Polyimide (PI) is a class of aromatic high-performance polymers whose chains are linked through imide groups. That is why the polymer has no melting point and decomposes only above its glass transition temperature of 360 to 410 °C (DuPont). As a film, polyimide withstands 220 to 240 °C continuously according to UL file E39505, 400 °C for short periods, and reaches a dielectric strength of 303 kV/mm at 25 µm with a density of 1.42 g/cm³. This article explains the chemistry and property values of the material class, sorts brands and trade forms, and shows how GOBA punches polyimide film from its range of insulation materials from 25 µm thickness and slits it into widths from 5 to 1,600 mm with a tolerance of plus minus 0.1 mm.
- Kapton is DuPont's brand, Upilex is UBE's, Apical is Kaneka's. Vespel and Tecasint are sintered sheet stock, Aurum is a thermoplastic polyimide, and Ultem is a polyetherimide, a relative rather than a member.
- The weak point of polyimide is water: the film absorbs 1.8 % moisture at 50 % relative humidity, and steam or alkali degrade the condensation polymer.
- Polyimide only pays off above 180 °C or where partial discharge occurs; below that, PET, PEN or aramid paper are the more economical choice.
Why polyimide has no melting point
The imide group that gives the material class its name is a nitrogen atom attached to two carbonyl groups, and in polyimides it almost always sits inside a five-membered ring between two aromatic rings. These rings are rigid, they leave the chains very little freedom to move, and that is exactly where the combination of temperature limit and electrical strength that no other film reaches comes from.
Polyimide is produced by polycondensation of an aromatic dianhydride with an aromatic diamine. The Kapton type is formed from pyromellitic dianhydride (PMDA) and 4,4'-diaminodiphenyl ether (ODA), as DuPont describes it in specification GS-96-7. UBE builds Upilex-S on biphenyltetracarboxylic dianhydride (BPDA) instead, usually with p-phenylenediamine (PPD) as the diamine. Both are polyimides, but the stiffer BPDA chain yields a film with more than twice the tensile strength, an expansion coefficient of 12 instead of 20 ppm/°C and less than half the water absorption. Anyone who treats polyimide as one material with one datasheet misses that difference.
The film is made in three steps, and the last one is the one that matters:
- Dianhydride and diamine react in a polar solvent such as N-methylpyrrolidone or dimethylacetamide to form polyamic acid, a precursor that is still soluble.
- That solution is cast as a thin layer onto a belt and dried.
- During imidization, thermally above 300 °C or chemically with acetic anhydride, the imide ring closes with the release of water. From this point the polymer is insoluble and infusible.
That is where the line in every datasheet under melting point comes from: none. A fully aromatic polyimide has a glass transition, but it does not flow above it, it decomposes. Film and sintered sheet therefore cannot be remelted or welded once made. That separates the classic polyimides from the thermoplastics, even though they are not chemically crosslinked like a thermoset. They sit between the two groups.
What properties does polyimide have?
The table puts the two film types and one sintered stock shape side by side. All values come from manufacturer documents: DuPont Kapton HN Technical Data Sheet and Summary of Properties, UBE Upilex brochure, DuPont Vespel SP-1 Product Information Sheet.
| Property | PMDA-ODA film (Kapton HN, 25 µm) | BPDA film (Upilex-S, 25 µm) | Sintered PI (Vespel SP-1) |
|---|---|---|---|
| Density | 1.42 g/cm³ | 1.47 g/cm³ | 1.34 g/cm³ |
| Tensile strength at 23 °C | 231 MPa | 520 MPa | 72 MPa |
| Tensile strength hot | 139 MPa at 200 °C | 290 MPa at 300 °C | 37 MPa at 260 °C |
| Modulus | 2.5 GPa | 9.1 GPa | 2.5 GPa (flexural) |
| Thermal expansion | 20 ppm/°C | 12 ppm/°C | 50 ppm/°C |
| Melting point | none, Tg 360 to 410 °C | none | none, sintered only |
| Thermal conductivity | 0.12 W/m·K | 0.29 W/m·K | 0.29 W/m·K |
| Dielectric strength | 303 kV/mm | 272 kV/mm (6.8 kV at 25 µm) | not specified |
| Permittivity at 1 kHz | 3.4 | 3.5 | not specified |
| Moisture absorption | 1.8 % at 50 % RH, 2.8 % after 24 h in water | 0.8 % at 60 % RH, 1.4 % after 24 h in water | 1.0 to 1.3 % at 50 % RH |
| Flammability | UL 94 V-0, oxygen index 37 % | UL 94 V-0, oxygen index 66 % | self-extinguishing |
Three rows deserve a second look. The dielectric strength of around 300 kV/mm applies to 25 µm and falls with thickness, at 125 µm DuPont still quotes 154 kV/mm. The tensile strength of Upilex-S is well over twice that of Kapton HN, which matters in flexible circuits and thin wraps. And the sintered sheet is the mechanically weakest member of the family: 72 MPa tensile strength is a third of the film, because sintering produces no chain orientation. Anyone buying polyimide as sheet stock is buying heat resistance and sliding properties, not strength.
The temperature figure needs the same reading as for any polymer. Trade listings usually say "up to 400 °C", and that is a short-term value. Design work relies on the thermal index, which UL lists under file E39505 at 200 to 220 °C for mechanical and 220 to 240 °C for electrical properties. DuPont adds the service life of a 25 µm film in air until its elongation at break has dropped from 70 to 1 %:
- 250 °C: 8 years
- 275 °C: 1 year
- 300 °C: 3 months
- 350 °C: 6 days
- 400 °C: 12 hours
UBE quotes a heat life of 290 °C over 20,000 hours for Upilex-S, measured on tensile strength. Under IEC 60085 polyimide film therefore falls into thermal class R at 220 °C, and in the GOBA material overview it sits in the top class at 220 °C and above. Oxygen is the limiting factor: in helium the film lasted at least ten times longer in DuPont's tests.
Chemically, polyimide is practically immune to organic solvents, DuPont knows of no solvent for the finished film. Against 2-normal hydrochloric acid Kapton HN retains 82 % of its tensile strength, whereas sodium hydroxide and steam degrade it. Gamma radiation of 10⁷ Gy lowers tensile strength from 207 to 152 MPa and elongation from 80 to 42 %, a high figure for nuclear and space applications. UV light in vacuum leaves the film its full tensile strength after 1,000 hours, whereas UV combined with oxygen and water embrittles it: in Florida weathering the elongation dropped to near zero after around 4,000 hours. For uncovered outdoor use polyimide is therefore the wrong choice.
Moisture and hydrolysis: the weak point of polyimide
DuPont states it in the Summary of Properties itself: Kapton is made by a condensation reaction, so its properties are affected by water. That sentence appears in no German distributor guide, although for washing machine motors, pumps and hydrogen systems it is the single most important piece of information.
The figures for a 25 µm type HN film: at 0 % relative humidity the dielectric strength is 339 kV/mm, at 50 % it is 303 kV/mm, at 100 % it is 268 kV/mm. Over the same range the permittivity rises from 3.0 to 3.8 and the dissipation factor from 0.0015 to 0.0035. The film expands by 22 ppm per percent of relative humidity, which on precision parts matters more than thermal expansion does in everyday conditions. In boiling water the tensile strength falls within 400 hours from 231 to around 175 MPa and then holds steady, while the elongation drops from 70 to 40 % and afterwards slowly further to around 25 % after 2,300 hours. Sodium hydroxide and steam continue to degrade the polymer, which is why nuclear plants have to shield Kapton in accident zones.
The BPDA chemistry has the advantage here. UBE quotes 0.8 % equilibrium moisture for Upilex-S and describes the film as virtually unaffected by long immersion in boiling water. For an insulating part that works in a humid housing or under condensation, that is a sound reason to change type, and a reason to consider PET or aramid paper wherever the temperature allows.
Polyimide as film, sheet, varnish and moulding compound: the supply forms
Searches for polyimide mean very different products. The table sorts them and says for each form whether GOBA supplies it.
| Supply form | Manufacturers and brands | Typical thicknesses | Used for | GOBA |
|---|---|---|---|---|
| Film, cast | DuPont Kapton, UBE Upilex, Kaneka Apical, Kolon, Chinese generics | 7.5 to 127 µm from the mill, distributors up to 0.5 mm | slot, phase and layer insulation, flexible circuits, cable wraps | roll, cut-to-size, die-cut part |
| Adhesive tape | 3M, Nitto, Scapa, CMC, private labels | carrier 25 to 50 µm, total 55 to 160 µm | masking during soldering, fixing, layer insulation | as die-cut and kiss-cut part, no roll goods |
| Sheet, rod, tube, sintered | DuPont Vespel, Ensinger Tecasint, Saint-Gobain Meldin | sheets from a few millimetres, machined | plain bearings, seals, vacuum parts, fixtures in ovens | no |
| Thermoplastic polyimide (TPI) | Mitsui Aurum | injection moulding, extrusion, film | moulded parts with Tg 245 °C, wire coating | no |
| Varnish and wire enamel | Elantas, UBE U-Varnish | layers of a few micrometres | magnet wire class 220 and above, coating of circuit boards | no |
| Foam | Evonik Solimide, BASF Basotect is melamine, not PI | blocks and sheets | acoustic and thermal insulation in aircraft and ships | no |
| Heating foil | Freek, IHP, HeatXperts, Sinomas | 0.2 mm without, 0.3 mm with adhesive | surface heating up to 200 °C | only the insulating film for it |
| Label | Brady, Avery Dennison, Labelident | film plus adhesive, around 50 to 100 µm | marking circuit boards through the soldering process | no |
Anyone searching for polyimide sheet almost always ends up with sintered material. DuPont forms Vespel SP-1 from polyimide powder under pressure and heat, and Ensinger explicitly calls Tecasint non-melting and processable only by sintering. According to Ensinger these stock shapes withstand up to 300 °C continuously and are available filled with graphite, PTFE or molybdenum disulphide, and DuPont quotes 0.20 % deformation for Vespel SP-1 under a 14 MPa load at 50 °C. Their field is plain bearings, valve seats and vacuum parts, in other words structural components, not sheet insulation.
Thermoplastic polyimide is the exception that proves the rule. With Aurum, Mitsui built a chain whose glass transition sits at 245 °C and which can still be injection moulded and extruded, because flexible ether groups are built into the backbone. The price is the temperature limit: continuous use up to 240 °C instead of the service life a cast film still has over years at 250 °C. Polyetherimide (PEI, Ultem from SABIC) takes that idea further and, with a glass transition temperature of around 217 °C, is an amorphous high-performance thermoplastic that belongs to the family but is not a polyimide in the strict sense. Polyamide-imide (PAI, Torlon) sits alongside as well. Anyone reading "polyimide" in a specification should check which of the three classes is meant.
Heating foils and labels are finished products with polyimide as the carrier film. In a heating foil an etched resistance pattern lies between two polyimide layers, and the manufacturers quote continuous temperatures up to 200 °C and power densities up to 0.8 W/cm² for it. A polyimide label survives five minutes at 260 °C in the reflow oven and then runs up to 180 °C. GOBA supplies the insulating film for both as cut-to-size material, not the finished heating element and not the printed label.
Kapton is the brand, polyimide is the material
Kapton is DuPont's registered trademark for polyimide film and has been in series production since 1955. The name has become so established that many drawings write "Kapton" and mean polyimide. That has consequences for purchasing: a generic PI film is not automatically equivalent. Distributors openly promote own brands as substitutes and admit in doing so that the film is only uniaxially oriented, as SynFlex does for SynTherm H. Anyone holding a UL listing, a customer release or a drawing that specifies the brand needs the brand. Anyone who only needs the material class can work with Apical from Kaneka or Upilex from UBE, both of which are rated V-0 under UL 94. The type series HN, VN, FN, CR and MT with their property values and the adhesive tape are covered in the article on Kapton. GOBA carries Kapton as a DuPont brand in its material overview and does not silently substitute generics when a brand is specified.
Polyimide compared with PET, PEN, aramid paper, PTFE and PEEK
Polyimide sits at the top of a range. The question is rarely whether it is better. The question is whether the application actually draws on its reserves. The table sets out the alternatives, with the reason people switch in practice.
| Material | Continuous temperature | Thermal class | Dielectric strength | Why people switch |
|---|---|---|---|---|
| Polyimide film (Kapton, Upilex) | 220 to 240 °C | R | 303 kV/mm at 25 µm | reference, expensive, moisture-sensitive |
| PET film (Hostaphan, Mylar) | 130 °C | B | around 180 kV/mm thin | fraction of the price, enough for most motors |
| PEN film (Teonex) | 155 °C | F | up to 300 kV/mm at 25 µm | intermediate step when PET runs too hot and PI is too expensive |
| Aramid paper (Nomex 410) | 180 to 220 °C | H to R | 18 to 34 kV/mm | tear-resistant and impregnable, but electrically ten times weaker per millimetre |
| PTFE film | 260 °C | C | around 60 kV/mm | chemistry and friction, but creeps under sustained pressure |
| PEEK | 250 °C | C | around 20 kV/mm on a 1 mm specimen | injection mouldable, rigid, electrically weak |
Sources: DuPont Kapton HN TDS, cmc.de for Teonex, DuPont for Nomex 410, manufacturer and literature values for PTFE and PEEK as also given in the article on thermoplastics. The aramid row shows the real argument: Nomex withstands almost the same temperature as polyimide, but only a tenth of the field strength per millimetre. Where the slot is narrow and the voltage high, polyimide film wins on build height; where there is room and the layer has to do mechanical work, aramid paper wins. Between 130 and 155 °C, PEN is the film many designers overlook. The overview of all materials above 180 °C is provided by the article on high-temperature insulation.
When polyimide is really necessary
Our recommendation is clear: up to 130 °C use PET film, up to 155 °C PEN, up to 180 °C aramid paper or a Nomex laminate, and only above that does polyimide pay off. Three cases justify reaching for polyimide film below that line. First, build height: where a PET film would need 125 µm and the slot only leaves 50 µm, polyimide delivers the field strength at half the thickness. Second, partial discharge on inverter drives, for which DuPont built the CR type, a film that lasts over 100,000 hours at 20 kV/mm instead of 200. Third, soldering processes and radiation, where 260 to 400 °C occur briefly and no other film gets through.
The objection from purchasing is the price, and it is valid. Polyimide film costs several times a PET film of the same thickness, and the moisture sensitivity comes on top. The answer is a laminate. For thermal class H, GOBA carries NKN, a composite of Nomex, Kapton and Nomex in which the expensive polyimide layer sits only in the middle and blocks the voltage, while the aramid paper on the outside does the mechanical work and takes up the impregnating resin. This construction brings polyimide into motors for electromobility without tipping the cost calculation of the slot insulation. How such composites are built is described in the article on laminate composites.
Die-cut parts and cut-to-size formats from polyimide film at GOBA
Polyimide film arrives at our plant as a master roll and leaves it as a slit roll, a cut-to-size format or a die-cut part. In stamping and forming the lower material thickness is 0.023 mm. That puts the common 25 µm film just inside punching capability, while 12.7 µm and 7.5 µm remain roll goods or are laminated onto a carrier first. At the upper end the range reaches 3.0 mm, which is irrelevant for polyimide itself but relevant for NKN and other laminates with a polyimide layer. The maximum size of a stamped or bent part is 1,000 x 2,000 mm.
The tooling is decided by quantity and tolerance. For samples and small batches we work with steel rule dies, for tight tolerances and long tool life with solid steel tools on the automatic press. We pre-emboss fold lines in the same stroke, because polyimide tears on from a damaged edge once the part is bent later. Typical parts made from polyimide film are:
- punched slot liners and phase insulation for traction and high-temperature motors, often as the middle layer of an NKN laminate
- top sticks and layer insulation where the build height has to stay below 50 µm
- separating layers and cell connector insulation in battery insulation
- insulating washers and pads between semiconductor and heat sink in power electronics
- masking parts for soldering and coating processes, as kiss-cut parts on the liner
Slitting is done by shear cutting between two circular blades, in widths from 5 to 1,600 mm and with a width tolerance of plus minus 0.1 mm. That is tighter than DuPont slits the film at the mill: specification GS-96-7 quotes plus minus 0.13 mm up to 38 mm width, plus minus 0.76 mm up to 102 mm and plus minus 1.5 mm above that. Anyone needing narrow wraps for end windings or strips for busbars gets them more accurately from contract slitting than from the standard roll, wound on cores with 25, 55, 76 or 152 mm inner diameter. Two things we watch in handling: the thickness tolerance of the film itself, which DuPont specifies at 21.6 to 29.2 µm for 25 µm and which adds up when many layers are stacked, and the moisture in storage, because a film carrying 2 % water runs differently in the tool than a dry one.
GOBA Takeaway
Polyimide is the material class that closes the temperature scale at the top in sheet insulation: no melting point, 220 to 240 °C continuous, 300 kV/mm at 25 µm. Its price and its moisture sensitivity are the reason to use it only where that reserve is actually drawn on, and otherwise to pack it as a thin middle layer into a laminate. Kapton is the brand, polyimide is the material, and the drawing should keep the two apart.
Send us the drawing, thermal class, voltage and quantity, and we will tell you whether polyimide film, an NKN laminate or a PET film is the more economical choice, and produce the part from our insulation materials as a roll, cut-to-size format or die-cut part.
