Electric devices in outdoor applications, under high thermal stress or in "dirty" environments require specialized powerful insulation surfaces so that the normal creepage distances specified in the standard don’t start to increase indefinitely.
The extraordinary stress in these environments leads to a faster degradation of the insulating materials. Microcracks form, in which dust can settle. Add moisture into the mix and the result can be the creation of, conductive paths, which ultimately lead to destruction of the material and loss of insulation integrity. To prevent this from happening to e.g. transformers, electric motors or even to the insulation in electronic assemblies located in metallic housings, product standards stipulate minimum lengths for creepage distances. These lengths depend on the durability of the insulation material’s surface.
The durability of an insulating material against surface leakage currents is measured using the so-called "comparative tracking index" (cti) which measures resistance to damage caused by leakage currents by placing conductive liquid between two surface electrodes.
All high-performance material films (PEEK, PEI, PI, PES) are subject to this kind of damage and perform considerably worse in comparison to e.g, polyester film.
However, various product standards specify that materials must possess an RTI (relative temperature index) of F (155°C) or H (180°C). Ultimately this swings things in the favour of high-performance plastics. On top of this, the added standards surrounding HWI (hot wire ignition), HAI (high arc ignition) and UL 94 VTM 0 only restrict material selection even further.
Fluoropolymer materials such as PTFE, ETFE and FEP might come to mind as a possible solution, but these are usually only used in specialised fields in part due to the highly aggressive gases that they emit in the event of a fire.
All that being said, DuPont has a solution to the challenges that devices in e.g. outdoor use face: Kapton® FN films.
Technically speaking the Kapton®HN film comes with a layer comprised of a thermoplastic fluoropolymer designed specifically for outdoor use. Because Kapton® FN is a Kapton® film that is coated with FEP on one or both sides, a cti value of 1 is achieved. In addition to this, the material has whole list of other appealing properties (see E39505):
- UL 94 VTM 0
- HWI = 0
- HAI = 4
- RTI = 220
and the Kapton@ 300FN929 is even capable of achieving cti = 0.
Kapton@ FN is the ideal film for anyone who is looking to restrict air gaps or creepage distances, as it allows significantly shorter creepage distances than the standard Kapton@ HN material.
Kapton@ CRC, MT are also produced in the F-version (FCRC, FMT, FMT+). Tey are available upon request.