G7 Phenolic Glass Silicone Properties & Data
G7 laminate properties are defined by NEMA LI-1 and military specification MIL-I-24768/17. The combination of woven E-glass reinforcement and silicone resin produces a material whose most notable characteristics are thermal stability at 425–485°F continuous, dielectric strength of 350 V/mil, and arc resistance exceeding 180 seconds — values that no epoxy-glass or melamine-glass laminate can match at sustained elevated temperatures.
At a glance:
- Continuous use: 425–485°F (218–252°C)
- Dielectric strength: 350 V/mil (perpendicular to laminations)
- Arc resistance: >180 seconds
- Flexural strength (LW): ~50,000 psi at room temperature
- Water absorption: <0.25% (24-hour immersion)
- Specific gravity: 1.70–1.80 g/cc
Mechanical Properties
G7's mechanical performance at room temperature is moderate compared to epoxy-glass (G10 and FR4). Its significant advantage appears at elevated temperature: silicone resin retains far more of its room-temperature stiffness and strength above 350°F than epoxy or melamine binders.
Strength and Stiffness
Elevated-Temperature Mechanical Retention
One of the primary justifications for specifying G7 over less expensive laminates is mechanical property retention at temperature. The following table shows approximate retention percentages at 400°F (204°C) compared to room-temperature baselines:
| Property | RT Value | Retention @ 400°F | Notes |
|---|---|---|---|
| Flexural Strength (LW) | 50,000 psi | ~60–70% | Still structural at temperature |
| Flexural Modulus | 2.8 × 10⁶ psi | ~55–65% | Minimal sag in horizontal spans |
| Compressive Strength | 30,000 psi | ~65–75% | Suitable for compression-loaded spacers |
By comparison, G10 and FR4 flexural strength falls to roughly 30–40% of room-temperature value at 400°F, and the resin visibly softens above 300°F under load. Phenolic-glass laminates also drop sharply above 300°F.
Hardness and Wear
G7 has a Rockwell M hardness of approximately 100–110. The glass fiber surface is abrasive and will wear mating parts if used as a dynamic bearing without lubrication. G7 is not a bearing material; it is a structural insulator. For bearing applications requiring high temperature, consider filled PTFE or polyimide composites.
Electrical Properties
Electrical insulation is the primary design driver for G7. The following properties are per NEMA LI-1 and ASTM test methods.
Dielectric and Insulation Data
10¹² Ω"], ["Volume Resistivity — Condition A", ">10¹¹ Ω·cm"], ["Surface Resistivity — Condition C (wet)", ">10¹⁰ Ω"], ["Volume Resistivity — Condition C (wet)", ">10⁹ Ω·cm"], ["Arc Resistance (D-495)", ">180 seconds"], ["Comparative Tracking Index (CTI)", "Group IIIb (PTI 100–175 V)"], ]} />
The 350 V/mil dielectric strength perpendicular to laminations is measured at 90° to the glass cloth planes — the critical direction for sheet insulators under throughbore voltage stress. Parallel dielectric strength (along the laminations) is lower, as expected for a layered construction.
Arc Resistance: Why It Matters
Arc resistance above 180 seconds means the surface of G7 laminate resists carbonization and conductive track formation when exposed to sustained electrical arcing. This is critical in:
- High-voltage switchgear where arc flash may contact insulator surfaces
- Missile fuzing circuits where surface tracking could cause unintended continuity
- Dry-type transformer barriers operating at voltages above 5 kV
G10 and FR4 typically achieves 60–120 seconds of arc resistance; the halogenated flame retardants in FR4 actually reduce arc resistance relative to unfilled laminates.
Frequency Performance
G7's dielectric constant of 4.1–4.8 at 1 MHz and dissipation factor of 0.010–0.020 are acceptable for low-frequency isolation and power applications but are not optimal for microwave frequencies. PTFE-glass laminates with Dk ~2.2–2.4 are preferred for RF structures above 1 GHz. G7's electrical profile suits 50 Hz–100 MHz power and control insulation.
Thermal Properties
The z-axis CTE of 30–40 ppm/°C is important for hardware design. Through-bolt joints and precision bore components must account for this expansion when cycling from cold soak (-65°F) to operating temperature.
Flame and Smoke Behavior
Silicone resins produce a non-conductive silicon dioxide (silica) ash when exposed to direct flame. G7 is self-extinguishing under many conditions, but it does not carry a UL 94 V-0 rating — the silica ash can drop off in small fragments rather than producing a stable char that self-quenches. For UL 94 V-0, specify FR4 [ ["Water Absorption (24-hr immersion)", "<0.25%"], ["Water Absorption (post-boil)", "<0.80%"], ["Acid Resistance (10% H₂SO₄)", "Good — slight surface etch only"], ["Alkali Resistance (10% NaOH)", "Moderate — avoid prolonged exposure"], ["Solvent Resistance", "Resistant to most hydrocarbons, ketones"], ["UV Resistance", "Fair — surface chalking with long outdoor exposure"], ["Fungus Resistance", "Excellent — MIL-STD-810 compliant"], ]} />
Low water absorption ensures that dielectric properties measured in the dry condition are largely retained in humid environments. The silicone matrix does not absorb water as aggressively as polyester or standard phenolic resins.
Physical and Dimensional Properties
| Property | Value |
|---|---|
| Color | Natural (cream/tan) |
| Surface Finish | Sanded or unsanded (specify) |
| Lamination Direction | Warp/fill designation available |
| Machinability | Good — standard carbide tooling |
| Density | 1.70–1.80 g/cc (~0.062–0.065 lb/in³) |
| Thickness tolerance | ±10% per NEMA LI-1 |
Property Comparison: G7 vs. G9 vs. G10 and FR4
For full versus analysis, see the G7 vs. G9 comparison and G10 and FR4 vs. G7 comparison.
180 sec", ">180 sec", "60–120 sec"], ["Flexural Strength (RT)", "50,000 psi", "55,000 psi", "60,000 psi"], ["Water Absorption", "<0.25%", "<0.40%", "<0.10–0.20%"], ["Specific Gravity", "1.70–1.80", "1.75–1.90", "1.80–1.95"], ]} />
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