Selecting the right electrical insulation material can make or break a project. Among the many options available, fish paper — technically known as vulcanized fiber — stands out as a cost-effective, mechanically robust solution that has served the electrical industry for over a century. Whether you are designing motor slot liners, transformer barriers, or switchgear components, understanding fish paper’s properties, standards, and sourcing options is essential for making informed procurement decisions.
This guide draws on decades of insulation material expertise. If you are exploring how paper performs as an electrical insulator, fish paper occupies a unique middle ground — offering better mechanical strength than kraft paper at a fraction of the cost of aramid materials like Nomex.
What Is Fish Paper and Why Is It Called That?
Fish paper is the commercial name for vulcanized fiber, a dense, homogeneous material produced by chemically treating cotton rag or wood pulp paper with zinc chloride. The vulcanization process partially dissolves and re-bonds the cellulose fibers into a continuous, non-delaminating sheet — creating a material that is far tougher than the paper it started from. The result is a hard, durable laminate with excellent electrical insulation properties and surprising mechanical resilience.
The unusual name has nothing to do with aquatic life. Historically, the material earned the nickname “fish paper” because its smooth, flexible surface and greyish color resembled the skin of certain fish. An alternative theory points to early manufacturing facilities located near fisheries, though the surface-appearance explanation is more widely accepted among industry veterans. For a deeper dive into the naming history, see this detailed article on the origins of fish paper and its applications.
Today, fish paper is manufactured in sheets, rolls, and custom die-cut shapes. Common designations in the electrical industry include 6520 fish paper and 6521 fish paper laminates, which combine vulcanized fiber with polyester film for enhanced dielectric performance. SIDA supplies both standard 6520 fish paper sheets and 6520-6521 lami
nated fish paper products tailored to specific voltage and mechanical requirements.
What Are the Electrical and Mechanical Properties of Vulcanized Fiber?
Vulcanized fiber occupies a distinctive position in the insulation materials landscape — it combines moderate dielectric performance with above-average mechanical toughness. The key electrical property is dielectric strength, which typically ranges from 150 to 250 V/mil (approximately 6 to 10 kV/mm) depending on thickness grade. This makes fish paper suitable for low to medium-voltage applications, though it falls short of the dielectric ratings achieved by high-performance aramid papers.
On the mechanical side, fish paper truly shines. Its tensile strength can reach 8,000 to 12,000 psi (55 to 83 MPa) in the machine direction, with flexural strength making it highly resistant to cracking during punching, shearing, and forming operations. The material exhibits excellent abrasion resistance and can be cleanly die-cut into complex geometries — slot liners, end laminations, and insulating washers — without fraying or delaminating at the edges. These mechanical advantages explain why fish paper remains popular for motor winding insulation, where it must withstand winding tension and vibration over the equipment’s service life.
Temperature performance is the primary constraint. Vulcanized fiber is rated for Class A (105°C) continuous operation. Prolonged exposure above this threshold leads to gradual embrittlement and loss of dielectric integrity. Engineers designing for Class B (130°C) or higher environments should evaluate alternative materials. For a broader comparison of insulation paper materials and their applications, consult our comprehensive material selection guide.
What NEMA and IEC Standards Cover Fish Paper?
Compliance with recognized standards is non-negotiable in electrical manufacturing. Fish paper is primarily governed by NEMA (National Electrical Manufacturers Association) specifications in North America, with parallel references in the IEC system for international projects. The two most relevant NEMA grades for vulcanized fiber are NEMA F and NEMA FR, which define minimum mechanical and electrical performance thresholds.
NEMA Grade F represents the general-purpose commercial grade, suitable for most electrical insulation applications including slot liners, phase barriers, and insulating washers. NEMA Grade FR adds a flame-retardant treatment that significantly reduces flame propagation — increasingly important in consumer electronics, appliances, and any application subject to UL 94 flammability requirements. Grade FR typically commands a 15-25% price premium over standard Grade F but is often mandatory for OEM-specified components.
The table below summarizes the key NEMA grade distinctions:
| Property | NEMA Grade F | NEMA Grade FR |
|---|---|---|
| Flame resistance | Standard | Flame-retardant (UL 94 V-0 capable) |
| Dielectric strength | 150-200 V/mil | 150-200 V/mil |
| Tensile strength (MD) | 8,000-12,000 psi | 7,500-10,000 psi |
| Water absorption (24h) | 40-60% | 35-55% |
| Continuous temperature | 105°C (Class A) | 105°C (Class A) |
| Typical applications | Motors, transformers, switchgear | Appliances, consumer electronics, enclosed devices |
IEC equivalents are referenced under IEC 60667 (vulcanized fibre for electric
al purposes), which defines dimensional tolerances, test methods, and classification. Always confirm the specific IEC designation with your testing laboratory, as regional interpretations may vary. Buyers should request a certificate of conformance (CoC) with every shipment to ensure lot-level traceability.
Fish Paper vs Nomex vs Kraft: When to Use Each
Choosing between fish paper, Nomex (aramid paper), and kraft paper requires balancing three variables: thermal class, dielectric requirements, and budget. Each material serves a distinct niche, and selecting the wrong one leads to either over-engineering costs or premature field failures.
Nomex aramid paper dominates high-temperature applications with its Class H (180°C) rating and excellent dielectric stability. It is the default choice for traction motors, aerospace systems, and premium industrial transformers. However, its cost is typically 5-8 times higher than fish paper on a per-square-meter basis — a factor that quickly adds up in high-volume motor production lines where thousands of slot liners are consumed monthly.
Kraft paper, particularly diamond-dotted (DDP) kraft, is the workhorse of oil-immersed transformer insulation. It provides adequate dielectric performance at very low cost when operating in an oil-filled environment that suppresses partial discharge. Kraft’s limitations become apparent in air-cooled or open-frame applications, where its low mechanical strength and hygroscopic nature lead to moisture uptake and dielectric degradation. For engineers weighing the drawbacks of paper insulation, moisture sensitivity is consistently the top concern.
The decision matrix below helps engineers match the application to the material:
| Criterion | Fish Paper (Vulcanized Fiber) | Nomex (Aramid) | Kraft Paper |
|---|---|---|---|
| Temperature class | Class A (105°C) | Class H (180°C) | Class A (105°C) |
| Dielectric strength | Moderate (150-250 V/mil) | High (450-800 V/mil) | Moderate (200-400 V/mil in oil) |
| Mechanical durability | Excellent | Very good | Low to moderate |
| Relative cost | Low ($) | High ($$$$$) | Very low ($) |
| Best for | Motor slot liners, punch-formed parts, switchgear barriers | Aerospace, traction motors, premium transformers | Oil-immersed transformer layer insulation |
| Key limitation | Temperature ceiling, moisture absorption | Cost | Low mechanical strength, moisture sensitivity |
For the majority of fractional and medium-horsepower motor applications operating at Class A or B temperatures, fish paper provides the optimal cost-performance ratio. It handles the mechanical demands of automated winding processes — slot insertion forces, vibration, and thermal cycling — at a price point that keeps the bill of materials competitive. Nomex enters the conversation only when the thermal envelope exceeds 130°C or when end-customer specifications explicitly mandate aramid insulation.
How to Order Fish Paper: Thickness, Sheet Size and Price Factors
Ordering fish paper requires specifying several parameters to ensure the material matches your production process. The most critical dimensions are thickness, sheet size, and grade. Standard thicknesses range from 0.005 inches (0.13 mm) for fine slot liners in small motors up to 0.062 inches (1.57 mm) for structural insulating barriers. The most commonly stocked gauges are 0.010″, 0.015″, 0.020″, and 0.031″ — these cover roughly 80% of electrical insulation applications.
Sheet dimensions typically follow industry-standard formats: 48″ x 96″ (4 ft x 8 ft) full sheets, 24″ x 48″ half sheets, or custom-cut panels. Many buyers also order slit coils in widths from 1/2″ to 48″ for automated stamping lines. When specifying coil stock, define the machine direction (MD) — fish paper’s mechanical properties are anisotropic, with tensile strength approximately 20-30% higher along the machine direction compared to the cross direction. Slot liners should be oriented so the machine direction runs parallel to the slot wall for maximum insertion strength.
Pricing factors include: raw pulp market fluctuations (cotton prices directly affect vulcanized fiber cost), NEMA grade selection (FR command a premium over F), thickness (thicker gauges cost disproportionately more due to longer vulcanization dwell times), and order quantity. Minimum order quantities (MOQ) vary by supplier — some mills enforce a 500 kg minimum, while distributors may break packs down to 50 kg for prototype or low-volume needs. Lead times for standard grades are typically 2-4 weeks from Asian manufacturers, with expedited air freight options adding 15-25% to the landed cost. For engineers evaluating paper materials for motor insulation, fish paper’s combination of mechanical robustness and competitive pricing makes it a compelling choice for high-volume production environments.
Frequently Asked Questions
Can fish paper be used in oil-filled transformers?
Fish paper is generally not recommended for oil-immersed transformer applications. Vulcanized fiber absorbs moisture and transformer oil, which causes dimensional swelling and a progressive reduction in dielectric strength. Oil-filled transformers rely on kraft paper or pressboard insulation, which are specifically engineered for oil compatibility and demonstrate stable dielectric performance over decades of immersion. If your application requires oil exposure, consider insulation paper types rated for oil-filled environments instead of fish paper.
What is the shelf life of fish paper, and how should it be stored?
Vulcanized fiber has an indefinite shelf life when stored under proper conditions, though manufacturers typically recommend a 2-year shelf life from the date of manufacture for quality assurance purposes. Storage requirements are straightforward but important: keep the material in its original vapor-barrier packaging until use, store in a dry environment at 10-30°C with relative humidity below 60%, and avoid direct sunlight or proximity to heat sources that could accelerate embrittlement. If sheets have absorbed ambient moisture, a controlled drying cycle at 60-80°C for 4-8 hours can restore dielectric properties before fabrication.
Is fish paper environmentally friendly or recyclable?
Fish paper is fully biodegradable and landfill-safe, as it consists entirely of cellulosic fibers with no synthetic binders, resins, or halogenated flame retardants. The zinc chloride used during vulcanization is washed out during manufacturing, leaving a chemically inert final product. Production scrap can be recycled through standard cardboard and paper waste streams in most regions. This environmental profile gives fish paper a sustainability advantage over glass-epoxy laminates and aramid materials, which require specialized disposal pathways. For companies tracking Scope 3 emissions, fish paper’s cotton-rag composition and simple manufacturing process typically yield a lower carbon footprint than synthetic alternatives.
References
- NEMA Standards Publication LI 1-1998 (R2011) — Industrial Laminated Thermosetting Products. National Electrical Manufacturers Association.
- IEC 60667-1:2020 — Vulcanized Fibre for Electrical Purposes, Part 1: Definitions and General Requirements. International Electrotechnical Commission.
- M. J. Madou, “Fundamentals of Microfabrication and Nanotechnology,” 3rd ed., CRC Press, 2011, Chapter 8: Cellulosic Materials in Electrical Applications.
- UL 94 Standard for Safety — Tests for Flammability of Plastic Materials for Parts in Devices and Appliances. Underwriters Laboratories, 2023.
- A. Küchler, “High Voltage Engineering: Fundamentals — Technology — Applications,” Springer Vieweg, 2018, Section 3.4: Solid Insulating Materials.
About SIDA: SIDA is a premier manufacturer and supplier of electrical insulation materials, serving transformer OEMs, motor manufacturers, and electrical equipment producers in over 30 countries. Our product portfolio includes 6520 fish paper sheets, laminated vulcanized fiber, pressboard, DDP paper, Nomex alternatives, and custom die-cut insulation components. We offer both standard NEMA grades and custom formulations tailored to your specific voltage, mechanical, and dimensional requirements.
For technical inquiries, samples, or a quotation, contact our team at jessie.feng@sidanm.com or call +86-15958243831. Reach us on WhatsApp at wa.me/8615958243831. Visit sidanm.com to browse our complete insulation materials catalog and request technical datasheets.
