Vulcanised fibre is a laminated material of pure cellulose: several plies of unsized paper are swollen in zinc chloride solution or sulphuric acid, pressed into a homogeneous sheet, leached and dried, without any resin or adhesive. The red standard sheet has a density of about 1.2 g/cm³, withstands 7.7 to 9.1 kV/mm depending on thickness, lasts 80 seconds in the arc test and is UL listed electrically for a continuous 115 °C (ESPE, datasheet Vulcanized Fibre, UL file E55656). German suppliers list it in thermal class Y at 90 °C (Müller Ahlhorn, Flexiso VF). This article covers manufacturing, properties, temperature, moisture, standards and applications, compares vulcanised fibre with pressboard and Nomex and explains how we punch it into insulation and molded parts to drawing.
Vulcanised fibre from GOBA: supply forms, thicknesses and enquiry
GOBA supplies Vulcanised fibre from a single piece, as material or as a finished part to your drawing.
- Supply forms
- 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
- As sheet, cut-to-size or as die-cut and formed part to your drawing.
- 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
Manufacturing: paper plies, zinc chloride and months of leaching
The process goes back to a British patent granted to Thomas Taylor in 1859 (Wikipedia, Vulcanized fibre). The base is an unsized raw paper made from cotton linters, rags or wood pulp. The sheet is made in three steps:
- Six to ten paper webs run together through a parchmentising bath, in the older process a zinc chloride solution of about 70 %, in the newer one sulphuric acid. The fibre surface swells and forms hydrated cellulose, which bonds the plies into one piece without a binder (Wikipedia, Vulkanfiber).
- The webs are pressed and leached in progressively weaker baths until no more than 0.05 % zinc chloride remains. For the thickest boards this takes 18 months to two years, and leaching too fast tears the plies apart (Wikipedia, Vulcanized fibre).
- The sheet is dried to 5 to 6 % moisture and calendered flat. It shrinks by about 10 % in the machine direction, 20 % across and 30 % in thickness.
For electrical insulation, what matters is how much of the parchmentising agent stays in the material. The Karl Popp datasheet gives less than 0.01 % zinc chloride and a pH of 6.2 in the aqueous extract for red hard grade. Sulphuric acid fibre leaves no detectable residue (Wikipedia). Continuous lines produce webs up to 2.4 mm thick, thicker boards are built up from several plies of vulcanised fibre (ESPE: laminated from 2.6 mm).
Vulcanised fibre properties: strength, breakdown and arc
Vulcanised fibre comes in two basic grades. The commercial grade is red, grey or black and is meant for mechanical and electrical parts. The electrical grade, known as fishpaper, is thinner, more flexible and designed for dielectric strength. IEC 60667-3-1 distinguishes exactly these two types. The table shows typical values from the datasheets of a UL listed manufacturer, converted from inches and psi.
| Property (test standard) | Commercial grade | Electrical grade (fishpaper) |
|---|---|---|
| Available thickness | 0.25 to 25 mm | 0.13 to 1.6 mm |
| Density (ASTM D619) | 1.20 g/cm³ | 1.20 g/cm³ |
| Tensile strength MD / CD (ASTM D638) | 124 / 62 N/mm² | 145 / 69 N/mm² |
| Flexural strength MD / CD (ASTM D790) | 179 / 110 N/mm² | 200 / 110 N/mm² |
| Compressive strength (ASTM D695) | 241 N/mm² | 241 N/mm² |
| Dielectric strength at 0.4 mm (ASTM D149) | 9.1 kV/mm | 15.7 kV/mm |
| Dielectric strength at 1.6 mm | 7.9 kV/mm | 8.5 kV/mm |
| Dielectric strength at 3.2 mm | 7.7 kV/mm | 7.9 kV/mm |
| Arc resistance (ASTM D495) | 80 s | 125 s |
| Comparative tracking index CTI | 400 to 599 V | 400 to 599 V |
| Water absorption in 24 h (ASTM D570) | 66 % | 63 % |
| Continuous temperature electrical / mechanical (UL) | 115 / 110 °C | 115 / 110 °C |
| Flammability | UL 94 HB | UL 94 HB |
Source: ESPE Manufacturing, datasheets Vulcanized Fibre and Fishpaper Properties, UL file E55656. Buyers often misread three things. First, these are typical values. Müller Ahlhorn guarantees at least 6.0 kV/mm for its vulcanised fibre, Popp at least 90 N/mm² in the machine direction and 50 N/mm² across. Design with these minimum values. Second, dielectric strength per millimetre falls with thickness: the commercial grade withstands 9.1 kV/mm at 0.4 mm and only 7.7 at 3.2 mm. Why this applies to all solids is explained in the article on electrical breakdown strength. Third, vulcanised fibre has only half the tensile strength across the machine direction. For a die-cut part that carries tension at a web, the machine direction therefore belongs on the drawing.
Thermal conductivity is 0.29 W/(m·K) at 65 °C, thermal expansion about 20 x 10-6 /K in the machine direction and 31 x 10-6 /K across (ESPE). What a CTI of 400 to 599 V means for clearances and creepage distances is covered in the article on tracking resistance.
Which temperature vulcanised fibre withstands: 90, 105 or 115 °C
Hardly any material carries so many different temperature ratings. Each one describes a different case:
- 70 °C is given by a maker of union gaskets for water and heating (Kirchhoff, gasket 18 x 24 x 1.5 mm).
- 90 °C corresponds to thermal class Y, in which Müller Ahlhorn rates its vulcanised fibre.
- 0 to 105 °C is Popp's continuous rating for untreated, unimpregnated material, with a flash point above 300 °C.
- 110 °C mechanical and 115 °C electrical are the continuous ratings UL lists for the ESPE grades.
- 130 °C applies continuously under oil, 180 °C briefly during processing (Wikipedia, Vulkanfiber).
Our recommendation: for an insulation system, calculate with class Y, that is 90 °C. The thermal classes to IEC 60085 rate the whole system, and cellulose slowly loses elasticity above that. Use 115 °C only if the drawing calls for a UL listed grade. If the winding runs in class B (130 °C) or higher, vulcanised fibre is the wrong material. A second limit applies at the cold end: dry vulcanised fibre tolerates frost, wet fibre loses its ply bond below 0 °C (Popp).
Vulcanised fibre and moisture: swelling, dimensional change and tolerance
Vulcanised fibre is cellulose and absorbs water. Ex works it contains 5 to 6 % moisture (Popp), in a water bath it absorbs 63 to 66 % of its weight in 24 hours (ESPE). The plies survive this: Popp tests 24 hours in water at 80 °C and another 24 hours at 90 °C without delamination. The dimensions do not. For every percentage point of moisture change the sheet grows by 1.0 % in thickness, 0.1 % in the machine direction and 0.25 % across (ESPE).
A worked example shows what this means for a drawing. A die-cut part is 100 mm long across the machine direction. If the material moisture rises from 5 to 8 %, it grows by 0.75 mm. The general tolerance to DIN ISO 2768, class m, allows ±0.3 mm for this dimension. We therefore recommend three entries: a reference climate for dimensional inspection, the machine direction for long parts and packed storage until assembly. Electrically, moisture has a second effect, it lowers the insulation value (Wikipedia, Vulcanized fibre). For parts in a humid environment without a housing, a film is the better choice.
In a gasket the swelling is wanted. A fibre washer absorbs some water, swells into the roughness of the sealing face and seals. It is a single-use part: after the union is undone, a new one goes in (Wikipedia, Vulcanized fibre).
Chemical resistance: oil and solvents yes, strong acids and alkalis no
Vulcanised fibre tolerates the media found in motors and gearboxes. Popp lists oils, petrol, tar oils, acetone, alcohol, toluene, benzene, chloroform and fatty acids as not attacking it. In water, salt solutions, glycol and weak acids the material swells without decomposing. It is attacked by:
- Caustic soda above 5 %
- Nitric acid above 10 % and hydrochloric acid above 8 %
- Hydrofluoric acid and hydrogen peroxide
Resistance to weak acids and alkalis only applies to permanent immersion, not to alternating soaking and drying (Popp).
Standards for vulcanised fibre: from DIN 7737 to DIN EN IEC 60667
Anyone who finds DIN 7737, DIN 7738 or DIN VDE 0312 on old drawings is ordering to standards from 1959 that have been withdrawn. DIN Media recommends the DIN EN IEC 60667 series instead, edition 2023-03, as VDE 0312:
- Part 1 (VDE 0312-100) with definitions and general requirements. It divides vulcanised fibre into general type and fishpaper type.
- Part 2 (VDE 0312-200) with the test methods: thickness, apparent density, tensile strength, flexural strength, water absorption, electric strength up to 3 mm thickness, arc resistance to IEC 61621, chloride and ash content, flexibility, moisture, ply adhesion and shrinkage.
- Part 3-1 (VDE 0312-301) with the limit values for flat sheets.
The series only covers single-piece sheets. Bonded multiple plies are excluded (IEC 60667-2, clause 1). American drawings usually specify ASTM D710, tested to ASTM D619.
Where vulcanised fibre is used
Vulcanised fibre sits wherever a part has to insulate and at the same time take pressure, impact or an arc, at temperatures below 90 °C:
- Arc quenching chambers, arc shields and baffles in circuit breakers, plus fuse tubes and lightning arresters (Müller Ahlhorn, Redseal, ESPE)
- Terminal stars in electric motors, slot insulation in small armatures (Müller Ahlhorn, ESPE)
- Insulating plates, terminal block covers, coil spool heads and washers (ESPE)
- Flat gaskets and fibre washers for unions on water and heating pipes, more in the article on sealing technology
- Strain reliefs and threaded rods (Müller Ahlhorn)
- Backing discs for abrasives, release films for polyester laminates and deep-drawn parts such as welding masks (Wikipedia)
In our die-cut parts for the electrical industry we use vulcanised fibre mainly for mechanically loaded spacers and covers. An overview of all materials for this is given in the article on die-cut parts for the electrical industry.
Vulcanised fibre or pressboard, PET film, Nomex: the choice
Vulcanised fibre competes with three materials that GOBA also punches. Temperature almost always draws the line, mechanics come second.
| Material | Continuous temperature | Dielectric strength | Strength in use |
|---|---|---|---|
| Vulcanised fibre | 90 °C (class Y) | 6 to 15.7 kV/mm | Toughness, pressure, arc, parts up to 25 mm thick |
| Pressboard | 105 °C (class A) | up to over 10 kV/mm | Layer and slot insulation in class A, transformers |
| PET film | 130 °C (class B) | up to 180 kV/mm (thin film) | Thin separating layers, 0.3 % water absorption |
| Nomex 410 | 220 °C (UL) | 18 to 34 kV/mm | Windings in classes F and H |
The values for the comparison materials come from our articles on pressboard, polyester film and Nomex. Our recommendation: for layer and slot insulation in windings, use pressboard, PET film or Nomex depending on the class. Choose vulcanised fibre for the part next to it that is pressed, bolted or hit by the arc: the end plate, the washer, the quenching chamber.
The most common objection is that vulcanised fibre is a material of the past. For windings above 90 °C that is true. For parts below this limit there is no reason to switch: the sheet takes 241 N/mm² of pressure, lasts 80 to 125 seconds in the arc test, can be punched, bent, embossed, deep-drawn and cut with a CO2 laser (Wikipedia, Müller Ahlhorn) and consists of renewable cellulose without resin.
How GOBA processes vulcanised fibre into die-cut parts
We punch vulcanised fibre to your drawing from sheets up to 3.0 mm thick, and parts can be up to 1,000 x 2,000 mm in size. For samples and small batches we use steel rule dies, for high volumes and tight tolerances solid steel tools on the automatic punching press. How the tools differ is shown on the page stamping and forming. Without a specific requirement we work to DIN ISO 2768-m, which means ±0.3 mm at an edge length of 50 mm.
For a quote we need the continuous temperature at the part, the test voltage, the drawing with thickness and tolerance and, for long parts or parts under tension, the machine direction. If the temperature is above 90 °C, we suggest a material from our range of insulation materials that carries the class.
