Presspaper Insulation: Thickness, Density & Grade Specs

TL;DR:

• Presspaper (transformer insulation paper) is a cellulosic insulating material defined by IEC 60641 — the global standard that classifies it into three density grades (Low ≤1.0 g/cm³, Medium 1.0–1.2 g/cm³, High ≥1.2 g/cm³) with distinct electrical and mechanical performance envelopes.
• Thickness selection is the first-order design decision: 0.05–0.5 mm for layer/turn insulation, 0.5–2.0 mm for barrier and inter-winding insulation, and 2.0–8.0 mm for structural spacers and end blocks — the wrong thickness means either dielectric failure or unnecessary material waste.
• A presspaper datasheet contains 8 critical parameters that engineers must verify: nominal thickness tolerance, apparent density, tensile strength (MD and CD), elongation at break, moisture content, ash content (conductive contaminant indicator), oil absorption, and dielectric strength in oil — skipping any one can lead to field failures that cost transformer OEMs six-figure warranty claims.
• Real RFQ example: a presspaper 3052 inquiry with specified thickness, density, roll width, and compliance requirements — showing exactly how SIDA processes a technical inquiry from quote to shipment.

Presspaper insulation is a high-purity cellulosic sheet material manufactured specifically for electrical insulation in oil-immersed transformers, reactors, and switchgear. Governed by the IEC 60641 family of standards, presspaper provides the essential dielectric barrier between energized conductors at different potentials — it is the material that prevents turn-to-turn short circuits in transformer windings while withstanding thermal aging at 105°C (Class A) continuous operating temperature in mineral oil. This article explores what presspaper is and where it is used in transformer construction, how thickness and density grades are specified under IEC 60641, how to interpret the critical parameters on a presspaper datasheet, and walks through a real customer RFQ for presspaper grade 3052 to demonstrate how engineering specifications translate into procurement decisions.

What Is Presspaper and Where Is It Used in Transformers?

Raw Material and Manufacturing

Presspaper is produced from high-alpha-cellulose wood pulp — typically unbleached softwood kraft — processed through a papermaking machine followed by calendering (hot-pressing between polished rolls) to achieve the target density, surface smoothness, and thickness uniformity. Unlike ordinary kraft paper, electrical-grade presspaper undergoes additional purification to minimize ionic contaminants (particularly sodium, potassium, and chloride ions) that would otherwise compromise dielectric performance in oil. The pulp is refined to controlled fiber lengths, formed into a wet sheet on a Fourdrinier wire, pressed to remove water, dried to below 1% moisture, and then calendered under heat and pressure. For higher-density grades (Medium and High per IEC 60641), multiple passes through the calender stack or post-impregnation with resin binders may be employed. The result is a smooth, void-free sheet with predictable electrical properties and consistent thickness — typically within ±5% tolerance for precision grades.

Primary Functions in Transformer Insulation

Presspaper serves four distinct functions inside an oil-filled transformer. First, layer insulation: thin sheets (0.05–0.25 mm) are interleaved between winding layers to provide turn-to-turn and layer-to-layer dielectric separation. Second, barrier insulation: thicker sheets (0.5–3.0 mm) form cylindrical or inter-phase barriers that segment the oil duct structure and control the electric field distribution between high-voltage and low-voltage windings. Third, end insulation: presspaper rings, collars, and angle pieces insulate the winding ends from the transformer core yoke. Fourth, mechanical support: thick presspaper laminates (2.0–8.0 mm, sometimes laminated into densified pressboard) serve as spacer blocks, clamping plates, and lead supports where moderate compressive loads must be carried without compromising insulation integrity. Each function imposes different thickness, density, and mechanical property requirements — a single transformer may use three or more different presspaper grades.

Relationship to Pressboard and Kraft Paper

Presspaper, pressboard, and kraft paper share the same cellulosic base material but differ in manufacturing process and thickness range. Kraft paper (typically 0.03–0.15 mm) is the uncalendered base — thinner, more porous, and used for cable insulation and general wrapping. Presspaper (0.05–8.0 mm) is calendered kraft — densified by hot pressing to improve dielectric strength and reduce porosity. Pressboard (0.8–8.0 mm, and laminated up to 100+ mm) is the thick, high-density variant made by laminating multiple presspaper plies under heat and pressure — used for structural insulation components. Understanding the distinction between presspaper and pressboard is essential because specifying the wrong material form for a given application leads to either over-spending on unnecessarily dense board or under-specifying a critical barrier.

Related product: Pressboard PSP 3050 — pre-compressed pressboard for structural transformer insulation components.

What Thicknesses Are Available and How to Choose?

Standard Thickness Ranges and Tolerances

Presspaper is commercially available in thicknesses from 0.05 mm (50 microns) to 8.0 mm, with standard sheet/roll formats ranging from 1000 mm to 3200 mm width depending on the mill. The IEC 60641-2 standard specifies nominal thickness series and acceptable tolerances. For precision-wound transformer applications, thickness tolerance is typically ±5% for grades ≤0.5 mm and ±7% for thicker grades — tighter tolerances (e.g., ±0.01 mm for sub-0.3 mm sheets) are available at premium pricing but often require mill-specific qualification. The most commonly stocked thicknesses at SIDA are 0.08, 0.13, 0.18, 0.25, 0.5, 0.8, 1.0, 1.5, 2.0, and 3.0 mm, with thicker laminates (4.0–8.0 mm) produced to order. Thickness uniformity across a single sheet or roll is critical — localized thin spots create weak points in the dielectric barrier, and IEC 60641 mandates that individual point measurements not deviate more than ±8% from the nominal value for electrical grades.

Selection by Application: Layer, Barrier, and Structural

The fundamental rule of thickness selection is: thinner for electrical (where voltage stress dictates the requirement), thicker for mechanical (where compressive load or physical separation is the driver). For layer and turn insulation in distribution transformers (≤36 kV), 0.08–0.25 mm is typical — the thickness is selected based on the BIL (Basic Insulation Level) rating and the number of layers, with each 0.1 mm of presspaper in oil providing approximately 5–8 kV of dielectric withstand. For barrier and inter-winding insulation in power transformers (up to 245 kV), 0.5–2.0 mm is standard, with multiple layers sometimes used to build up the required creepage distance. For structural spacers, clamping plates, and end blocks, 2.0–8.0 mm is specified — here the thickness is driven by mechanical load-bearing requirements, with the insulating function as a secondary consideration. Over-specifying thickness adds cost, weight, and thermal impedance (thicker paper blocks oil flow and heat dissipation); under-specifying means a dielectric puncture that can total a transformer.

Roll vs Sheet Format Considerations

Presspaper is supplied in rolls (typically 500–1000 linear meters for thin grades) or cut sheets (for grades above 0.5 mm that cannot be tightly wound without cracking). The format decision affects transformer manufacturing workflow: rolls enable automated slitting and winding insertion for high-volume production, while sheets are used for hand-layup of barrier insulation and custom-cut spacer components. Roll width is a critical specification — it must match the transformer winding machine’s maximum web width, with common widths being 1000, 1250, 1600, and 2000 mm. SIDA stocks multiple roll widths to support different OEM production lines and offers slitting services for custom widths down to 10 mm. Sheet format orders specify length × width (e.g., 1220 × 2440 mm or 1000 × 2000 mm), with the mill’s maximum sheet size dictating whether larger barriers must be fabricated from multiple pieces.

What Density Grades Does IEC 60641 Define?

The Three IEC 60641 Density Classes

IEC 60641-3 (the specification part of the standard) divides presspaper and pressboard into three density grades based on apparent density in g/cm³. Low Density (LD): ≤1.0 g/cm³ — the most flexible and conformable grade, with the highest oil absorption rate (15–25% by weight), preferred for layer insulation where conformability to winding contours and rapid oil impregnation during vacuum filling are priorities. Medium Density (MD): 1.0–1.2 g/cm³ — the general-purpose balance point, offering improved tensile strength (80–120 MPa MD at 0.25 mm) and dielectric strength (40–60 kV/mm in oil) while retaining adequate flexibility for most winding applications. This is the most commonly specified grade across the transformer industry. High Density (HD): ≥1.2 g/cm³ — approaching pressboard territory, with maximum mechanical strength (tensile >120 MPa MD), lowest oil absorption (<10%), and best compressive creep resistance, used for structural components and high-mechanical-stress barrier applications.

How Density Affects Electrical and Mechanical Properties

Density is the master variable that controls the trade-off between electrical and mechanical performance. As density increases: dielectric strength in oil improves (from approximately 35 kV/mm at LD to 55+ kV/mm at HD for 0.25 mm sheet), tensile strength approximately doubles from LD to HD, elongation at break decreases (HD grades are stiffer and less conformable — a disadvantage for tight winding radii), and oil absorption drops significantly. The oil absorption effect is operationally important: a low-density presspaper absorbs oil rapidly during the vacuum impregnation cycle, reducing processing time, while a high-density grade resists oil penetration and may require extended vacuum drying cycles. Transformer designers select density grade based on the dominant requirement: electrical-only applications favor low-to-medium density for impregnation speed and conformability; combined electrical-mechanical applications favor medium-to-high density for structural integrity.

Grade Designation Examples (3050, 3052, 3055)

IEC 60641 uses a numeric grade designation system where the first two digits typically indicate the material type and the last two digits indicate the density variant. Grade 3050 (commonly called “pressboard PSP 3050”) is a medium-density pre-compressed board specified for structural applications. Grade 3052 is a medium-density electrical-grade presspaper optimized for barrier and layer insulation — this is the workhorse grade most frequently specified in transformer OEM drawings. Grade 3055 is a higher-density variant used where enhanced mechanical properties are required. The specific properties for each grade — thickness range, density, tensile strength, shrinkage, and dielectric strength — are tabulated in IEC 60641-3-1 and must be verified against the manufacturer’s actual test data, as mills may achieve the standard’s minimum values through different fiber blends and processing routes. SIDA’s transformer insulation paper catalog covers the full IEC 60641 grade range with mill test certificates provided as standard.

How to Read a Presspaper Datasheet: Key Parameters

The 8 Critical Parameters to Verify

A presspaper datasheet lists 15–20 measured properties, but 8 are critical for engineering acceptance. (1) Nominal thickness and tolerance: verified by micrometer measurement per IEC 60641-2 at 10+ points across the sheet — reject if any spot deviates >±8%. (2) Apparent density: measured by weighing a precisely dimensioned sample — confirms the material falls within the specified IEC density class. (3) Tensile strength (MD and CD): reported in MPa — machine-direction (MD) strength is typically 20–40% higher than cross-direction (CD); both must exceed the minimum for the grade. (4) Elongation at break: typically 2–6% — lower values indicate brittleness that may cause cracking during winding. (5) Moisture content: must be ≤8% as-delivered — higher moisture means the paper has absorbed ambient humidity and may have degraded dielectric properties. (6) Ash content: the single most important purity indicator — must be ≤1.0% for electrical grades (IEC 60641), with premium grades achieving ≤0.5%. Ash represents non-cellulosic conductive residues; every 0.1% above specification increases the risk of partial discharge inception under high electrical stress. (7) Oil absorption: measured as percentage weight gain after 24-hour oil immersion at 105°C — confirms impregnation behavior. (8) Dielectric strength in oil: the acceptance test — kV/mm measured on conditioned samples in transformer oil per IEC 60243-1.

Red Flags on a Datasheet

Three patterns on a presspaper datasheet should trigger deeper investigation. First, tensile strength that meets the minimum but elongation is below 2%: this indicates an over-calendered sheet that will crack when formed around winding radii below 15× the sheet thickness — a common cause of latent insulation defects that only manifest after thermal cycling. Second, ash content near the 1.0% limit with moisture at the high end of the range: the combination suggests the mill’s pulp washing process may be inconsistent, and the ionic contamination could be higher than the ash number alone indicates — request a conductivity of aqueous extract test (IEC 60554-2) for confirmation. Third, dielectric strength reported only at room temperature: presspaper’s dielectric strength decreases by approximately 15–25% at 105°C compared to 23°C. A datasheet that omits the elevated-temperature dielectric curve is hiding data that matters for operational conditions. Always request full test reports, not just summary datasheets, when qualifying a new presspaper supplier.

Compliance Documentation: Mill Test Certificate vs COA

Two documents accompany every presspaper shipment, and engineers should understand the difference. The Mill Test Certificate (MTC) reports actual measured values for the specific production lot — it is traceable to the batch number and includes the test date, equipment used, and operator identification. The Certificate of Analysis (COA) is a simpler document that confirms the material “conforms to” the specified standard without necessarily reporting individual measurements. For critical transformer insulation applications — particularly for power transformers above 10 MVA or voltage classes above 72.5 kV — always require an MTC with full traceability, not merely a COA. SIDA provides MTCs as standard with all presspaper shipments, including third-party lab verification for customers who require independent validation per their quality assurance protocols.

Customer Quote Example: RFQ for Presspaper 3052

The Inquiry

A South American transformer manufacturer submitted the following RFQ to SIDA in Q1 2026: “Presspaper grade 3052 per IEC 60641-3, thickness 0.25 mm ±5%, density medium (1.0–1.2 g/cm³), roll width 1600 mm, roll length minimum 500 linear meters, quantity 24 rolls (approximately 6,000 kg total). Application: layer insulation for 36 kV class distribution transformers. Required documentation: Mill Test Certificate with full traceability per batch. Delivery: CIF Callao, Peru. Please quote lead time and best price.” This is a representative real-world inquiry — it specifies the IEC grade, the critical dimensions (thickness, width, roll length), the application context, the documentation requirement, and the delivery terms.

Quote Breakdown and Engineering Review

SIDA’s engineering team first verified that 0.25 mm 3052 presspaper is within the mill’s standard production range (it is — this is a stock thickness). The 1600 mm width is standard; no slitting surcharge applies. The 500-meter roll length was confirmed as achievable without in-roll splices — an important requirement because splices create thickness discontinuities that can jam automated winding insertion machines. Density was verified against the mill’s production capability: the target 1.1 g/cm³ nominal with ±0.05 tolerance is well within capability. The 36 kV distribution transformer application was noted as appropriate for medium-density 3052 — no up-specification needed. SIDA quoted $2.85/kg FOB with 45-day lead time, ocean freight to Callao at approximately $1,800 for the 6-pallet shipment, and provided a preliminary MTC template for the customer’s quality department to pre-approve before production release.

Production and Quality Assurance

Upon order confirmation, SIDA’s production workflow for this presspaper order followed these steps: (1) mill production scheduling — the order was allocated to a production slot within 14 days; (2) in-process QC sampling at the mill — thickness profile was measured at 12 cross-web positions every 100 meters, density checked on every third master roll; (3) slitting to 1600 mm width from the master roll with edge-trim QC inspection; (4) final Mill Test Certificate generation with batch traceability linking each of the 24 rolls to specific production data; (5) packaging — moisture-barrier wrapping with desiccant, palletized on fumigated ISPM-15 wood pallets, each pallet labeled with roll number, batch number, gross/net weight, and mill production date; (6) pre-shipment inspection photos shared with the customer via email. The total order-to-delivery cycle was 52 days including ocean transit. The customer’s incoming QC verified two random rolls against the MTC data — all parameters within ±3% of the certificate values — and the material was released to production without rejection.

Read more: Pre-Compressed Pressboard: Technical Guide & Buyer Checklist — for structural-grade board specifications.

FAQ

What is the difference between presspaper and pressboard?

Presspaper is a single-ply calendered cellulose sheet, typically 0.05–8.0 mm thick, produced on a papermaking machine and hot-pressed to the target density. Pressboard is manufactured by laminating multiple plies of presspaper under heat and pressure to achieve thicknesses from 0.8 mm up to 100+ mm. Presspaper is used for winding insulation (layer, turn, barrier) where thin, flexible material is required; pressboard is used for structural components (spacers, clamping plates, angle rings) where thickness and compressive strength are the primary requirements. In practice, the boundary is around 2–3 mm — below this thickness, material is produced as presspaper; above it, as pressboard or laminated board.

Does presspaper require drying before use in transformer manufacturing?

Yes — always. Presspaper as-delivered contains 6–8% moisture (equilibrium with ambient humidity). Before oil impregnation, the completed transformer winding (with all its presspaper insulation) must undergo a vacuum drying cycle — typically 12–48 hours at 105–110°C under vacuum (<1 mbar) — to reduce moisture content below 0.5%. Residual moisture in presspaper is one of the most common root causes of dielectric failure in new transformers because water dramatically reduces the partial discharge inception voltage. The drying cycle is not optional; it is a mandatory part of the transformer manufacturing process specified in IEC 60076 and must be validated by measuring the final moisture content of the insulation system.

Can presspaper be used in air-insulated (dry-type) transformers?

No — presspaper is designed specifically for oil-immersed applications. Its dielectric properties depend on oil impregnation: the oil fills the microscopic voids in the cellulose fiber structure, replacing air (dielectric strength ~3 kV/mm) with transformer oil (dielectric strength ~15–20 kV/mm). Without oil impregnation, presspaper’s dielectric performance is substantially degraded and it does not meet the insulation requirements for dry-type transformers. For dry-type applications, materials such as Nomex aramid paper, glass-reinforced epoxy laminates, or MICA-based insulation are the appropriate choices.

Summary

Presspaper remains the foundational insulating material in oil-immersed transformer design — over a century after its introduction, no synthetic alternative matches its combination of dielectric reliability, thermal aging characteristics, mechanical workability, and cost-effectiveness for layer, barrier, and end insulation. The IEC 60641 standard provides the engineering framework for specifying presspaper: thickness from 0.05 to 8.0 mm, three density grades (Low, Medium, High) each tuned to a different balance of electrical and mechanical performance, and a datasheet structure that captures the 8 critical parameters every transformer engineer must verify before accepting a material lot. Understanding how to read those parameters — and recognizing the red flags that signal process inconsistency at the mill — is the difference between a 30-year transformer service life and an in-service dielectric failure. SIDA supplies the full range of IEC 60641-compliant presspaper grades, from thin layer insulation on rolls to thick structural sheets, with Mill Test Certificates as standard on every order and engineering support to help customers select the optimal grade and format for their specific transformer design.

Contact SIDA for Presspaper Insulation:
📞 +86-15958243831
📧 jessie.feng@sidanm.com
💬 WhatsApp: https://wa.me/8615958243831
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Read more:

• Previous: Pressboard vs Presspaper vs Transformerboard: Which One Does Your Project Actually Need?
• Next: Transformer Insulation Paper: How to Choose the Right Grade, Supplier & Get the Best Price

References

1. IEC 60641-1:2008 — Pressboard and presspaper for electrical purposes — Part 1: Definitions and general requirements. International Electrotechnical Commission.
2. IEC 60641-3-1:2008 — Pressboard and presspaper for electrical purposes — Part 3: Specifications for individual materials — Sheet 1: Requirements for pressboard and presspaper. International Electrotechnical Commission.
3. IEC 60243-1:2013 — Electric strength of insulating materials — Test methods — Part 1: Tests at power frequencies. International Electrotechnical Commission.
4. IEC 60076-3:2018 — Power transformers — Part 3: Insulation levels, dielectric tests and external clearances in air. International Electrotechnical Commission.
5. IEC 60554-2:2001 — Cellulosic papers for electrical purposes — Part 2: Methods of test. International Electrotechnical Commission.
6. Moser, H. P. & Dahinden, V. (1999). Transformerboard II: Properties and Application of Transformerboard of Different Fibres. H. Weidmann AG. Available from: https://www.weidmann-electrical.com