IEC 60093 |
1.00E+14 |
? |
|
Creep Resistance (Comparative Tracking Index) |
IEC 60112 |
600 |
- |
|
Thermal properties |
|||
|
Thermal conductance |
ISO 22007 |
0,24 |
W/K m |
|
Specific heat |
IEC 1006 |
0,96 |
J/g K |
|
Linear thermal expansion along/cross to direction of flow |
ISO 11359 |
130-200 |
10-6/K |
|
Melting point |
ISO 11357 |
327 |
°C |
|
Heat distortion temperature A |
ISO 75 HDT/A (1,8 MPa) |
50 |
°C |
|
Heat distortion temperature B |
ISO 75 HDT/B (0,45 MPa) |
121 |
°C |
|
Short time use temperature |
|
300 |
°C |
|
Continuous use temperature |
|
260 |
°C |
|
Minimal use temperature |
|
-200 |
°C |
|
Other properties |
|||
|
Humidity absorption at 23°C/50% |
ISO 62 |
<0,1 |
% |
|
Water absorption |
ISO 62 |
<0,1 |
% |
|
Flammability UL 94 |
IEC 60695-11-10 |
V-0 |
- |
|
Limiting oxygen index |
ISO 4589 |
95 |
% |