Linen Phenolic Properties: NEMA L & LE Data

Linen phenolic properties are governed by NEMA LI 1 and ASTM D709. The two linen grades — L and LE — share the same phenolic resin matrix but differ in the weave density and uniformity of their linen reinforcement. That difference produces measurable variation in dielectric strength, moisture absorption, and achievable surface finish, while mechanical values remain nearly identical between grades. This page presents full property tables, explains how the finer LE weave drives specific performance advantages, and identifies where linen phenolic outperforms cotton grades.

At a glance:

• Tensile strength 10,500 psi LW / 9,500 psi CW — anisotropic by approximately 10 %
• Dielectric strength 350 V/mil (LE, perpendicular to laminations)
• Continuous use temperature 250 °F (121 °C) — higher than most engineering thermoplastics in this price range
• Density 1.34 g/cc — roughly half the weight of aluminum
• Moisture absorption (24 hr, ASTM D570): ≤ 1.5 % for LE, ≤ 1.8 % for L
• Coefficient of thermal expansion 12–14 × 10⁻⁶ /°F in-plane

How the Linen Weave Affects Properties

Fiber Geometry and Resin Interface

Linen (flax) fiber has a polygonal-to-circular cross-section and a naturally smooth surface compared to the twisted, ribbon-like morphology of cotton fiber. When woven into cloth and impregnated with phenolic resin, linen yields:

• Higher fiber packing density — fewer void spaces at the fiber-resin boundary
• More uniform ply thickness — leads to tighter control of finished laminate thickness
• Lower surface roughness after machining — the cut surface exposes smoother fiber ends rather than the irregular fuzziness typical of cotton phenolic

These structural differences directly explain why NEMA LE consistently outperforms NEMA CE (cotton electrical grade) in dielectric strength and moisture resistance, even when both use identical resin systems.

Grade L vs. Grade LE Weave Density

Grade L uses a medium-weave linen, balancing cost and mechanical performance. Grade LE uses a finer, more tightly woven linen cloth that undergoes stricter quality inspection before impregnation. The finer weave in LE:

• Reduces interstices between yarns, lowering pathways for moisture ingress
• Creates a more consistent resin-to-fiber ratio across the laminate cross-section
• Results in a higher and more reproducible dielectric strength value

For a direct grade selection comparison including cost trade-offs, see the NEMA L vs. LE grade guide.

Full Mechanical Property Data

Anisotropy in Woven Laminates

All woven-fabric laminates are orthotropic — properties differ by direction. Lengthwise (LW) properties are measured parallel to the warp direction of the reinforcing cloth; crosswise (CW) properties are measured parallel to the fill (weft) direction. For linen phenolic, the LW/CW ratio for tensile strength is approximately 1.1:1, which is tighter than glass-cloth laminates but still meaningful for design.

Design implication: Orient the part so the primary stress axis aligns with the LW direction. For gears, the tooth root bending load should ideally act along the LW axis of the blank from which the gear is cut.

Impact Resistance

At 0.50 ft·lb/in notched Izod, linen phenolic is notably brittle by polymer standards. Do not apply it where impact or shock loading is the primary design driver. Where impact resistance matters alongside electrical insulation, consider cotton canvas phenolic, which offers approximately 1.0–1.2 ft·lb/in notched Izod due to its heavier reinforcement.

Electrical Properties

Dielectric Strength in Context

NEMA LE's 350 V/mil perpendicular dielectric strength positions it between NEMA CE cotton electrical grade (~300 V/mil) and NEMA G-10 glass epoxy (~400–500 V/mil). For 60 Hz power-frequency applications up to a few kilovolts, LE is often the cost-optimal choice: it machines more easily than G-10, is cheaper than G-11, and delivers adequate dielectric margin.

For RF or high-frequency electronics, the dissipation factor of 0.04–0.06 at 1 MHz limits linen phenolic's suitability. G-10 and FR-4 (loss tangent ≈ 0.02) is the standard choice at those frequencies.

Effect of Moisture on Electrical Properties

Phenolic laminates are hygroscopic. After 24-hour immersion, LE absorbs up to 1.5 % moisture by weight. Moisture reduces dielectric strength; conditioning samples to 50 % RH equilibrium before electrical testing is required under NEMA LI 1 for accurate comparison. In high-humidity environments, encapsulate or seal linen phenolic electrical parts wherever the full dry dielectric strength is critical.

Thermal Properties

Temperature Limits

Continuous use at 250 °F is well-defined: parts operating indefinitely in this range retain their design mechanical strength. Brief excursions to 300 °F are tolerable; sustained exposure at or above the resin's glass transition temperature (~320 °F) will cause irreversible softening and permanent deformation.

Linen phenolic is not rated V-0 or V-1 under UL 94 — the material is HB (self-extinguishing at certain thicknesses). If a flame-retardant rating is mandatory, specify FR-4 (glass epoxy) instead.

CTE Mismatch with Metals

The in-plane CTE (12–14 × 10⁻⁶ /°F) is close to aluminum (13 × 10⁻⁶ /°F), making linen phenolic a good companion material in aluminum-framed instruments and housings where thermal expansion compatibility reduces fastener stress during thermal cycling. The through-thickness CTE is two to three times higher; account for this in blind-hole thread design by specifying adequate thread engagement.

Physical and Chemical Properties

Density Advantage

At 1.34 g/cc, linen phenolic is roughly 50 % the density of aluminum (2.70 g/cc). This matters in applications like fishing reel side plates, where minimizing rotational mass improves casting distance and reduces fatigue. Compared to nylon 6/6 (1.14 g/cc) or acetal (1.41 g/cc), linen phenolic is similar in density but far stiffer and more dimensionally stable under heat.

Chemical Environment Notes

Linen phenolic holds up well in light machine-oil environments, dry cutting conditions, and dilute acidic service. The phenolic resin is attacked by strong sodium hydroxide (caustic), concentrated nitric acid, and chlorinated solvents (methylene chloride, TCE). Do not specify linen phenolic for submerged service in industrial cleaning solutions.

Comparison: Linen vs. Cotton Phenolic Properties

For applications where cost is the primary driver and surface finish is not critical, cotton phenolic is often adequate. When the application demands tighter tolerances, a smoother finished surface, or better electrical properties, the small cost premium of linen LE is typically justified. The full side-by-side analysis is on the cotton vs. linen phenolic comparison page.

Request Material or Custom Parts

Federal Materials maintains NEMA L and LE inventory in sheet, rod, and tube. Reach out with target dimensions for a prompt quote.

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