High-Temperature Insulation

In electrical engineering, high-temperature insulation means insulating materials that block voltage continuously above class B (130 °C): class F at 155 °C, H at 180 °C, N at 200 °C, R at 220 °C and above, as defined in IEC 60085. The order by temperature and price is PET up to 130 °C, PEN up to 155 °C, aramid paper up to 180 °C, then polyimide and mica, while PPS, PEEK and PTFE cover special cases involving chemicals, moisture or sliding requirements. This article places nine material groups side by side with documented continuous, short-term and breakdown values per class and shows the forms in which GOBA has been slitting and punching them from its range of insulation materials since 1959: slit rolls from 5 to 1,600 mm wide at plus minus 0.1 mm, punched parts from 0.023 to 3.0 mm thick up to 1,000 x 2,000 mm.

• Four temperature figures appear on datasheets and mean four different things: short-term limit, continuous service temperature, temperature index TI to IEC 60216 (20,000 hours) and RTI to UL 746B.
• Polyimide only pays off above 180 °C or where partial discharge occurs; below that, aramid paper or an NKN laminate with a polyimide core layer is the more economical choice.
• Thermal insulation made of ceramic fibre or calcium silicate is a different subject: it slows heat flow but does not block voltage.

Why the threshold is 130 °C

The threshold of 130 °C is not arbitrary. Up to that point, which is thermal class B, biaxially oriented polyester film handles almost every sheet insulation job in motors, transformers and appliances, and it does so at a price no other insulating material can match. Above that, PET first loses its orientation, then its mechanical strength and finally its electrical strength. That threshold is where the selection covered on this page begins.

That draws the lower boundary on the temperature scale, but the term also needs a boundary to the side, because it is used in two senses, and confusing them costs money at the ordering stage. In furnace construction and plant engineering, high-temperature insulation means thermal insulation: ceramic fibre, calcium silicate and insulating wool slow the flow of heat out of a hot zone. In electrical engineering the same term means electrical insulation at high temperature: aramid paper, polyimide film or mica block voltage while the winding or the heating element gets hot. GOBA manufactures for the second meaning. The first is covered in the article on thermal insulation, and the overview of both worlds is in the article on insulating material.

High-temperature insulation by thermal class: which material carries which class

IEC 60085 sorts insulating materials and insulation systems into classes by their permissible continuous operating temperature. For selection, what matters is the mapping of class to material, and in practice it looks like this:

Classes N and R were added in newer editions of the standard; many datasheets still show the old umbrella class C for everything above H. The complete table from Y to C and the rule that every sustained 10 kelvin above the limit halves the service life are covered in the article on thermal classes. One point matters for ordering: the class of a material does not say which class the finished insulation system of film, wire, impregnating resin and adhesive achieves. The system class is tested on the complete assembly to IEC 60034-18 or UL 1446, which is why a film datasheet usually shows a temperature index rather than a class.

Property values of heat-resistant insulating materials compared

The table below puts the materials above class B side by side with the values that count in design. All figures come from manufacturer datasheets or UL files, and the source is given in every row.

Three rows deserve a second look. PPS carries two different RTI values, 200 °C for the electrical and 160 °C for the mechanical properties, and a datasheet that quotes only the 200 hides half the story. PEEK shows the thickness effect: as sheet above 762 µm the RTI is 260 °C, as film between 50 and 127 µm it is 200 °C, because thin films age faster owing to their surface-to-volume ratio (Victrex). Anyone who reads a sheet datasheet and applies it to a 50 µm film is off by 60 kelvin. And for mica the temperature applies to the mineral: the sheet depends on at most 10 % silicone binder, the mica tape on its glass carrier and its adhesive. Which limit the composite actually carries is covered in the article on mica tape.

Short-term limit, continuous service temperature, TI and RTI: four figures, one datasheet

The most common mistake in selecting heat-resistant insulation happens while reading the datasheet. Distributor pages advertise the highest number that can be substantiated, and that is almost always a short-term value. Four figures circulate, and they are not interchangeable.

The temperature index is the most honest of the four figures because it carries a time. 20,000 hours is roughly two and a half years of continuous operation, and the usual end point is half of the initial breakdown voltage (ZVEI, Insulation Bulletin No. 1). A material operated at its TI has therefore used up half of its electrical reserve after two and a half years. Its companion is the halving interval HIC: the temperature interval in kelvin that halves the time to the end point, usually around 10 kelvin for organic insulating materials.

How far apart short-term and continuous values sit is shown by DuPont for Kapton HN in a dedicated service life table: the film lasts 8 years at 250 °C, 1 year at 275 °C, 3 months at 300 °C, 6 days at 350 °C and 12 hours at 400 °C, in each case until elongation at break has dropped to 1 % (DuPont, Kapton Summary of Properties). The advertised 400 °C and the design figure of 250 °C describe the same material. Aramid paper follows the same pattern: Nomex 410 holds 12 kV/mm for several hours at 400 °C and is at the same time recognised as a 220 °C material. For design work, take the TI or the electrical RTI, subtract 20 to 30 kelvin of headroom, and use the temperature at the hottest point of the winding, not at the housing.

Which high-temperature insulation for which application?

The application sets the class, the class narrows down the materials, and only then do build height, voltage and price decide. From our production, these are the typical assignments:

• Class H motors in ovens, dryers and paint lines: slot liners and phase insulation from Nomex 410 or NKN laminate, slot closures from aramid paper. Designing to class H and operating to class F keeps the 25 kelvin headroom that, by the 10 kelvin rule, roughly multiplies service life by five.
• Traction drives with hairpin windings: high current density, tight slots and inverter operation with steep edges. Here polyimide works as the CR grade against partial discharge or as the middle layer in NKN, because the slot leaves no room for thick layers.
• Heating appliances and white goods: ovens, tumble dryers, hair dryers and strip heaters carry their heating coils on rigid mica sheets, because heat and voltage act on the same component there. Transformers and motors inside the appliance usually stay with PET or NMN.
• Dry-type transformers and transformers of classes F and H: layer and barrier insulation from aramid paper, there with the continuous stress of at most 1.6 kV/mm that DuPont recommends against partial discharge.
• Power electronics: insulating layers between semiconductor and heat sink from thermally conductive polyimide such as Kapton MT, where alongside temperature the thermal conductivity decides the thickness.
• Cables, leads and sleeves in hot zones: silicone glass fibre and PTFE. That field is covered in the article on insulating sleeving.

Selecting high-temperature insulation: the order by temperature and price

Our recommendation is a staircase, not a list. You climb it from the bottom and stop on the first step that carries the temperature at the hottest point with headroom. Every step above that costs a multiple per square metre without giving the application anything in return.

1. Up to 130 °C: PET film. Class B covers the majority of industrial motors, appliance drives and mains transformers. Only a real measured value above 130 °C at the insulation point justifies the next step.
2. Up to 155 °C: PEN film or NMN laminate. PEN delivers class F with the same converting behaviour as PET and four times the hydrolysis resistance; NMN delivers it with the edge stability of aramid paper when the liner is inserted into the slot.
3. Up to 180 °C, with headroom to 200 °C: aramid paper such as Nomex 410 or NKN laminate. This is the class H step and the reference material for oven motors, dryers and dry-type transformers.
4. Up to 240 °C: polyimide film. Kapton HN only pays off here, or earlier when the build height rules out a 125 µm PET film and 50 µm of polyimide carries the same voltage.
5. Above that: mica. From 250 °C only PTFE and polyimide with limited service life remain among organic materials, and from 300 °C only the mineral is left. Rigid mica sheets carry heating coils at 500 to 700 °C.

Three materials deliberately sit beside this staircase. We take PPS when chemicals or hydrolysis come into play, because its tensile strength stays stable in sodium hydroxide and ammonia where polyester and polyimide fail. PEEK belongs where a film has to withstand heat, wear and radiation at the same time, for instance as a permanent label or release film in tooling. PTFE is excellent electrically and thermally, but it cold-flows under sustained pressure and drops out of clamped assemblies. Which of these materials we carry as roll goods and which class the specific grade carries is listed in the overview of our insulation materials.

Is polyimide too expensive? The objection and the laminate answer

The objection comes from purchasing, and it is justified: polyimide film costs a multiple of a PET film of the same thickness, and a slot liner made of 125 µm solid polyimide wrecks the costing of a series motor. The wrong conclusion would be to work with PET at 180 °C and hope for headroom. The right one is called laminate.

In NKN laminate the polyimide film sits only as a 25 µm middle layer between two layers of calendered aramid paper. The middle blocks the voltage, the paper on the outside carries the mechanical load during insertion, creasing and impregnation. The laminate reaches class H at a total thickness of 0.2 to 0.4 mm, and the polyimide share of the material price shrinks to a fraction. That is exactly why many class H motors contain Kapton without it showing up as a cost driver on the bill of materials. Where class F is enough, NMN with a PET core does the same job even more cheaply. As a rule of thumb for the enquiry: solid polyimide only for partial discharge, build height below 50 µm or continuous temperature above 200 °C, otherwise laminate.

High-temperature thermal insulation: where ceramic fibre and calcium silicate belong

Anyone looking for high-temperature insulation for a furnace, an exhaust line or a combustion chamber needs thermal insulation, and there different materials and different figures apply. Ceramic fibre made of aluminium silicate carries classification temperatures from 1,260 °C to EN 1094, calcium silicate boards insulate up to 1,000 °C, glass wool up to around 400 °C and stone wool up to around 700 °C. None of these materials has a thermal class, because none of them is tested for blocking voltage.

One material group stands in both worlds: mica. As a rigid sheet it insulates the heating element electrically and at the same time provides thermal insulation with a conductivity well below that of ceramics. A ceramic fibre paper, by contrast, remains heat protection even when it sits next to a coil. Material choice and layer thickness for plants and pipework are covered in the article on thermal insulation, and the property itself is explained in the article on thermal conductivity.

How GOBA slits and punches high-temperature insulating materials

Selection ends at the datasheet; production begins there. At our plant, aramid paper, polyimide film, laminates, glass fabric and mica arrive as master rolls or sheets and leave as slit rolls, cut-to-size formats or punched parts. Slitting is done by shear cutting between two circular knives in widths from 5 to 1,600 mm with a width tolerance of plus minus 0.1 mm, wound onto cores with 25, 55, 76 or 152 mm inside diameter. Anyone who needs narrow Nomex or polyimide tapes for end windings gets them from our contract slitting with tighter tolerances than the manufacturer's standard roll.

In stamping and forming, material thickness ranges from 0.023 to 3.0 mm, and the maximum size of a punched or bent part is 1,000 x 2,000 mm. For samples and small runs 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 so the part stays in its final position at the customer, and we form complex geometries under heat rather than forcing them cold.

Three materials behave differently in the tool than PET. Polyimide does not fray, but it tears further from a damaged edge when the part is later creased, so the cut edge has to be clean and the fold line pre-embossed. Glass fibre fabrics are abrasive to knives and tools, so at our plant they run on lines designed for abrasive webs. Rigid mica sheets can be punched, and according to the manufacturer datasheet the tool needs a hold-down so the sheet does not chip or delaminate during the cut. Aramid paper, in turn, has a machine direction and a cross direction with different values; DuPont therefore recommends orienting slot liners in the direction with the higher strength, and we fix that with the punching layout on the web. For constructions that do not exist as catalogue goods, such as a laminate with a different layer sequence, a self-adhesive mica part or a small-series cut-to-size format, talk to us about custom products.

GOBA Takeaway

High-temperature insulation starts where PET ends, and it does not end at polyimide. In between lie PEN, PPS, aramid paper and the laminates; above them mica; beside them PEEK and PTFE for special cases. Selection follows the temperature at the hottest point, read as a temperature index or RTI with 20 to 30 kelvin of headroom, never as a short-term value from an advertisement. Anyone who selects this way ends up with aramid paper or a laminate in most cases and saves solid polyimide for the cases where there is no substitute.

Send us the drawing, continuous temperature, voltage and quantity. We will tell you which step of the staircase your part needs, whether a laminate rescues the costing, and in which form we slit or punch it from our insulation materials.

FAQ on High-Temperature Insulation

Which insulation is heat resistant up to 200 °C?

For a continuous temperature of 200 °C the candidates are aramid paper such as Nomex 410 (recognised by UL as a 220 °C material), PPS film with an electrical RTI of 200 °C, PEEK film and polyimide film. In motors and transformers, aramid paper or an NKN laminate is the economical choice; solid polyimide only pays off with partial discharge or very low build height.

Which insulation withstands 300 °C continuously?

At 300 °C, the only organic materials left are PTFE at the edge of its 260 °C continuous rating and polyimide, which according to DuPont turns brittle at 300 °C after three months. Only inorganic insulating materials carry this temperature permanently: rigid mica sheets of muscovite up to 500 °C and of phlogopite up to 700 °C, plus glass fibre and ceramics. That is why heating coils in ovens and dryers sit on mica.

What is the difference between continuous service temperature and temperature index?

The continuous service temperature is a manufacturer's assessment without a standardised time basis. The temperature index TI to IEC 60216 is the temperature at which a property reaches its end point after 20,000 hours, usually half of the initial value. The RTI to UL 746B works similarly but separates electrical and mechanical properties. Design work relies on TI or RTI, not on the short-term limit from advertising.

When is PET film no longer enough?

PET film carries thermal class B with a continuous temperature of 130 °C. If the temperature at the hottest point of the insulation exceeds that, or if the 20 to 30 kelvin headroom is to be maintained, you move to PEN film or NMN laminate for class F and to aramid paper or NKN laminate for class H.

Is high-temperature insulation the same as thermal insulation?

No. Thermal insulation made of ceramic fibre, calcium silicate or insulating wool slows heat flow and has no thermal class, because it is not tested for blocking voltage. High-temperature insulation in the electrical sense blocks voltage under heat, for example as aramid paper, polyimide film or mica. Mica can do both, which is why it sits under heating elements.

Which thermal class do Kapton and Nomex have?

Nomex 410 is used in class H insulation systems (180 °C) and is recognised by UL as a 220 °C material. Kapton HN carries a temperature index of 220 to 240 °C for its electrical properties in UL file E39505 and is assigned to class R and above. Which class the finished system achieves is decided by testing the complete assembly with wire, resin and adhesive.