Glass Phenolic Properties — Mechanical & Electrical Data
Glass phenolic properties vary significantly across NEMA grades because each grade uses a different resin system: phenolic (G3), melamine (G5/G9), silicone (G7), or epoxy (G11). Understanding which properties are driven by the glass reinforcement versus the resin matrix allows engineers to select the right grade without over-engineering the solution. This page consolidates flatwise mechanical, electrical, and thermal data for all five commercial grades.
At a glance:
- Glass reinforcement provides 35,000–65,000 psi flatwise flexural strength across all grades
- G7 silicone resin delivers the highest continuous-use temperature at 425°F (218°C)
- G11 epoxy resin achieves the best balance of mechanical and electrical properties up to 285°F
- Dielectric strength ranges 200–600 V/mil; G9 leads in arc resistance (>120 s, ASTM D495)
- Water absorption is low (0.05–0.25%), making all grades suitable for moderate-humidity environments
- Through-thickness (edgewise) properties are 40–60% of flatwise values — design accordingly
- Density across all grades: 1.70–1.90 g/cc; heavier than paper phenolics, lighter than aluminum
Mechanical Properties
Flexural Strength
Flexural strength in glass phenolic laminates is dominated by the woven glass fabric reinforcement. Because E-glass has a tensile modulus of ~10 million psi and the woven cloth runs in both the warp and fill directions, the laminate resists bending in both principal in-plane axes. Flatwise flexural strength is the property most relevant to structural applications such as bus bar supports, mounting panels, and machined brackets.
G11's epoxy resin bonds more intimately with the glass fiber surface than phenolic or silicone resins, producing fiber-matrix adhesion that translates to the highest flatwise flexural values in the family. G7's silicone resin sacrifices some room-temperature mechanical performance to gain its high-temperature stability.
Tensile and Compressive Strength
Impact and Hardness
Dimensional Stability and Density
The low water absorption of all glass phenolic grades — particularly G11 at 0.05–0.10% — stems from the dense, cross-linked resin network and the low surface energy of the glass cloth. In contrast, paper-base phenolics can absorb 0.3–0.8% water in 24 hours, leading to swelling and dimensional change in humid environments.
Thermal Properties
Continuous-Use Temperature
Operating temperature is the primary grade-selection criterion when the application runs above 250°F:
The silicone polymer backbone of G7 is fundamentally more thermally stable than phenolic, melamine, or standard epoxy resins. Silicone Si–O bonds have bond dissociation energies around 110 kcal/mol, compared to ~90 kcal/mol for the C–C and C–O bonds in organic resins. This is why G7 retains mechanical and dielectric properties at temperatures that cause G3 and G5 to carbonize.
Glass Transition Temperature (Tg)
G7 silicone does not exhibit a conventional glass transition because the silicone backbone remains rubbery over an exceptionally wide temperature range. This means G7 softens less abruptly than organic resin grades when temperatures climb — a safety advantage in fault scenarios.
Thermal Conductivity and CTE
Glass phenolic is a thermal insulator — its conductivity is 500–1,000× lower than copper — which is precisely why it is used for bus bar spacers, standoffs, and insulating rails between conductors. The large mismatch between in-plane and through-thickness CTE means thermal cycling can create interlaminar shear stress; grade and thickness selection should account for this in applications with wide temperature swings.
Electrical Properties
Dielectric Strength
Dielectric strength in glass phenolic laminates depends on thickness (thinner specimens test higher per unit thickness), frequency, humidity, and temperature. All values below are measured per ASTM D149 on conditioned specimens at room temperature unless noted.
G11 achieves higher dielectric strength than phenolic-resin grades because epoxy seals surface and internal voids more completely during cure, leaving fewer paths for partial discharge to initiate breakdown.
Dielectric Constant and Dissipation Factor
G7 has the lowest dissipation factor in the family, making it preferred when high-frequency dielectric losses contribute to heat generation in coil forms or condenser bushings. Applications in RF or microwave service should specify G7 or G11, not G3 or G5.
Resistivity and Arc/Track Resistance
120 s"], ["Arc Resistance (ASTM D495) — G11", "60–100 s"], ["CTI (Comparative Tracking Index) — G9", "400–600 V"], ["CTI — G11", "175–250 V"], ]} />
G9 leads in arc and track resistance because the melamine resin system thermally decomposes without forming a conductive carbon track — the char is predominantly nitrogen-containing oxides rather than carbon. G11 epoxy, while stronger mechanically, forms a more conductive char when arced, producing lower CTI values.
Comparing Properties Across All Five Grades
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Properties vs. Paper-Base Phenolics
The mechanical advantage of glass reinforcement is most apparent when comparing to NEMA X (cotton paper) or XX (fine paper) grades. Paper-base phenolics achieve 15,000–25,000 psi flatwise flexural strength — roughly half of G3 and less than a third of G11. However, paper grades are easier to punch-stamp without delamination, lighter (1.35–1.40 g/cc), and cost less per pound.
For structural loads above moderate service, arc-prone environments, or temperatures above 220°F, glass reinforced grades are the correct choice. See the glass phenolic comparisons guide and the paper vs. glass comparison for a full head-to-head.
Grade-Specific Property Considerations
G7 Silicone — Understanding the Trade-offs
G7's silicone resin gives it unique high-temperature capability but comes with trade-offs that matter at the design stage:
- Lower room-temperature stiffness: Silicone is intrinsically less stiff than epoxy or phenolic, so G7's flexural modulus is the lowest in the family (~2.5–3.0 × 10⁶ psi vs. 3.0–3.5 × 10⁶ psi for G11).
- Higher cost: Silicone resin costs significantly more than phenolic or even epoxy; G7 typically carries a 50–100% premium over G3.
- Machining: G7 produces a slightly waxy chip; surface finish is good but cutting edges must be kept sharp to avoid fuzzing.
Detailed temperature-dependent property retention for G7 silicone phenolic is covered in the G7 silicone phenolic material hub.
G11 — The G10 Upgrade Path
G11 uses the same E-glass fabric as G10 and FR4 but with a higher-Tg epoxy resin. The mechanical properties at room temperature are nearly identical, which makes G11 a direct upgrade path when an existing G10 design starts exhibiting creep or softening at elevated temperatures. See the G10 and FR4 vs. glass phenolic comparison for test-data-based guidance on the upgrade decision.
Get Glass Phenolic Sheet, Rod, or Tube
All five NEMA grades are stocked in sheet, rod, and tube at standard dimensions. Custom cuts and close-tolerance blanks are available. Our materials team can confirm which grade meets your dielectric strength, temperature, and mechanical requirements before you order.
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