TL;DR:
- Crepe paper is kraft paper that has been mechanically micro-creped to achieve 35%–60% elongation — 5–10× more stretchable than flat kraft paper, making it the standard for transformer layer insulation and winding wraps.
- The elongation percentage (35%, 50%, 60%) directly determines conformability: higher numbers suit uneven conductor surfaces and tight bend radii, while lower grades prioritize maximum dielectric strength per layer.
- Crepe paper tubes replace flat wrap in critical winding sections — they eliminate the risk of layer unwrapping during thermal cycling and provide uniform radial pressure distribution across the conductor surface.
- Compared to flat kraft paper at the same thickness, crepe paper offers 3–5× higher tear resistance, deeper oil impregnation, and 15–25% higher partial discharge inception voltage in oil-filled transformer windings.
- Ordering correctly means specifying three parameters simultaneously: thickness × width × elongation grade — missing any one leads to material that will not process or perform as expected.
Crepe insulation paper is a mechanically texturized kraft paper engineered to stretch 35% to 60% beyond its original length before tearing — a property that makes it the dominant layer insulation material in oil-filled distribution and power transformers worldwide. Unlike flat kraft paper, which tears under the mechanical stress of winding tension and thermal expansion, crepe paper’s micro-folded structure absorbs tension without compromising dielectric integrity. This article explores the material science behind crepe paper grades and elongation percentages, when and why transformer windings need crepe paper tubes instead of flat wrap, and a head-to-head performance comparison with conventional kraft paper, providing engineers and procurement professionals with the technical foundation to specify and source crepe paper with confidence.
What Is Crepe Paper and How Does It Differ from Kraft Paper?
The Micro-Creping Manufacturing Process
Crepe paper starts from the same electrical-grade kraft paper base stock — virgin wood pulp with high alpha-cellulose content, processed through sulfate (kraft) pulping to maximize fiber length and mechanical strength. The critical difference lies in the creping stage: the base paper sheet, still at 30–40% moisture content, is fed onto a heated Yankee dryer cylinder and scraped off by a doctor blade set at a specific angle. This mechanical action compresses the paper longitudinally, creating a dense network of microscopic folds and ridges — typically 20–40 crepe folds per linear centimeter for electrical-grade material. The creping process reduces the paper’s machine-direction tensile modulus by 60–80% compared to uncreped kraft, converting a stiff, inextensible sheet into a compliant, stretchable insulation layer. Post-creping, the paper is supercalendered to control thickness uniformity (±5% tolerance for transformer-grade material, per IEC 60554-3-5) and slit to customer-specified widths.
Physical Properties That Define Transformer-Grade Crepe Paper
Three properties distinguish electrical-grade crepe paper from general-purpose industrial grades: controlled porosity for complete oil impregnation (Gurley porosity typically 200–600 seconds for 0.05 mm thickness), high dielectric strength (≥8 kV/layer for 0.075 mm dry, per IEC 60554-2), and low extractable contaminants — specifically chloride (<50 ppm) and sulfate (<100 ppm) levels that, if exceeded, accelerate copper conductor corrosion in oil-filled transformers. The paper's basis weight for transformer use typically ranges from 40 to 120 g/m², corresponding to thicknesses of 0.05 mm to 0.20 mm. Unlike flat kraft, crepe paper's elongation value is the primary ordering parameter — thickness and width alone are not sufficient specifications. Related product: SIDA electrical-grade kraft paper is the base stock for crepe paper production.
What Crepe Paper Percentages (35%, 50%, 60%) Mean for Your Application
35% Elongation: Maximum Dielectric Density for Uniform Windings
At 35% elongation, crepe paper retains the highest fiber density among the three grades — approximately 0.85–0.95 g/cm³ apparent density — which directly correlates to the highest dielectric strength per layer. This grade is specified for transformer windings with uniform conductor geometry: rectangular magnet wire, foil-wound secondaries, and layer-to-layer insulation in distribution transformers where the winding surface is smooth and predictable. The trade-off is conformability: at 35% elongation, the paper cannot fully conform to radius variations below approximately 3× paper thickness without risking micro-voids at the inner bend radius. For applications where the winding geometry is simple and consistent across production runs, 35% elongation delivers the most reliable dielectric performance at the lowest material cost.
50% Elongation: The General-Purpose Transformer Workhorse
The 50% elongation grade represents the most commonly stocked and specified crepe paper for power and distribution transformers. It delivers a balance of adequate dielectric strength (typically 7–9 kV/layer at 0.075 mm dry) and sufficient stretch to wrap around conductor radii as tight as 2× paper thickness without bridging or tearing. For procurement teams, specifying 50% elongation as the default grade across transformer models simplifies inventory management — one grade covers approximately 80% of layer insulation applications. This grade is also the standard substrate for crepe paper tubes, where the tube-winding process itself consumes 5–10% of the available elongation during spiral wrapping around the mandrel. The broad availability of 50% elongation crepe paper across Chinese mills means shorter lead times and more competitive pricing compared to the 35% and 60% grades.
60% Elongation: When Winding Geometry Demands Maximum Flexibility
At 60% elongation — the practical upper limit for electrical-grade crepe paper — the material can stretch to over 1.6× its relaxed length, making it suitable for the most demanding transformer winding geometries: stepped-lap core joints, transposed conductor bundles (CTC), lead exits with tight bend radii, and irregularly shaped busbar insulation. However, the mechanical gains come with a dielectric trade-off: the creping process at 60% elongation reduces fiber-to-fiber contact points, lowering the dielectric strength by approximately 10–15% compared to the 35% grade at the same thickness. Engineers compensate by specifying a 0.025–0.050 mm thicker grade for the 60% elongation material, achieving equivalent turn-to-turn dielectric protection with full conformability. This grade is also preferred for repair and retrofill applications where the original winding geometry was not designed for flat paper.
Crepe Paper Tube: When and Why Transformer Windings Need Them
Flat Wrap vs. Tube: The Mechanical Integrity Argument
Flat crepe paper applied as a multi-layer spiral wrap around a transformer conductor is simple and cost-effective — but it introduces a failure mode that tubes eliminate: layer unwrapping. During thermal cycling (20°C to 120°C in normal oil-filled operation), the conductor expands and contracts, and each adjacent wrap layer experiences differential movement. Over thousands of cycles, the outermost layers of a flat wrap can begin to unwind, creating loose paper that blocks oil flow channels or, in the worst case, migrates into the winding gap and causes a turn-to-turn short. Crepe paper tubes solve this by being pre-formed on a mandrel with a continuous spiral seam — the tube maintains its cylindrical geometry as a single structural unit, distributing radial forces uniformly and eliminating the unwrapping failure mode entirely. Independent testing at 1,000 thermal cycles (20°C ↔ 120°C, per IEEE C57.100 test methodology) showed zero layer delamination in tube-form insulation versus detectable loosening in 3 of 10 flat-wrap samples.
Common Tube Dimensions and Installation Best Practices
Standard crepe paper tube dimensions cover conductor diameters from 3 mm to 80 mm, with wall thicknesses ranging from 0.25 mm (single-layer, small-diameter leads) to 3.0 mm (multi-layer, high-voltage bushing insulation). Tubes are specified by internal diameter × wall thickness × length — the ID must exceed the conductor’s maximum thermal-expansion diameter by 0.1–0.3 mm to prevent stress cracking during operation. For installation, tubes are typically slid over the conductor after brazing or crimping operations, then secured at each end with a cotton or polyester tying cord. In oil-filled transformers, the tube’s spiral seam is not adhesively bonded — it relies on winding tension and oil saturation to maintain closure, which preserves the oil’s ability to penetrate every fiber layer for complete impregnation and heat dissipation.
Crepe Paper vs Kraft Paper: Real Performance Comparison
Mechanical Performance: Elongation and Tear Resistance
The defining mechanical difference between crepe paper and flat kraft paper is elongation at break. Standard transformer-grade flat kraft (typically 0.05–0.25 mm thick) offers 2–4% elongation in the machine direction — it stretches just enough to be wound without immediate tearing, but not enough to accommodate the 0.2–0.5% conductor expansion that occurs from cold-start to full-load temperature. Crepe paper, at 35–60% elongation, provides a 10–30× safety margin. Tear resistance (Elmendorf method, ASTM D689) shows a similarly dramatic gap: crepe paper at 0.075 mm thickness registers 350–500 mN tear force versus 80–120 mN for flat kraft at the same thickness — approximately 3–5× improvement. This matters in manufacturing: operators can wind crepe paper at higher tension and speed without tear-related rejects, improving shop-floor throughput by an estimated 15–25% in high-volume transformer production lines.
Electrical Insulation: Dielectric Strength and Partial Discharge Behavior
At identical thickness and basis weight, flat kraft paper holds a modest dielectric strength advantage: 9–11 kV/layer at 0.075 mm versus 7–9 kV/layer for 50% elongation crepe paper, per IEC 60554-2 test conditions. The gap exists because the creping process introduces microscopic low-density zones within the paper structure. However, in oil-impregnated service conditions, this gap narrows significantly for two reasons: first, crepe paper’s higher porosity accelerates and deepens oil penetration, and oil-impregnated paper achieves 2–3× the dielectric strength of dry paper; second, crepe paper’s conformability eliminates the air pockets that form between a flat paper layer and a non-planar conductor surface — and it is those air pockets, not the paper’s bulk dielectric rating, that often become partial discharge initiation sites. Field PD measurements on identical winding geometries show PD inception voltages 15–25% higher for crepe-insulated windings versus flat-kraft-insulated windings after oil impregnation, despite flat kraft’s higher dry dielectric rating. Browse our flat insulation options: SIDA kraft paper for transformers and pressboard PSP 3050.
Cost, Availability, and When Flat Kraft Still Makes Sense
Crepe paper carries a 20–40% price premium over equivalent-thickness flat kraft paper, driven by the additional creping and supercalendering steps and the lower production speeds of the creping process. For applications where the conductor geometry is flat and uniform — large rectangular busbars, planar winding sections, simple layer-to-layer barriers between smooth foil layers — flat kraft paper remains technically sufficient and cost-optimal. The decision rule used by experienced transformer designers: if the winding surface has any curvature with a radius below 10× the paper thickness, specify crepe; if all surfaces are planar and the winding temperature swing exceeds 80°C, specify crepe; otherwise, flat kraft is acceptable. For buyers procuring both material types, SIDA supplies the full range — contact our technical team for grade recommendations specific to your winding design.
How to Order Crepe Paper: Thickness, Width & Elongation Specs
The Three Mandatory Parameters and Why Each Matters
Every crepe paper purchase order must specify three values: thickness (mm), slit width (mm), and elongation grade (35%, 50%, or 60%). Thickness determines the dielectric withstand voltage per layer and the overall build-up of the insulated conductor — a 0.025 mm deviation from nominal changes the turns ratio clearance in a tightly wound coil. Slit width must match the winding machine’s paper feed system within ±0.5 mm; wider tolerance causes feed jams or edge folding that can scrap an entire winding. Elongation grade, as discussed above, is determined by the winding geometry and thermal cycling requirements of the specific transformer design. Additionally, buyers should specify: surface finish (calendered or uncalendered — calendered reduces dusting during high-speed winding), core diameter (standard 76 mm or 152 mm for automated dereeling equipment), and roll length in meters (which, combined with thickness and width, determines the roll outer diameter and must be compatible with the winder’s dereeling stand clearance).
Quality Certifications and Acceptance Tests to Specify
Transformer-grade crepe paper should be ordered with a compliance certificate referencing IEC 60554 (Cellulosic Papers for Electrical Purposes), specifically Part 3-5 (Crepe Paper). The certificate of analysis for each production batch should report: thickness (micrometer, ±5% of nominal), basis weight (g/m², ±7%), elongation at break (machine direction and cross direction, per ISO 1924-2), tensile strength (kN/m, MD and CD), dielectric strength (kV/layer, dry, per IEC 60243-1), moisture content (≤8% as-shipped, Karl Fischer method), ash content (≤1.0%, IEC 60554-2), conductivity of aqueous extract (≤10 mS/m, IEC 60554-2), and chloride/sulfate ppm levels. For export orders to China, request a GB/T 19264 compliance statement; for North America, ASTM D1305 and IEEE C57.12.00 references should appear on the technical datasheet. Reputable suppliers — including SIDA — provide full certificate of analysis traceability by production batch number and manufacturing date, which is essential for transformer OEM audit compliance and end-user acceptance testing.
Frequently Asked Questions
Can crepe paper be used in dry-type transformers?
Yes, but with important caveats. Crepe paper’s primary dielectric mechanism relies on oil impregnation — without oil, its dielectric strength is approximately 40–50% lower than the oil-impregnated rating. For dry-type transformers, specify crepe paper that has been pre-impregnated with a compatible varnish or resin (usually epoxy or polyester-based), or use it only in combination with a VPI (vacuum pressure impregnation) process. The elongation benefit still applies in dry-type applications for winding ease, but the dielectric design must account for the dry (unimpregnated) rating, not the oil-impregnated rating.
What is the shelf life of crepe insulation paper?
When stored in original moisture-barrier packaging at 15–30°C and 30–60% relative humidity, unopened crepe paper rolls have a shelf life of 24 months from the date of manufacture. Once opened, the paper should be used within 30 days or re-conditioned — dried at 105°C for 24 hours per IEC 60554-2 guidelines — before use, because cellulose rapidly absorbs atmospheric moisture that reduces dielectric strength and can introduce water into the transformer oil system. Always inspect for visible mold, discoloration, or brittleness before use — these indicate moisture ingress that cannot be fully reversed by re-drying.
How does crepe paper compare to Nomex or aramid paper for transformer insulation?
Crepe paper and Nomex (aramid) serve overlapping but distinct roles in transformer insulation. Crepe paper offers 4–6× lower material cost, better oil impregnation (cellulose is naturally hydrophilic and wicks insulating oil efficiently), and full biodegradability. Nomex offers a 220°C continuous thermal rating versus crepe paper’s 105°C (thermal class A in oil) — making it the choice for high-temperature or overload-prone transformers — and retains higher mechanical strength at elevated temperatures. In practice, many transformer designs use crepe paper for bulk layer insulation and reserve Nomex for turn-to-turn insulation in hot-spot zones such as lead exits and top-of-winding regions. The two materials are complementary, not direct substitutes.
Summary
Crepe insulation paper — with its 35–60% elongation capability, oil-compatible cellulose chemistry, and availability in both flat-wrap and pre-formed tube formats — remains the workhorse layer insulation for oil-filled transformers ranging from 10 kVA distribution units to 500 MVA power transformers. The key to successful specification is understanding that elongation grade is the primary selection parameter: 35% for uniform windings where dielectric density matters most, 50% for the broad majority of general-purpose transformer applications, and 60% for complex geometries and retrofill work where conformability trumps all other considerations. When ordering, pair the three mandatory parameters — thickness, width, elongation — with IEC 60554-3-5 acceptance criteria, and confirm that your supplier provides full batch-level certificate of analysis traceability. For transformer OEMs and service centers seeking a reliable crepe paper supply chain with complete documentation support, SIDA delivers consistent quality backed by in-house quality control testing on every production batch.
Contact SIDA for Crepe Insulation Paper and Transformer Insulation Materials:
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Browse our complete transformer insulation product range: Kraft Paper | Pressboard PSP 3050 | G4 Transformerboard High Density | Mica Tape for VPI
References
- IEC 60554-3-5:2001 — Specification for Cellulosic Papers for Electrical Purposes — Part 3: Specifications for Individual Materials — Sheet 5: Crepe Paper. International Electrotechnical Commission. https://webstore.iec.ch/publication/2463
- IEEE C57.12.00-2021 — IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers. IEEE Power and Energy Society. https://standards.ieee.org/standard/C57_12_00-2021.html
- Prevost, T. & Oommen, T.V. (2006). “Cellulose Insulation in Oil-Filled Power Transformers: Part I — History and Development.” IEEE Electrical Insulation Magazine, 22(1), 28–35. https://ieeexplore.ieee.org/document/1593375
- Moser, H.P. & Dahinden, V. (1999). Transformerboard II: Properties and Application of Transformerboard of Different Fibers. Weidmann AG. ISBN 978-3-9522555-0-2.
- Lundgaard, L.E., Hansen, W., Linhjell, D., & Painter, T.J. (2004). “Aging of Oil-Impregnated Paper in Power Transformers.” IEEE Transactions on Power Delivery, 19(1), 230–239. https://ieeexplore.ieee.org/document/1265803
- ASTM D1305-16(2020) — Standard Specification for Electrical Insulating Paper and Paperboard — Sulfate (Kraft) Layer Type. ASTM International. https://www.astm.org/d1305-16r20.html
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