G-7 Silicone Glass Laminate: When Operating Temperatures Exceed 200°C

What G-7 actually is

G-7 is the silicone member of the NEMA glass-laminate family. It is continuous woven E-glass cloth impregnated and bonded with a silicone (organopolysiloxane) resin. That Si-O-Si backbone—not a carbon-chain epoxy and not an aromatic polyimide—is the source of its behavior. Under heat, where epoxies oxidize and embrittle, the silicone network stays cross-linked and keeps its electrical properties.

The standards place it cleanly. G-7 maps to IEC SI GC 201 / SI GC 202 under IEC 60893; the German DIN equivalent is HGW 2572; and the military specification is MIL-I-24768/17 (GSG). On a shelf it looks like any other tan or cream glass sheet, which is exactly why the certification—never the colour—is what you verify.

The 200°C threshold: where G-7 earns its place

NEMA assigns G-7 to Class H (180°C) continuous duty. Suppliers routinely quote 200-220°C continuous and up to about 250°C for short-term excursions. The practical service window therefore sits in the 180-220°C band—the region that begins where G-11 (155°C) ends. Push past roughly 240-260°C sustained, and polyimide (G-15) takes over.

This is the honest reading of the title: G-7 is the bridge laminate. When your operating temperature has clearly exceeded what any epoxy can hold, but you are not yet in the polyimide stratosphere, G-7 is the standard silicone-glass choice. It is the rigid laminate benchmark for the ~200°C Class C duty in transformers, motors, and furnace hardware.

What makes silicone different from epoxy and polyimide

Three traits separate G-7 from the other two high-temperature families:

• Arc and tracking resistance: silicone resists carbon tracking under high-voltage arcing. G-7 posts arc-resistance values of ≥180-240 seconds (ASTM D495)—roughly double epoxy and far above commodity grades. That is why it lives inside arc chutes and switchgear barriers.

• Flame and smoke: silicone chars rather than burns. G-7 is UL 94 V-0, self-extinguishing, and generates minimal smoke and toxic fumes—a property epoxy and polyimide also meet on flame class, but silicone delivers with the lowest fire-load penalty.

• Dielectric retention at heat: the resin holds its insulation integrity through 200°C. This is the quiet advantage—less about peak numbers, more about not collapsing when the temperature does.

One trait it does not share with polyimide: G-7 is not a low-outgassing material. Silicone can release mobile fractions in vacuum, so it is not qualified for ASTM E595 vacuum duty. Where vacuum, outgassing limits, or a space-rated environment applies, polyimide-glass (G-15 / PIGC 301) remains the correct choice.

Electrical and mechanical properties

The numbers tell a consistent story—strong at heat, modest at the bench:

• Dielectric strength: about 350 V/mil (≈13.8 kV/mm). Lower than epoxy's 400-500 V/mil and polyimide's 700-850 V/mil—but stable at temperature, which is the point.

• Dissipation factor: ≤0.003 at 1 MHz. This ultra-low loss is why G-7 appears in high-frequency and RF supports where epoxy's losses would heat the part.

• Arc resistance: ≥180-240 s (ASTM D495)—the standout figure.

• Flexural strength: typically 150-185 MPa (≈22,000-27,000 psi)—noticeably below epoxy G-10/G-11. G-7 is chosen for thermal and arc endurance, not room-temperature structure.

• Moisture absorption: very low (≤0.1-0.35%), thanks to silicone's hydrophobicity; it holds insulation resistance better than epoxy in humid or salt-spray service.

Where G-7 wins

The applications cluster around heat plus arc plus continuous duty:

• Dry-type transformer coil forms and spacers—the dominant use; the silicone mandrel survives the winding's thermal rise for the life of a Class H unit.

• High-voltage bushings and switchgear arc chutes / barriers—direct exposure to switching arcs and short-circuit energy.

• Class H motor slot wedges, phase barriers, and end-turn supports—isolating conductors at sustained high temperature.

• Induction-furnace coil bobbins and oven / furnace fixtures—standing in 300-400°F enclosures.

• High-frequency and radar supports—where the ≤0.003 dissipation factor matters more than raw strength.

G-7 vs G-11 vs G-15: the high-temperature decision

The selection rule

Use this filter before opening a data sheet:

• Choose G-11 (epoxy) when service stays at or below ~155°C and mechanical load dominates. It is the cheapest and strongest of the three.

• Choose G-7 (silicone) when service reaches 180-220°C continuous, arc or flame exposure is part of life, and the part does not need to carry heavy structural load. It is the arc-hardened, lower-cost middle step.

• Choose G-15 (polyimide) when you need structural strength at temperature, a 240-260°C ceiling, or a vacuum / low-outgassing environment. Pay the premium only when one of these demands it.

When G-7 is the wrong call

Knowing where G-7 fails is as useful as knowing where it wins:

• Structural load at room temperature: G-7's flexural strength is roughly a third of epoxy's. If the part must carry real mechanical load below 155°C, G-11 or G-10 does the job for a fraction of the cost. Specifying G-7 here buys heat capability you never use.

• Vacuum or low-outgassing duty: silicone is not ASTM E595-qualified. Inside a vacuum chamber or spacecraft, reach for polyimide (G-15) instead.

• Blind "upgrade" from FR-4 under 130°C: swapping FR-4 for G-7 just to feel safer is waste. Below 130°C, FR-4 actually holds slightly higher mechanical strength than G-7 and costs far less.

• High clamp load at temperature: G-7's lower modulus means it creeps more under sustained compression at heat. For bushing cores or bolted barriers where dimensional stability under load at 200°C is critical, verify the deflection budget against the actual load before committing.

None of these are defects in the material; they are mismatches between the property and the demand. The data sheet settles all of them before the part is built.

Machining and fabrication notes

G-7 machines as a rigid laminate with a hard, abrasive silicone matrix. Use carbide or diamond-tipped tooling with good dust extraction; the resin is more abrasive than epoxy and dulls tools faster. Its surface is low-energy, so paints, inks, and adhesives release poorly—plan bonding steps accordingly, and rely on mechanical fixation where possible. As with any glass laminate, seal edges to protect the interlaminar bond.

Verify the grade, not the colour

The closing caution is the same one that protects every laminate spec: a tan sheet on a shelf proves nothing. Always read the resin system and the designation on the lot certification—IEC SI GC 201/202; DIN HGW 2572; MIL-I-24768/17—and confirm "silicone," not "epoxy" or "polyimide." We have seen silicone sheets described only as "high-temp glass" with no resin stated; the certification is what you specify against, especially when the thermal and arc stakes are real.

Frequently asked questions