Phenolic Glass Silicone (G7) — Material Hub

G7 phenolic glass silicone is a woven glass cloth laminate bonded with silicone resin, rated for continuous service at 425–485°F (218–252°C) — the highest sustained-temperature rating of any standard NEMA laminate grade. It meets MIL-I-24768/17, delivers a dielectric strength of 350 V/mil, and resists arc tracking for more than 180 seconds, making it the standard choice for aerospace insulators, high-frequency coil forms, missile components, and transformer spacers where heat and electrical stress coexist.

At a glance:

• Continuous use temperature: 425–485°F (218–252°C)
• Dielectric strength: 350 V/mil
• Arc resistance: >180 seconds
• Water absorption: <0.25% (24-hour immersion)
• Governing standard: NEMA G7 / MIL-I-24768/17
• Available forms: sheet, rod, tube
• Natural (cream/tan) color; no approved pigmentation in mil-spec grades

What Is G7 Laminate?

G7 is a thermoset composite built from multiple plies of woven E-glass cloth, each ply saturated with silicone resin and then consolidated under heat and pressure. The silicone binder is what differentiates G7 from every other NEMA glass-fabric grade: silicone's inherent thermal stability — rooted in the silicon-oxygen backbone — allows continuous service at temperatures that degrade epoxy (G10 and FR4, G11), melamine (G9), and polyester laminates.

Silicone Resin vs. Other Binders

Silicone resin retains mechanical cohesion and electrical insulating properties after prolonged thermal cycling — a characteristic absent from epoxy laminates, which begin to embrittle above 300°F under sustained load.

Glass Cloth Construction

The reinforcement in G7 is woven E-glass fabric, the same fiber used in G10 and FR4, but the weave weight and silane coupling agent at the glass-resin interface are optimized for silicone adhesion. The result is a material with:

• Flexural strength: ~50,000 psi (lengthwise, room temperature)
• Flexural modulus: ~2.5–3.0 × 10⁶ psi
• Tensile strength: ~30,000 psi in-plane
• Compressive strength (flatwise): ~30,000 psi

These values are lower than G10 and FR4 at room temperature, but G7's mechanical properties remain substantially more stable at 400°F, where G10 and FR4 has softened considerably.

Electrical Properties

Electrical insulation is the primary engineering driver for G7 selection. The material's silicone matrix provides an unusually wide operating range without the dielectric degradation that plagues many thermosets.

Dielectric Performance

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The 350 V/mil dielectric strength is measured perpendicular to laminations, the direction most relevant for sheet insulators. Arc resistance exceeding 180 seconds is critical for high-voltage switchgear and missile fuzing systems, where arc tracking across a surface can cause catastrophic failure.

Low Water Absorption Impact

G7's water absorption of <0.25% after 24-hour immersion (vs. ~0.10–0.20% for FR4 under the same conditions) may appear similar, but the silicone matrix does not swell or delaminate on drying — it returns to baseline electrical properties. Epoxy-glass laminates can show measurable dielectric constant shift after wet conditioning; G7 does not exhibit the same recovery lag.

Thermal Properties

Continuous Use Ratings

MIL-I-24768/17 specifies a minimum temperature index of 200°C (392°F) for G7. In practice, most fabricators and users reference a continuous use range of 425–485°F (218–252°C), accounting for mechanical load. The material will survive short excursions above 500°F without structural collapse, though prolonged exposure beyond 485°F accelerates resin oxidation.

Thermal Cycling Resistance

Aerospace and missile applications subject laminates to repeated thermal cycles from -65°F to +400°F. G7's coefficient of thermal expansion (CTE) in the z-axis is approximately 30–40 ppm/°C — higher than G10 and FR4 — so through-thickness dimensional change must be budgeted when precision bore tolerances are required after temperature cycling.

Flame and Smoke

G7 does not meet UL 94 V-0 under standard test conditions because silicone resins produce a powdery silica ash on combustion rather than a self-extinguishing char. For applications requiring UL 94 V-0, G10 and FR4 with a halogenated flame retardant is the correct choice. G7 is used in military and aerospace contexts that apply MIL-spec qualification rather than UL 94.

Available Forms and Sizes

G7 is commercially available in three fabricated forms, all carrying the NEMA G7 and MIL-I-24768/17 designations when produced to specification:

Sheet

Sheet is the most common form, used for insulators, spacers, and barrier boards. Standard dimensions:

• Thickness: 0.031 in. to 4.0 in. (thicker slabs on inquiry)
• Width × Length: 24 × 36 in., 36 × 48 in., and 48 × 96 in. are typical mill sizes
• Tolerances: ±10% on thickness per NEMA LI-1

Rod

G7 rod is pultruded or turned from sheet and used for standoff insulators, bushings, and coil forms:

• Diameters: 0.25 in. to 6.0 in.
• Lengths: typically 4-ft and 8-ft stock

Tube

Tubes and cylinders carry the same NEMA G7 rating and are used for coil forms, transformer bobbins, and conduit insulators:

• Wall thickness: 0.062 in. and up
• ID/OD combinations on standard or custom programs

Key Standards and Qualifications

• NEMA LI-1, Grade G7 — the commercial laminate standard defining test methods and minimum property requirements
• MIL-I-24768/17 — the military specification covering glass-silicone laminated sheet; this is the qualification required for military procurement
• ASTM D709 — covers test specimen preparation for laminated thermosetting materials, referenced in property verification

Buyers procuring G7 for defense programs must ensure their supplier's stock is certified to MIL-I-24768/17, not merely NEMA G7 labeled. The mil-spec requires lot testing and traceability that NEMA commercial stock does not mandate.

Compared to Other High-Temperature Laminates

G7 competes primarily with G9 (Phenolic-Glass-Melamine) and, for lower-temperature ranges, with G10 and FR4 standard phenolic-glass laminates.

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See the full G7 comparisons page for detailed side-by-side analysis and the G10 and FR4 vs. G7 versus page.

Common Applications

G7 appears wherever an insulating laminate must survive continuous high heat and maintain its dielectric properties:

• Aerospace insulators — coil forms, barrier boards, and spacers in avionics bays where temperatures exceed 350°F
• Missile components — guidance system insulators, fuze spacers, and structural insulators in propulsion zones
• Transformer spacers and coil forms — high-voltage distribution transformers, dry-type power transformers operating in elevated-temperature environments
• High-frequency insulator blocks — RF and microwave equipment where the moderate Dk and low loss tangent are acceptable trade-offs for thermal durability
• Industrial high-temperature machinery — oven conveyor guides, motor end plates, and press platens

See the G7 applications page for detailed use-case breakdowns.

Machining G7

G7 machines similarly to other glass-reinforced laminates — abrasive on tooling, prone to delamination if feed rates are excessive. Key considerations:

• Use carbide or diamond-tipped tooling; HSS dulls rapidly
• Sharp tools and moderate feed rates minimize fiber pullout
• Wet cutting is preferred for dust control; silica dust from glass is a respiratory hazard
• Rout, drill, mill, and turn standard shapes without special equipment

Detailed parameters and tooling recommendations are in the G7 machining guide.

Frequently Asked Questions

Can G7 replace G10 and FR4? In most room-temperature electrical applications, G7 is an expensive substitute for G10 and FR4 with no advantage. G7 is the correct choice when continuous temperatures exceed 300°F or when arc resistance greater than 120 seconds is required.

Is G7 RoHS compliant? Yes — the silicone resin and glass fiber construction contains no regulated heavy metals or halogenated flame retardants. Confirm with your supplier's material certification.

Does G7 require special storage? No. Cured G7 laminate is stable at ambient conditions. Unlike B-staged prepreg, finished G7 sheet has no shelf-life concern.

See all questions in the G7 FAQ.

For a quote on certified NEMA G7 / MIL-I-24768/17 sheet, rod, or tube, visit the G7 line card page or contact our technical sales team.

More related guides

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

Applications

• Electrical Insulation
• Aerospace Interior

Industries

• Aerospace
• Electrical Power

Compare to other materials

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