Mica tape is a flexible insulating tape made of mica paper that is laminated onto a backing of glass fabric or plastic film and bonded with epoxy, polyester or silicone resin. It is wound in several layers around conductors, coils and stator bars of high-voltage motors and generators, where it forms the main insulation that withstands partial discharge. In fire-resistant cables it keeps the circuit closed during a fire. Commercial tapes are 0.06 to 0.26 mm thick, carry 30 to 240 g/m² of mica and reach thermal class 155 (F) or 180 (H) in the finished insulation system (Von Roll, Insulating Systems for High-Voltage Rotating Machines). This article explains the construction, the resin-rich and VPI processes, the tape types to IEC 60371 and the difference from micanite, and it shows which details belong in the specification when you source mica tape through our insulation materials.
Mica tape from GOBA: supply forms, thicknesses and enquiry
GOBA supplies Mica tape from a single piece, as material or as a finished part to your drawing.
- Supply forms
- Roll and slit roll, Cut-to-size
- Thicknesses
- To your specification. GOBA processes materials from 7 µm to 3 mm.
- Versions
- In principle every mica tape on request, with muscovite or phlogopite, on glass fabric or film backing.
- Slit rolls
- Widths from 5 to 1,600 mm, tolerance ±0.1 mm, cores of 25, 55, 76 or 152 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.
Mica as an insulating material: why high-voltage windings need a mineral
In a high-voltage winding, partial discharges ignite in voids and at edges, small sparks that erode organic insulating materials layer by layer. Mica is a sheet silicate without a carbon chain and hardly erodes under these discharges. In the tape its flakes form a barrier that the discharge cannot get past. That is why mica is the standard for high-voltage machines above 6 kV, as the article on corona discharge shows. How partial discharges arise and how they are measured is explained in the article on partial discharge.
Two types of mica are made into tape. Muscovite, the potassium mica, is the electrically stronger one and withstands 25 to 200 kV/mm as a single flake (Amidon). Phlogopite, the magnesium mica, tolerates more heat and is the mica of fire-resistant cables (Cogebi, FIROX P). For the winding the voltage counts, for the cable in a fire the temperature. Mineralogy, temperature limits and density of both types are covered in the article on micanite, and which other materials insulate high voltage in the article on high-voltage insulation.
Construction: mica paper, backing and binder
Mica paper consists of very small mica flakes without any binder (IEC 60371-1). On its own it is too weak to be wound onto a coil under tension. The tape is therefore a composite of three layers, and some resin-rich tapes add an interleaving foil (Von Roll).
| Layer | Material | Function | Values (source) |
|---|---|---|---|
| Mica paper | muscovite, also calcined, or phlogopite | blocks the voltage, resists partial discharge | 30 to 75 g/m² in conductor tape, 120 to 240 g/m² in main wall tape (Von Roll) |
| Backing | glass fabric, PET film, PET fleece, polyimide film | takes the tensile load during wrapping | glass 23 g/m² or PET film 23 µm in cable tape (IEC 60371-3-8) |
| Binder | epoxy, polyester, silicone | bonds paper and backing, in resin-rich tape also the resin of the insulation | 8 ± 3 % or 16 ± 3 % for VPI (IEC 60371-3-5), 30 to 40 % for resin-rich (Krempel) |
The backing follows the application. In Von Roll’s main wall tapes glass fabric is the rule: it stays dimensionally stable and, as a fabric, porous enough for impregnation, which is why IEC 60371-3-5 includes an air porosity test. Polyester film makes the tape thin and smooth. Von Roll uses it for conductor tapes of 0.06 to 0.11 mm (Samicafilm) that reach a breakdown voltage of 5 to 8 kV per layer. Polyimide film lifts a 0.09 mm tape into thermal class 180 (H) (Samicatherm PI). For fire-resistant cables IEC 60371-3-8 permits glass fabric and plastic film, and the phlogopite tapes of cable suppliers carry glass fabric with silicone resin (Cogebi, FIROX).
Resin-rich or VPI: two routes to the finished mica tape insulation
On delivery a mica tape is not yet an insulation. Only at the motor or generator plant do the wrapped layers become a solid, void-free wall. There are two processes for this, and they differ in when the resin enters the tape.
| Feature | Resin-rich | VPI (vacuum pressure impregnation) |
|---|---|---|
| Resin content of the tape | 30 to 40 % thermoset binder (Krempel, KREMICA.therm) | 8 ± 3 % (low bond) or 16 ± 3 % (medium bond) (IEC 60371-3-5) |
| Condition on delivery | resin in B-stage, flexible, cures after wrapping (IEC 60371-3-6) | porous and almost dry, waiting for the impregnating resin |
| Curing | coil or bar in the heated press, conventional or hydrostatic (Von Roll) | impregnation of the wound coils under vacuum and pressure, then oven, for example 8 h at 140 to 150 °C (Von Roll) |
| Thickness | 0.16 to 0.26 mm (Samicatherm), 0.14 mm (Dongfang 5440-1D) | 0.13 to 0.18 mm (Samicapor) |
| Mica | 120 to 240 g/m² | 160 to 180 g/m² |
| Thermal class | 155 (F) (Dongfang), with polyimide film 180 (H) (Von Roll) | 155 (F) or 180 (H), depending on the impregnating resin (Von Roll) |
Resin-rich tape brings all of its resin with it, and every coil is wrapped and pressed individually. VPI tape is almost dry: the winding is wrapped first and then impregnated with resin under vacuum (Wikipedia, Kunstglimmer). Von Roll calls the simple winding process with dry coils a recognised benefit of VPI technology.
For purchasing this leads to one rule: never order a VPI tape without naming the impregnating resin. IEC 60371-3-5 states that the properties of the cured insulation depend primarily on the VPI resin. Some resins cure only with an accelerated tape, Von Roll specifies this for the epoxy anhydride resin Damisol 3407, and Krempel supplies its VPI tapes with and without accelerator. The thermal class also depends on the resin: the tape Samicapor 366.58 reaches class 155 (F) with accelerated epoxy resin and 155 (F) or 180 (H) with the polyesterimide system Samicabond. The thermal class is a property of the system of tape and resin. The basics are covered in the article on thermal classes of insulation.
Tape types by position: conductor, main wall, end winding, cable
In the insulation system of a high-voltage coil several mica tapes sit on top of each other, each with its own job. IEC 60371-3 describes the most important ones in separate sheets.
| Tape type | Construction | Thickness | Standard | Use |
|---|---|---|---|---|
| Conductor tape | mica paper on one or two PET films, epoxy, partly with hot-melt adhesive | 0.06 to 0.11 mm | IEC 60371-3-7 | turn and strand insulation, butt-lapped or overlapped (Von Roll, Samicafilm) |
| Main wall tape VPI | mica paper on glass fabric, little epoxy | 0.13 to 0.18 mm | IEC 60371-3-5 | main insulation against the laminated core (Samicapor, KREMICA.por) |
| Main wall tape resin-rich | mica paper on glass fabric with B-stage epoxy, also on PET film | 0.16 to 0.26 mm | IEC 60371-3-6 | main insulation of pressed coils and bars (Samicatherm, KREMICA.therm) |
| End winding tape | mica on glass fabric or on PET film with glass threads, highly flexible | 0.12 to 0.18 mm | by manufacturer | coil ends outside the slot (Samicaflex, Filosam) |
| Cable tape | phlogopite or muscovite on glass fabric or film, resin impregnated | 0.05 to 0.15 mm | IEC 60371-3-8 | circuit integrity of fire-resistant cables |
How these layers work together in the coil is shown in the articles on form-wound coils and turn insulation. In a cable a different standard applies. The mica tape sits as a layer on the conductor with the plastic insulation on top, and once the plastic has burnt in a fire, only the mica separates the cores. Nexans tests its marine cable MPRX 331 with mica tape and XLPE insulation to IEC 60331 for 90 minutes of circuit integrity. Cable tape is usually wrapped with 20 to 50 % overlap (Cogebi). More on the construction of cable cores is in the article on cable insulation.
Mica tape, micanite or heating tape: three products that are often confused
Micanite and mica tape are made from the same mica paper. Micanite arrives fully cured as a sheet, foil or moulded part and is punched or installed. Mica tape is wrapped and cured only at the customer’s plant. For a part to drawing micanite is therefore the material, for a winding or a cable core the tape.
The second confusion concerns electric heating tapes. A heating tape carries a heating conductor, is connected to mains voltage and has a power rating in watts. Mica tape carries no current, it insulates. Anyone who wants to heat pipes or protect them from frost is looking for a heating tape. Mica sheets as carriers of heating wires in hair dryers and toasters are covered in the micanite article.
One objection is that mica tape is oversized for low-voltage motors. That is true as long as no partial discharges occur in the winding: there aramid papers such as Nomex and polyimide films insulate the slot reliably. Once the winding sees partial discharges in service, typically in high-voltage machines above 6 kV, the calculation flips. Organic films erode, mica remains. A middle route is Nomex 418, an aramid paper with 50 % mica content that lasts much longer under corona than Nomex 410 (DuPont, see the article on corona discharge).
Specifying mica tape: these details define the tape
Mica tapes differ so much in mica type, backing, binder and resin content that “mica tape 0.15 mm” is not yet an order. With these details the tape is clearly described:
- Application: conductor insulation, main wall, end winding or cable.
- Process: resin-rich or VPI, for VPI the impregnating resin with manufacturer and type and whether an accelerated tape is needed.
- Thermal class of the insulation system, for example 155 (F) or 180 (H).
- Mica type, muscovite or phlogopite, and the mica weight in g/m².
- Backing: glass fabric, PET film, PET fleece or polyimide film, one or both sides.
- Thickness and, if specified, the sheet of IEC 60371-3.
We supply mica tape to these specifications and slit it from the master roll to the width your taping machine needs, in contract slitting between two circular knives. If you have the datasheet of your current tape, send it along. We compare mica weight, backing and resin content and offer you a tape with the same values.
