Ready Plastics

Paper Phenolic Properties — Mechanical & Electrical Data

Paper phenolic (NEMA grades XX, XXX, XXXP, XXXPC) delivers a reliable combination of electrical insulation, moderate mechanical strength, and dimensional stability at a cost point that glass-reinforced laminates cannot match. This page provides the full numerical property dataset across mechanical, electrical, thermal, and physical categories, organized by test method and grade where values differ between NEMA designations. All values are for dry-as-processed conditions unless noted; moisture conditioning reduces electrical properties significantly for Grade XX.

At a glance:

  • Dielectric strength (perpendicular, short-time): 600 V/mil in 1/16" sheet
  • Tensile strength flatwise: 12,000–18,000 psi depending on grade and ply count
  • Compressive strength flatwise: 25,000–35,000 psi
  • Flexural strength flatwise: 15,000–22,000 psi
  • Water absorption (24 hr, 1/16" specimen): 0.15–0.80% — Grade XXX lowest
  • Continuous service temperature: 120°C (250°F)
  • Volume resistivity (dry): >10¹² Ω·cm for Grade XXX

Mechanical Properties

Paper phenolic's mechanical behavior is dominated by its laminated structure. Properties measured flatwise (load applied perpendicular to the laminate plane, stress distributed across plies) are substantially higher than edgewise (load applied parallel to the laminate plane, stress concentrated on fewer plies). Always confirm the load orientation before applying handbook values.

Tensile Strength

The flatwise range spans grades; Grade XXX with its denser paper ply typically tests at the upper end (16,000–18,000 psi), while Grade XXXPC formulated for punchability tests at the lower end (12,000–14,000 psi). Tensile modulus is relatively consistent across grades at 1.0–1.4 × 10⁶ psi — stiff but not as stiff as glass-reinforced epoxy laminates, which run 2.5–3.5 × 10⁶ psi flatwise.

Compressive Strength

Compressive strength flatwise exceeds tensile strength — a common characteristic of fiber-reinforced thermosets. Paper phenolic performs well under sustained compressive load in dry conditions. Under elevated humidity and sustained stress, creep becomes a design consideration above 2,000 psi sustained flatwise loading.

Flexural Strength and Modulus

Flexural strength and modulus are the properties most frequently cited for panel and chassis structural assessments. Grade XXX and XX are similar in flexural performance; XXXP (post-formable) is slightly lower because the B-stage resin system does not fully cross-link under standard cure conditions — final flexural properties develop only after forming.

Impact Resistance

Paper phenolic is brittle. Notched Izod impact of 0.8–1.5 ft-lb/in is low compared to thermoplastics (nylon 6/6 is ~2.0 ft-lb/in notched; polycarbonate is >12 ft-lb/in) and well below glass-reinforced epoxy. Avoid shock-loaded or impact-prone applications. Where toughness is required, cotton phenolic or linen phenolic offers 2–3× higher impact resistance at modest cost increase. For high-impact electrical insulation, glass phenolic is the correct specification.

Shear Strength and Hardness

Rockwell M hardness of 95–110 confirms the rigid, hard surface of cured phenolic. This hardness is suitable for wear pads and sliding contact surfaces at low loads and speeds, but the brittleness makes the material prone to edge chipping in thin sections.

Electrical Properties

Electrical performance is the primary reason engineers specify paper phenolic over lower-cost structural materials. All electrical values are grade- and thickness-dependent; thinner specimens test higher in dielectric strength (V/mil) because the test voltage is divided over fewer mils of material.

Dielectric Strength

The 600 V/mil value at 1/16" is the most commonly cited figure and is appropriate for comparing paper phenolic to competing insulators. For thicker cross-sections, use the per-thickness values rather than extrapolating linearly — dielectric strength does not scale proportionally with thickness.

Parallel (edgewise) dielectric strength of 200 V/mil is significantly lower because current can track along ply interfaces more easily than through the laminate cross-section. Design terminal boards and bus bar supports so the electrical path is always perpendicular to laminates when possible.

Dielectric Constant and Dissipation Factor

Grade XXX's lower dissipation factor makes it preferable in circuits where dielectric losses are a design concern at power frequencies. At RF frequencies (>1 MHz), paper phenolic's loss tangent rises and becomes unsuitable for RF-critical signal routing — G10 and FR4 or PTFE-based laminates are the correct choices for RF applications.

Resistivity

10¹² Ω·cm (Grade XXX)"], ["Volume resistivity, dry (ASTM D-257)", ">10¹¹ Ω·cm (Grade XX)"], ["Surface resistivity, dry (ASTM D-257)", ">10¹² Ω (Grade XXX)"], ["Volume resistivity, after moisture conditioning", "10⁸–10¹⁰ Ω·cm (Grade XX)"], ["Volume resistivity, after moisture conditioning", "10¹⁰–10¹² Ω·cm (Grade XXX)"], ]} />

The difference in resistivity after moisture conditioning is the most practical reason to specify XXX over XX in humid environments. Grade XX volume resistivity can drop 3–4 decades under moisture conditioning — a significant degradation. Grade XXX maintains volume resistivity within 1–2 decades, remaining acceptable for most low-voltage insulation service.

Arc Resistance

Arc resistance above 120 seconds is generally acceptable for switchgear panels in low-voltage service. Paper phenolic chars rather than tracking, which limits the spread of conductive carbonized paths under arcing — a more favorable failure mode than tracking-prone materials.

Thermal Properties

Temperature Ratings

The gap between continuous service temperature (120°C) and glass transition (175–200°C) represents the safety margin built into the rating. Operation above 135°C degrades mechanical properties, accelerates resin oxidation, and increases moisture sensitivity. For applications routinely exceeding 120°C, phenolic-glass laminates (continuous service to 155°C) or high-temperature thermosets are required.

Thermal Conductivity and Expansion

Thermal conductivity of 0.25–0.35 W/m·K is low — paper phenolic is an effective thermal insulator as well as an electrical insulator, which is useful in bus bar support and heat-generating component mounting. CLTE is anisotropic: the perpendicular-to-laminate direction expands more than the in-plane direction. For precision parts with significant temperature cycling, account for differential expansion in both planes.

Physical Properties

Density of 1.30–1.40 g/cc makes paper phenolic heavier than common thermoplastics (nylon: 1.14 g/cc; acetal: 1.41 g/cc) but lighter than glass-reinforced laminates (G10 and FR4: ~1.85 g/cc). The lower density reduces component weight in chassis and panel assemblies compared to glass laminates.

Property Comparison: Paper Phenolic Grades

10¹¹", ">10¹²", ">10¹¹", ">10¹²"], ["Tensile strength flatwise (psi)", "12,000–16,000", "14,000–18,000", "12,000–15,000", "12,000–14,000"], ["Flexural strength flatwise (psi)", "15,000–20,000", "17,000–22,000", "14,000–18,000", "14,000–18,000"], ["Post-formable?", "No", "No", "Yes (160–180°C)", "No"], ["Cold-punchable?", "Marginal", "Marginal", "No", "Yes"], ["Primary use case", "General insulation", "Humid environment electrical", "Formed insulators", "Die-punched terminals"], ]} />

Hardness and Surface Properties

Rockwell M hardness of 95–110 reflects the rigid, hard character of the cured phenolic resin matrix. This hardness is suitable for light-duty wear pad and sliding contact applications at low loads and speeds, but the brittleness makes the material prone to edge chipping under repeated impact. For applications combining electrical insulation with a wear function — such as a cam follower that also insulates the cam follower shaft from the frame — confirm that contact stresses remain below approximately 500 psi and sliding velocity stays below 50 ft/min to prevent surface fracture and progressive chipping.

Surface roughness of the as-pressed face (formed against the laminator's polished press platens) is typically 63–125 µin Ra. This is a smooth matte finish — neither mirror-polished nor rough. For electrical applications where surface tracking resistance is a concern, the smooth as-pressed surface is preferred over machined surfaces; machined faces expose fiber cross-sections that provide moisture ingress paths and slightly higher surface conductivity when wet.

Chemical Resistance Summary

Cured paper phenolic has good resistance to dilute mineral acids (10% H₂SO₄, 10% HCl) at room temperature, alcohols, aliphatic hydrocarbons, and most petroleum-based fluids. It is attacked by concentrated acids, strong alkaline solutions (sodium hydroxide above 10%), ketones (acetone, MEK), and chlorinated solvents. Prolonged contact with water degrades both the matrix (slow hydrolysis at elevated temperature) and the fiber-matrix interface.

For applications where the insulating component is exposed to cleaning solvents or maintenance fluids, verify compatibility against this profile before specifying paper phenolic. In most electrical panel environments the material contacts only ambient air, dust, and occasional contact-cleaner spray — all of which are acceptable.

Positioning vs. Competing Laminates

Paper phenolic's property numbers make most sense in context. Compared to G10 and FR4 cotton phenolic, the trade-off is reversed in the mechanical domain: paper phenolic has better dielectric strength (600 V/mil vs. 350–450 V/mil) but lower toughness. The paper version is the electrician's laminate; cotton is the mechanical engineer's laminate. Linen phenolic sits between them in most properties, with its woven-cloth construction providing moderate improvement in edgewise strength over paper without sacrificing as much dielectric performance as cotton.

Design Notes and Specifier Guidance

When pulling a property value for a design calculation, observe these precautions:

  1. Orientation matters. Flatwise and edgewise values can differ by 30–50%. Confirm which orientation applies to the load path in your design before computing stresses or deflections.

  2. Thickness is a variable in electrical properties. Dielectric strength in V/mil decreases as thickness increases. A 1" thick wall does not provide 600 × 1000 = 600,000 V breakdown; use thickness-specific test values and apply appropriate derating.

  3. Moisture conditioning reflects field conditions. Dry-as-processed electrical data represent best-case conditions. For indoor/humid service, specify grade-specific moisture-conditioned resistivity to avoid over-specification on dry values and under-specification in actual use.

  4. XXXP properties pre- and post-form. XXXP tested before forming will show slightly lower properties than a fully cured laminate; post-form values depend on the forming temperature, dwell time, and cooling rate. Confirm final properties with the laminator for critical applications.

For full grade definitions and procurement guidance, see the Paper Phenolic grades page. For comparison with glass-base laminates, see G10 and FR4 properties phenolic-paper-vs-glass comparison.

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