Kapton is the registered trademark of DuPont de Nemours for polyimide film. The film does not melt, withstands 240 °C continuously and reaches a dielectric strength of 303 kV/mm at 25 µm thickness (DuPont, Kapton Summary of Properties). GOBA carries Kapton as roll goods and supplies cut-to-size formats, die-cut parts and formed parts for electric motors, battery systems and power electronics. This article covers the brand: type series, property values, temperature limits and converting. The chemistry of the material is covered in the article on polyimide.
Kapton is a brand, polyimide is the material
Every Kapton film is made of polyimide, but far from every polyimide film is Kapton. Several PI films from other manufacturers are on the European market, and distributors offer them openly as substitutes. SynFlex, for example, positions its own SynTherm H against the Kapton range and points out itself that this film is only uniaxially oriented, which lowers the mechanical values. For a drawing, a UL listing or a customer release that explicitly names Kapton, that is not an equivalent substitute. GOBA carries Kapton as a DuPont brand in its material overview and supplies exactly that film when a brand is specified, with no silent substitution.
Which Kapton types are available?
DuPont supplies Kapton as a type series rather than a single film. The difference between HN and CR decides years of service life in traction motors, yet German distributor pages rarely name it.
| Type | What sets it apart | What it is built for |
|---|---|---|
| HN | standard film, 231 MPa tensile strength at 23 °C, 303 kV/mm at 25 µm | slot insulation, winding and layer insulation, cable wraps |
| VN | same base as HN, shrinkage of no more than 0.10 % after one hour at 200 °C | applications with tight dimensions after heat input, such as etched circuits |
| FN | HN with FEP coating, heat sealable, dielectric strength drops to 272 to 197 kV/mm | bonding without extra adhesive, moisture barrier, cable jacketing |
| CR | over 100,000 hours at 20 kV/mm and 50 Hz instead of 200 hours for HN, thermal conductivity 0.385 instead of 0.12 W/m·K | traction motors and inverter-fed drives with partial discharge |
| MT | formulated for heat transfer, roughly three times the conductivity of HN | insulation between semiconductor and heat sink |
The CR figures come from a test series DuPont ran together with ABB Industrie AG Switzerland and Siemens AG to IEC 343. The jump from 200 hours to more than 100,000 hours at the same voltage stress is why an inverter-fed drive with steep switching edges needs a different film than a mains-fed motor. The physics behind it is covered under partial discharge.
What property values does Kapton HN have?
| Property | Value (25 µm unless stated otherwise) |
|---|---|
| Dielectric strength | 303 kV/mm at 25 µm, 240 at 50 µm, 205 at 75 µm, 154 at 125 µm |
| Tensile strength | 231 MPa at 23 °C, 139 MPa at 200 °C |
| Elongation at break | 72 % at 23 °C |
| Dielectric constant at 1 kHz | 3.4 at 25 µm, 3.5 at 125 µm |
| Dissipation factor at 1 kHz | 0.0018 at 25 µm |
| Thermal conductivity | 0.12 W/m·K |
| Density | 1.42 g/cm³ |
| Melting point | none, the film decomposes thermally |
| Fire behaviour | UL 94 V-0, limiting oxygen index 37 % |
| Thermal class | C in the GOBA material overview, 220 °C and above |
All values come from the DuPont documents on Kapton (Summary of Properties and Bulletin GS-96-7, General Specifications). One point matters for sizing: dielectric strength falls as thickness rises. A 125 µm film blocks more voltage in total than a 25 µm film, but only about half as much per millimetre of material. Replacing one thin layer with a thick one gains less than the thickness suggests.
How heat resistant is Kapton really?
Distributors usually quote a range of -269 °C to +400 °C, and tape sellers quote 350 °C. Both describe short-term exposure. For sizing, the continuous rating counts, and according to UL file E39505 that is a thermal index of 200 to 220 °C for mechanical and 220 to 240 °C for electrical properties, depending on gauge and type.
DuPont publishes a service life table for this. It states how long a 25 µm HN film in air takes until its elongation at break has dropped from 70 % to 1 %, meaning the film has turned brittle:
- 250 °C: 8 years
- 275 °C: 1 year
- 300 °C: 3 months
- 325 °C: 1 month
- 350 °C: 6 days
- 400 °C: 12 hours
That makes the advertised figure readable. At 350 °C Kapton lasts six days, which is generous for a soldering step or a paint line and useless for motor insulation. Oxygen is the limiting factor: in DuPont's own tests the film lasted at least ten times longer in helium. How these limits translate into classes is covered under thermal classes.
Where Kapton is used
Kapton sits wherever a thin film has to block voltage and survive heat at the same time. In flexible printed circuits the film is the base material onto which the copper layer is laminated. In traction motors and in high-voltage insulation it handles slot lining, phase separation and end-winding insulation, usually as the CR type or as the middle layer of a laminate. In battery modules it separates cell connectors and busbars from the housing, a field described in more detail in the article on battery insulation.
Other fields sit outside classical electrical engineering. In aerospace, low outgassing in vacuum matters alongside temperature, which is why Kapton forms the multi-layer insulation on satellites. In 3D printing the film serves as an adhesion layer on the heated build plate. And in heating foils the etched resistance grid sits between two Kapton layers, which is the construction behind searches for Kapton heating foils. GOBA supplies the insulating material in those cases, not the finished heating element.
Kapton, PET film or aramid paper: which one when?
| Material | Continuous temperature | Thermal class | Dielectric strength |
|---|---|---|---|
| Kapton (polyimide) | up to 240 °C, short-term up to 400 °C | C | 303 kV/mm at 25 µm |
| PET film (Mylar, Hostaphan) | up to 130 °C, peaks to about 150 °C | B | around 180 kV/mm |
| Aramid paper (Nomex 410) | up to 180 °C | H | depends on thickness and calendering |
Our recommendation is inconvenient for anyone selling Kapton, but it holds: up to 130 °C use polyester film, up to 180 °C aramid paper such as Nomex 410, and only above that does Kapton pay off. Two exceptions apply. The first is build height: where a PET film would need 125 µm and the slot leaves only 50 µm, Kapton wins through the higher field strength per millimetre. The second is partial discharge in inverter-fed drives, where the CR type has no real alternative.
The most common objection is price per square metre, and it is fair: Kapton costs a multiple of a PET film of the same thickness. The way out is lamination. For thermal class H we carry NKN, a composite of Nomex, Kapton and Nomex in which the expensive polyimide layer sits in the middle and blocks the voltage while the aramid paper outside does the mechanical work. That construction is why many motors contain Kapton without breaking the costing. Which other materials come into question is shown in the overview on high-temperature insulation.
Kapton tape: silicone or acrylic adhesive
Most searches for Kapton mean the tape. It consists of a Kapton carrier film of 25 to 50 µm and an adhesive layer, giving total thicknesses of roughly 55 to 160 µm. The adhesive decides the temperature limit, not the film: silicone adhesive holds up to 400 °C short-term and thermal class H continuously, acrylic adhesive stops at around 200 °C. Acrylic still makes sense when painting, coating or bonding follows, because silicone migrates and spoils any later adhesion. For masking during soldering, take silicone. For paint line work, take acrylic.
The trade sells the tape in a handful of fixed constructions. Single-sided tape masks during soldering, painting and coating, double-sided tape holds parts that will later see heat. The values below come from the range data of CMC Klebetechnik, the data sheet for 3M 5413 and the Scapa 1801.
| Construction | Carrier film | Total thickness | Breakdown voltage |
|---|---|---|---|
| single-sided, acrylic adhesive | 25 µm | 55 to 60 µm | from 6 kV |
| single-sided, acrylic adhesive | 50 µm | 78 to 80 µm | 7 to 9 kV |
| single-sided, silicone adhesive | 25 µm | 60 to 69 µm | from 6 kV |
| single-sided, silicone adhesive | 50 µm | 100 µm | 12 kV |
| single-sided, silicone adhesive | 125 µm | 160 µm | from 12 kV |
| double-sided, acrylic adhesive | 25 µm | 60 µm | from 6 kV |
| double-sided, silicone adhesive | 25 µm | 140 µm | 7 kV |
On formats the market is remarkably uniform:
- 33 m roll length for single-sided tape, 20 m for double-sided
- standard widths of 6, 9, 12, 15, 19, 25, 30, 38, 50, 75 and 100 mm
- 76 mm core, the usual three-inch size
- master rolls up to 1,000 mm wide, from which the narrow rolls are slit
- film without adhesive in 25, 50, 75 and 125 µm, the same gauges as the carriers
How heat resistant is Kapton tape?
The figure printed on the roll almost always describes the carrier. Sizing runs on the adhesive. A silicone adhesive carries thermal class H continuously, meaning 180 °C, 3M states a service range up to 260 °C for tape 5413, and for the seconds in a solder bath the construction survives the advertised 400 °C. Acrylic adhesive stops at around 200 °C and loses tack before that. The film underneath would be nowhere near its limit at that point, at 250 °C it lasts eight years. For continuous duty above 200 °C the reliable number therefore sits in the adhesive data sheet.
Service life, removal and adhesive ageing
A pressure-sensitive adhesive only grips a dry surface free of grease and oil, which 3M writes explicitly into the application notes for the 5413. Unopened rolls stay usable for 12 months from manufacture and belong in the original carton at 15 to 27 °C and 40 to 50 % relative humidity. Heat in the warehouse costs tack before the tape has ever been applied. After the process, removal is what counts: both adhesives come off in one piece, wave soldering included, as long as the temperature limit was respected. Above it, removal turns into handwork. For parts that get assembled on takt time we die-cut the tape as a kiss-cut on the liner, so the fitter peels the part off without creasing the edge.
Against the two other tapes on the same motor shop bench, Kapton sits in a clear place. A PET tape such as 3M 1350F-1 is almost identical in build with a 25 µm carrier and 64 µm total thickness, but it carries a UL rating of 130 °C and stays a thermal class B part. A glass cloth tape such as 3M ET 69 puts 140 µm of woven glass under the silicone adhesive, reaches 178 µm total thickness and thermal class H and takes more mechanical load, which makes it the first choice for banding an end winding. Electrically it trails: 3,000 V breakdown voltage against 6,000 V for the thinnest Kapton tape. For masking and thin layer insulation take polyimide, for banding take glass cloth.
You will not get a roll of stock tape from us, that is what tape distributors are for. GOBA works the other side of this market: we slit Kapton to width, convert film supplied with an adhesive coating and apply tape to the part in a defined position, as a cut-to-size piece, as a kiss-cut part on the liner or as a finished die-cut part to your drawing, up to stamped and bent parts with pre-formed legs. For a masking tape at the solder bath that is the wrong route. For a series part that the fitter peels off the liner on takt time, it is the right one.
How GOBA converts Kapton
Kapton arrives here as a master roll and leaves as a finished part. Slitting runs as a shear cut between two circular blades, in widths from 5 to 1,600 mm and with a width tolerance of ±0.1 mm. That is tighter than DuPont slits the film at the mill: the manufacturer specification states ±0.13 mm up to 38 mm width, ±0.76 mm up to 102 mm and ±1.5 mm above that. Anyone who needs narrow tapes for end windings gets them more dimensionally accurate from contract slitting than from a standard slit roll.
In stamping and forming our lower limit is a material thickness of 0.023 mm. Kapton HN at 25 µm just falls within that range, while the 12.5 µm and 7.5 µm gauges stay roll goods or get laminated first. The maximum dimension of a stamped or bent part is 1,000 x 2,000 mm. For small batches and samples we work with steel rule dies, for tight tolerances and long tool life with solid steel dies on the stamping press.
Two things stand out when handling thin polyimide films. First the thickness tolerance of the film itself: DuPont specifies 21.6 to 29.2 µm for nominal 25 µm material, which adds up to more than 0.3 mm difference across a stack of 40 layers. Second the cut edge. Kapton does not fray like a nonwoven, but it tears on from a damaged edge once the part is folded. That is why we emboss fold lines during stamping rather than bending them afterwards. How finished insulating and molded parts come out of this is covered on the production page.
GOBA Takeaway
Kapton solves two specific problems: continuous temperatures above 180 °C and partial discharge in inverter-fed drives. As an all-purpose film it fails on cost. Anyone who only has to cover 130 °C pays for headroom they will never use. Anyone who genuinely needs the 240 °C will find no substitute. Send us the drawing with thermal class, voltage and quantity, and we will tell you whether Kapton, an NKN laminate or a PET film is the more economical choice.
