Insulating paper is a flat electrical insulating material made of cellulose or aramid fibres, alone or laminated with polyester or polyimide film, that separates the winding from the core and the phases from each other in electric motors, generators and transformers. In motor building it is also called insulation paper or electrical insulation paper. The thermal class ranges from 105 °C for pressboard through 155 °C for the laminates DMD and NMN to 220 °C for aramid paper: Nomex 410 is recognised by UL as a 220 °C insulation and typically withstands 33 kV/mm at 0.25 mm thickness (Arclin, Nomex 410 datasheet, April 2026). This article ranks the five paper families by thermal class and breakdown, shows where they sit in the motor and explains how we process insulating paper into insulating parts to drawing such as slot liners and slot closures.
Insulating paper from GOBA: supply forms, thicknesses and enquiry
GOBA supplies Insulating paper from a single piece, as material or as a finished part to your drawing.
- Supply forms
- Roll and slit roll, Sheet and board, Cut-to-size, Die-cut part to drawing, Formed part (folded, embossed or bent)
- Thicknesses
- To your specification. GOBA processes materials from 7 µm to 3 mm.
- Versions
- Pressboard, aramid paper (Nomex) and the laminates NMN, NKN and DMD.
- Slit rolls
- Widths from 5 to 1,600 mm, tolerance ±0.1 mm, cores of 25, 55, 76 or 152 mm
- Die-cut and formed parts
- Material thickness 0.023 to 3.0 mm, dimensions up to 1,000 × 2,000 mm
- Quantity
- From 1 piece, from sample to series
- Lead time
- Two weeks at most when the material is in stock
What we need for your quotation
- Drawing or dimensions
- Quantity
- Material and thickness
For rolls also width, length, core diameter, core material (for example plastic) and tolerances.
Insulating paper in a motor means five material families
The encyclopedia defines electrical insulation paper as a speciality paper of finely ground cellulose, usually resin impregnated, free of pores and of conductive impurities. Its examples are cable paper and the electrolytic paper in capacitors (Wikipedia, Elektroisolierpapier). In electrical machine building the term is broader. Anyone ordering insulating paper there means one of five flexible sheet insulations:
- Pressboard of unbleached sulphate pulp, the classic cellulose paper.
- Aramid paper of meta-aramid fibres, best known as Nomex 410.
- DMD, a polyester film with polyester fleece on both sides.
- NMN, a polyester film between two layers of aramid paper.
- NKN, a polyimide film between two layers of aramid paper.
Pressboard is tested to IEC 60641, the laminates to IEC 60626 (SynFlex, datasheets SynKraft E and SynTherm DMD). The laminates combine the dielectric strength of the film with a paper or fleece surface that absorbs impregnating resin and protects the film. How these materials fit into the wider group is shown in the article on flexible sheet insulation.
Insulating paper by thermal class: from 105 °C to 220 °C
The thermal class is the first selection criterion because it sets the continuous temperature the insulation system withstands over its service life. The classes to IEC 60085 are explained in the article on thermal classes. The table shows typical representatives with values from manufacturer and distributor datasheets.
| Insulating paper | Build-up | Thermal class | Breakdown (example) | Source |
|---|---|---|---|---|
| Pressboard | 100 % unbleached sulphate pulp | A, 105 °C | 11 kV/mm | SynFlex, SynKraft E |
| Pressboard laminate | Pressboard, polyester film, pressboard | B, 130 °C | min. 8 kV at 0.30 mm | Maier Kupferdraht, Trivolton |
| DMD | Polyester fleece, polyester film, polyester fleece | F, 155 °C (unimpregnated B) | 10 kV at 0.23 mm | SynFlex, Müller-Ahlhorn |
| NMN | Aramid paper, polyester film, aramid paper | F, 155 °C, up to 180 °C in UL systems | min. 12 kV at 0.22 mm | Maier Kupferdraht, DuPont |
| Aramid paper | 100 % meta-aramid | H, 180 °C, Nomex 410 at UL 220 °C | 33 kV/mm at 0.25 mm | Arclin, SynFlex |
| NKN | Aramid paper, polyimide film, aramid paper | H, 180 °C, up to 220 °C in UL systems | min. 8 kV at 0.20 mm | Maier Kupferdraht, DuPont |
The breakdown column does not compare like with like. Laminates are usually listed as a minimum voltage in kV for the total thickness, Nomex 410 as a typical field strength in kV/mm from a short-term test. Converting 33 kV/mm to 0.25 mm gives about 8 kV, less than the NMN minimum of 12 kV at 0.22 mm. That is no ranking, because minimum and typical values come from different test set-ups. Compare insulating paper within one datasheet series, or test both candidates in the same set-up.
Pressboard: the cellulose paper for class A and the transformer
Pressboard is densified paper of unbleached sulphate pulp. An electrical pressboard such as SynKraft E has a density of 1.20 g/cm³, withstands 11 kV/mm, reaches 80 N/mm² tensile strength lengthwise and shrinks 0.7 % lengthwise and 1.2 % crosswise. It contains up to 8 % moisture and comes as rolls from 0.1 to 1.0 mm or as sheets up to 3.0 mm thick (SynFlex, SynKraft E datasheet, test standard IEC 60641-2). Low-density pressboard such as Flexiso FI 11052 sits at 1.0 g/cm³ and 8.0 kV/mm (Müller-Ahlhorn, to IEC 60554-2).
Pressboard absorbs dip and trickle resin well and stops at 105 °C. In the motor that means slot insulation and slot closures for machines with low thermal load. In the transformer it serves as ground, layer and banding insulation. To reach class B with cellulose, use the laminate of pressboard and polyester film rated 130 °C. For large transformers pressboard comes as thick board, the transformerboard. Grades and standard types are covered in the article on pressboard.
Aramid paper: Nomex 410 and the 1.6 kV/mm limit
Aramid paper consists of meta-aramid fibres and reaches class H without film. The best known is Nomex 410, available from 0.05 to 0.76 mm at a density of 0.72 to 1.08 g/cm³. Its dielectric strength is 18 kV/mm at 0.05 mm, 33 to 34 kV/mm between 0.18 and 0.38 mm and 27 kV/mm at 0.76 mm. At 400 °C it still holds 12 kV/mm for several hours (Arclin, Nomex 410 datasheet). The current datasheet of April 2026 is published under the Arclin name, older editions under DuPont.
A different value counts for design. Arclin recommends that continuous stress on Nomex 410 in dry-type applications should not exceed 1.6 kV/mm, because above that partial discharges can start in the air at the paper. By this rule a 0.25 mm paper carries about 400 V continuously. The 33 kV/mm from the short-term test is a test value for comparing lots.
Besides Nomex there are other aramid papers, for example SynTherm YT of 100 % meta-aramid with a UL temperature index of 210 °C and an oxygen index above 28 % (SynFlex). Calendered and uncalendered types are compared in the article on aramid paper, the Nomex family in the article on Nomex.
Laminates: DMD for class F, NMN for reserve, NKN above 180 °C
In DMD a polyester film sits between two layers of polyester fleece. It reaches class F only with impregnated fleece, uncoated DMD counts as class B (Müller-Ahlhorn). SynTherm DMD comes in total thicknesses from 0.10 to 0.45 mm. Breakdown voltage rises from 4.5 kV to 22 kV, with the film accounting for 36 to 350 µm (SynFlex, SynTherm DMD datasheet, IEC 60626-2). Its smooth, stiff surface runs on insulating and inserting machines. Details are in the article on DMD laminate.
NMN replaces the fleece with aramid paper. Standard builds use calendered Nomex 464 or 416 at 50, 80 or 130 µm per layer (NMN 5, 8 and 13), with total thickness between 0.12 and 0.82 mm (Müller-Ahlhorn). As Trivoltherm N 50, NMN holds at least 12 kV at 0.22 mm and at least 23 kV at 0.47 mm (Maier Kupferdraht). The layer build-up is explained in the articles on NMN laminate and Trivoltherm.
The difference shows under overload. At 155 °C the polyester in both laminates embrittles over time. In NMN the aramid layer stays largely unaffected and keeps separating electrically, in DMD only the impregnating resin slows the degradation. The oxygen index of DMD is 18 to 24 %, of NMN 27 to 34 %. Above 21 % a material self-extinguishes in air (Ingeniería Eléctrica, March 2016).
Our recommendation: DMD for series motors of class F whose winding stays safely below 155 °C and whose impregnation runs evenly. NMN as soon as the motor sees overload, needs reserve or a failure becomes expensive. How evenly a DMD is impregnated is hard to verify at final inspection. In NMN the aramid paper carries the properties itself.
NKN swaps the polyester film for polyimide film (Kapton). Maier Kupferdraht lists it as class H with at least 8 kV at 0.20 mm and 11 kV at 0.40 mm and states 200 °C in the insulation system. DuPont refers to UL-recognised systems up to and including 220 °C. NKN belongs in motors and generators that run continuously above class H.
Where insulating paper sits in an electric motor
In the stator, insulating paper separates copper from iron or copper from copper at several points. Each point makes different demands on thickness, stiffness and surface:
- As slot insulation it lines every stator slot before the winding is inserted. Bending stiffness matters here so that the liner does not buckle on insertion and sits firmly in the slot with a slight spread (SynFlex).
- As a slot closure it closes the slot at the top and keeps the wires in the slot.
- As phase insulation it separates the phases in the end winding, often from conformable, uncalendered paper as in NMN 411 (Müller-Ahlhorn).
- In the transformer it lies as layer insulation between the winding layers and as core and cover insulation.
How these parts work together in the finished stator is shown in the article on stator insulation.
Selecting insulating paper: four questions in this order
- Which thermal class does the insulation system have? The paper must carry at least this class. A class F motor needs DMD, NMN or higher, pressboard is ruled out.
- Which voltage is applied continuously? Thickness follows from the permissible continuous stress, the short-term breakdown from the datasheet is only a test value. For aramid paper in air Arclin calculates with at most 1.6 kV/mm.
- How is it processed? Inserting machines need a smooth, stiff layer such as DMD or NMN. Manufacturers limit the thickness for feathered tapes, for SynTherm DMD to 0.23 mm.
- Which resin impregnates the winding? Paper and fleece surfaces must absorb it. In DMD the cured resin protects the film against oxidation and hydrolysis (SynFlex).
A common objection: why not use aramid paper for everything? The thermal class applies to the whole insulation system of wire, resin and sheet insulation. SynTherm VPV, for example, is class B as a material and reaches class F in the UL insulation system (SynFlex). Aramid paper therefore does not lift a class F motor to class H. It pays off where its properties are needed: continuous temperature above 155 °C, flame resistance or a layer that must keep separating when overheated.
How GOBA processes insulating paper
We stock pressboard, aramid paper (Nomex) and the laminates NMN, NKN and DMD and make slot insulation, slot liners, slot closures, intermediate slides and slot caps from them to your drawing. We slit roll stock in contract slitting to widths from 5 to 1,600 mm with a width tolerance of ±0.1 mm. We produce die-cut parts through stamping and forming with steel rule dies or solid steel tools in material thicknesses from 0.023 to 3.0 mm and up to 1,000 x 2,000 mm, folded, embossed or bent on request.
Which paper carries your thermal class and test voltage we decide with you on the basis of the drawing. Our range of papers, films and laminates is listed under insulation materials.
