Cotton Phenolic Properties — NEMA Grade C & CE Data

Cotton phenolic (NEMA LI-1 grades C and CE) delivers compressive strength up to 36,000 psi, a continuous use temperature of 250°F, and a coefficient of friction low enough for unlubricated bearing service — all from a material that costs a fraction of engineered thermoplastics and machines with standard carbide tooling. This page provides the complete property dataset for grades C and CE, covering mechanical, electrical, thermal, dimensional stability, and oil-impregnation characteristics.

TL;DR — Properties at a Glance

• Compressive strength (flatwise) 36,000 psi — on par with or exceeding SAE 660 bronze bearing allowable
• Tensile strength 10,000 psi lengthwise; 7,000 psi crosswise — anisotropic, plan your part orientation
• Continuous use 250°F; intermittent peak ~300°F before dimensional drift risk
• Grade CE adds meaningful electrical insulation without sacrificing compressive or flexural strength
• Water absorption 0.8–1.5% — enables water-lubricated (wet running) bearing operation
• Density 1.36 g/cc — significantly lighter than bronze (8.9 g/cc) or cast iron (7.2 g/cc)

Mechanical Properties

Cotton phenolic is an anisotropic laminate — properties measured parallel to the laminate plane (lengthwise, LW) differ from those measured through the thickness (flatwise, FW). Specify orientation on drawings whenever tensile or flexural loading is a design driver.

Compressive Strength in Bearing Context

The 36,000 psi flatwise compressive value is the figure most relevant to bushing and thrust washer design. This measurement reflects load perpendicular to the laminate plane — the orientation a bushing wall sees when radial load is applied. For comparison, ASTM B505 SAE 660 leaded bronze (commonly used for bushings) has a compressive yield of approximately 28,000–32,000 psi. Cotton phenolic equals or exceeds that threshold without the corrosion concerns in wet or chemically active environments.

Anisotropy and Part Orientation

The woven cotton fabric reinforcement creates a distinct difference between in-plane (LW/CW) and through-thickness (FW) properties. When designing a bushing from rod stock, the laminate planes run parallel to the rod axis — meaning bore wall loading is flatwise, and you get the full 36,000 psi compressive benefit. Sheet-cut bushing blanks orient the laminate differently; flatwise load becomes edgewise on the machined bore. Design engineers must verify laminate orientation relative to the load path before finalizing material selection.

Impact Resistance

At 1.0–1.5 ft·lb/in (Izod notched), cotton phenolic absorbs moderate shock but is not the choice for high-impact applications. Canvas phenolic (NEMA grade C/CE canvas) offers higher impact energy absorption due to its heavier fabric weave. For applications involving recurring shock loads — punch press guides, drop hammers — canvas grades or a thermoplastic option such as ultra-high-molecular-weight polyethylene may be more appropriate.

Electrical Properties

Grade C and grade CE share the same cotton fabric reinforcement. The difference lies in resin formulation: CE uses a modified phenolic that reduces ionic contamination and improves dielectric performance.

Grade CE is not a high-voltage insulating laminate in the class of G-10 or FR-4 fiberglass. Its dielectric properties are sufficient for low-to-medium voltage applications (typically below 5 kV in continuous service) and for personnel protection grounding applications per OSHA 29 CFR 1910.137 requirements at appropriate thickness. Consult your specific voltage and test standard before committing to CE for safety-critical electrical isolation.

Thermal Properties

Thermal Stability in Service

The 250°F continuous rating assumes moderate cyclic loading. Sustained operation above 275°F causes progressive post-cure cross-linking, which can induce dimensional change and surface cracking over time. In systems where the material is exposed to steam, superheated water, or localized hot spots from friction, monitor surface temperature directly rather than relying on bulk coolant temperature alone.

The UL 94 HB rating means cotton phenolic will sustain combustion when a flame is applied horizontally — it is not self-extinguishing. Cotton phenolic is not suitable for applications requiring UL 94 V-0, V-1, or V-2 flame ratings. For flame-retardant phenolic service, specify a glass-epoxy FR-4 or a UL-rated phenolic grade.

Dimensional Stability and Moisture

Water Absorption as a Feature

In most polymer engineering, water absorption is a defect — it swells the material and degrades properties. In cotton phenolic bushings, controlled water absorption is a design feature. Running in water-lubricated service (marine propeller shafts, pump impeller wear rings, water-turbine guide bearings), the cotton fibers absorb enough moisture to create a hydrodynamic film that reduces both coefficient of friction and wear rate relative to dry running. The key design consideration: allow 0.001"–0.003" of radial clearance over and above the standard metal bushing allowance to accommodate the dimensional change upon wetting.

Dry vs. Wet Running Clearances

A common engineering error is machining a cotton phenolic bushing to the same clearance as a bronze equivalent, then experiencing tight fits after moisture ingress. Standard practice: add 0.001"–0.002" per inch of bore diameter to the metal bushing clearance specification. The NEMA LI-1 standard provides absorptivity data per grade; use the 24-hour immersion value for worst-case wet-fit calculations.

Oil Impregnation Properties

Oil impregnation is a secondary process applied to cotton phenolic rod or bushing blanks to extend bearing life in dry or intermittently lubricated service.

Impregnation Process

Blanks are dried to remove residual moisture, then vacuum-impregnated with hot oil under pressure. The cotton fiber structure acts as a reservoir, holding oil within the fabric matrix rather than in machined grooves. Under load and shaft rotation, pressure and heat release oil at the sliding interface in controlled quantities. Oil-impregnated cotton phenolic is particularly effective in oscillating bearing applications — slow rotation, high load, limited access for external lubrication — such as agricultural equipment pivots, crane hook swivels, and construction machinery wear pads.

Physical and Chemical Resistance Properties

Chemical Resistance Overview

Cotton phenolic's chemical resistance is governed by the phenolic resin matrix — the cotton fiber itself offers little barrier to chemical attack, so the resin must protect the fiber. The following table summarizes compatibility:

Flammability and Smoke

Cotton phenolic carries a UL 94 HB (horizontal burn) rating — the material will sustain a flame when ignited in a horizontal orientation. It is not self-extinguishing. For applications requiring V-0 or V-2 flammability ratings, specify halogenated glass-epoxy laminates (FR-4 or equivalent). Cotton phenolic does not contain halogens and therefore does not generate halogenated combustion products when burned — a consideration in confined-space or electronics applications where halogen-free materials are preferred.

Density and Weight Advantage Over Metal

At 1.36 g/cc (0.049 lb/in³), cotton phenolic is:

• 85% lighter than carbon steel (7.8 g/cc)
• 85% lighter than bronze (8.9 g/cc)
• 50% lighter than aluminum (2.7 g/cc)
• 3% lighter than acetal Delrin (1.41 g/cc)

In large-diameter bushings or long shaft segments, the weight saving over bronze can be significant. A 4" diameter × 8" long cotton phenolic bushing weighs approximately 1.9 lb; the equivalent 660 bronze bushing weighs approximately 12.5 lb. In rotating equipment where imbalance is critical, and in portable or hand-operated systems, this weight difference has direct engineering value.

Hardness and Wear Rate

Cotton phenolic's Rockwell M hardness of 100–115 places it harder than most engineering thermoplastics (acetal: M80, nylon: M79–92) but softer than hardened steel shaft materials. This hardness differential is intentional in bearing design: the bushing wears preferentially rather than scoring the shaft. A worn bushing is a maintenance event; a scored shaft is a replacement event. The cotton fiber matrix moderates the wear mechanism — fiber ends at the bore surface carry part of the load, reducing peak resin-surface stress and extending the interval between dimensional re-cuts. Under clean water lubrication at moderate PV (below 10,000 psi·ft/min), cotton phenolic bushing wear rates of 0.001"–0.005" per thousand hours of service are common in documented marine and pump applications.

Summary: Grade C vs. CE Property Comparison

The mechanical property difference between C and CE is small — typically within 5–10% across tensile and flexural values. The electrical performance improvement is significant. Unless the application demands electrical isolation, grade C is the default selection for its marginally higher mechanical values and lower cost.

Related Guides

More related guides

Cross-cluster suggestions to help shoppers and engineers explore adjacent topics:

Compare to other materials

• Nylon Vs. Cotton Phenolic
• FR4 Vs. Cotton Phenolic
• Cotton Vs. Linen Phenolic

Frequently asked questions — Cotton Phenolic FAQ