This is because the extraordinary stress leads to faster degradation of the insulation material on the surface. Microcracks form, in which dust can settle. Together with moisture, this creates conductive paths that ultimately lead to the destruction of the material and the loss of the insulation effect. To avoid this, e.g. in transformers, electric motors or insulation of electronic assemblies with respect to metallic housings, product standards prescribe minimum lengths for creepage distances. Their length depends on the sensitivity of the surface of the material.
A measure of the sensitivity of an insulation material to these surface creepage currents is the so-called "Comperative Tracking Index" (cti). A measurement (conductive liquid between two surface electrodes) is used to measure the resistance to destruction by creepage currents.
All high-performance materials for films (PEEK, PEI, PI, PES) have their difficulties here and are considerably inferior to polyester film, for example.
However, various product standards prescribe property sets that include an RTI (relative temperature index) of F (155°C) or H (180°C). This quickly leads to the choice of high-performance plastics. When HWI (Hot wire Ignition) and HAI (High Arc Ignition) and UL 94 VTM 0 are added, the choice of material becomes even narrower.
Fluoropolymer materials such as PTFE, ETFE and FEP would be conceivable candidates, but are only used in special areas because of the highly aggressive gases produced in a fire.
However, DuPont has an answer to the requirements for equipment in outdoor use, for example: Kapton® FN films.
In principle, the Kapton®HN film is coated with a layer of a thermoplastic fluoropolymer. Kapton® FN is therefore a Kapton® film coated on one or both sides with, for example, FEP. As a result, a cti value of 1 is achieved. At the same time, the material has the very pleasant property set (see E39505): UL 94 VTM 0, HWI=0, HAI=4, RTI=220 and with Kapton@ 300FN929 even cti=0.
So if you want or need to narrow air and creepage distances: Kapton FN is an ideal film for this purpose, as it allows significantly lower creepage distances than the standard Kapton@ HN material.
Kapton@ F versions are also available for CR, MT and MT+ on request.
The following variants are available on request:
| Designation | FEP | HN | FEP |
| Kapton® 120FN616 | 2.5 µm | 25 µm | 2.5 µm |
| Kapton® 200FN919 | 12.7 µm | 25 µm | 12.7 µm |
| Kapton® 300FN929 | 12.7 µm | 50 µm | 12.7 µm |
| Kapton® 500FN131 | 25.2 µm | 76 µm | 25.4 µm |
| Flame class | V-0 |
| HWI | 0 |
| HAI | 4 |
| RTI elec. | 240°C |
| RTI Str. | 220°C |
| CTI | 0 |
Reduce creepage distances with Kapton®, FN