G10 vs G7: Epoxy Glass vs Silicone Glass for High-Temperature Insulation

G10 and G7 are both NEMA-designated glass-reinforced thermoset laminates, and both are used as structural electrical insulators — but the resin systems are entirely different, and that difference defines their service domains. G10 uses an epoxy resin binder; G7 uses a silicone resin. Silicone's exceptional thermal stability extends G7's continuous service rating to 180°C (Class H), compared with G10's 130°C (Class B) limit. G7 also retains superior electrical properties at elevated temperature and maintains more flexibility at low temperature. The tradeoff: G7 is significantly more expensive than G10, and its room-temperature mechanical strength is somewhat lower. When service temperature stays below 130°C, G10 is the practical and economical choice. When the application demands Class H insulation or performance in extreme thermal cycling environments, G7 is the appropriate upgrade.

TL;DR

• Resin system: G10 = epoxy; G7 = silicone — fundamentally different resin chemistry with major thermal consequences.
• Temperature: G10 rated to 130°C (Class B); G7 rated to 180°C (Class H) — a 50°C gap that matters in motors, transformers, and power electronics.
• Mechanical strength: G10 outperforms G7 at room temperature (tensile ~45,000 psi vs ~35,000 psi for G7).
• Electrical: Both are excellent insulators; G7 maintains better dielectric properties at elevated temperatures.
• Low-temperature flexibility: G7 silicone resin remains more flexible at cryogenic temperatures than epoxy-based G10.
• Cost: G7 is significantly more expensive than G10 — specify only where the temperature requirement justifies it.

Chemistry & Origin

The NEMA laminate designation system identifies both the reinforcement and resin by a single letter-number code. The "G" prefix denotes glass-fabric reinforcement across G7, G10, FR4, and G9. The number indicates the resin system and curing method:

• G10: woven E-glass + epoxy
• G7: woven E-glass + silicone

Silicone resins — polyorganosiloxanes — owe their thermal stability to the Si-O backbone, which has substantially higher bond energy than the C-C and C-O bonds in epoxy systems. This siloxane backbone remains stable to temperatures well above epoxy's thermal limit, enabling G7's Class H rating. The same silicon-oxygen backbone is responsible for G7's retained flexibility at cryogenic temperatures — a secondary advantage in aerospace and space-exposure applications.

Epoxy resins cure to a harder, more rigid network than silicone, which explains G10's higher room-temperature tensile and flexural strength. The greater epoxy crosslink density provides stronger mechanical performance in ambient conditions but degrades more rapidly as temperature approaches and exceeds 130°C.

Mechanical Properties

At room temperature, G10 is the stronger material. The highly crosslinked epoxy network produces tensile strengths 10–25% higher than G7's silicone-glass composite. For structural applications at ambient temperatures — mounting brackets, insulation spacers, machine components — G10's additional strength contributes to tighter safety margins or allows thinner sections.

At elevated temperatures, G7's relative performance improves dramatically. Above 150°C, G10's epoxy matrix begins to soften and lose structural integrity. G7 maintains useful mechanical properties throughout its 180°C continuous rating and retains some structural utility beyond that. For applications combining structural and insulating function at high temperature — coil support structures in high-temperature motors, bus-bar supports in furnace controls — G7 is the engineering-correct choice.

Electrical Properties

Both G7 and G10 are highly effective electrical insulators. G10's dry dielectric strength (>40 kV/mm) is marginally higher than G7's (~35 kV/mm), but this difference is rarely decisive in application design. The more important distinction is performance retention at temperature: G7's silicone resin maintains dielectric properties closer to its rated service temperature because silicone's electrical properties degrade less rapidly with thermal elevation than epoxy.

G10's moisture absorption (<0.10%) is slightly lower than G7's (<0.15%), giving G10 a marginal edge in humid ambient environments. In both materials, moisture absorption is far lower than in cotton or paper phenolic laminates.

Thermal Properties

The 50°C service temperature gap between G10 (130°C) and G7 (180°C) maps directly onto insulation class standards:

• G10: IEC/NEMA Class B (130°C)
• G7: IEC/NEMA Class H (180°C)

Motors, generators, and transformers designed to Class H insulation standards require all laminated insulating components to meet Class H — a formal engineering requirement, not a suggestion. When replacing or specifying G10 for a Class H application, G7 or another Class H laminate (such as silicone-bonded mica products) is required.

G7 also outperforms G10 in thermal cycling resistance. The silicone resin's greater flexibility absorbs differential thermal expansion stresses better than the more rigid epoxy, reducing delamination risk in applications with frequent thermal cycles.

Chemical Resistance

G10 resists dilute acids and alkalis, organic solvents, and oils — typical of epoxy-glass systems. G7's silicone resin provides good resistance to ozone, corona discharge, and oxidation at elevated temperature — important in high-voltage electrical equipment where ozone generation is a degradation concern. Both materials resist common dielectric fluids used in transformer applications.

Cost & Availability

G10 is a commodity laminate with wide distribution and competitive pricing. G7 is a specialty material with significantly higher cost — often 3–5× the cost of equivalent G10 — reflecting the more expensive silicone resin system and lower production volume. G7 is stocked by specialty thermoset laminate distributors but may require longer lead times than G10 in non-standard sizes.

When to Choose G10 vs G7

Choose G10 when:

• Continuous service temperature stays below 130°C (Class B applications).
• Cost is a significant factor and high-temperature performance is not required.
• Maximum room-temperature mechanical strength is needed.
• Standard shop stock availability and lead time matter.

Choose G7 when:

• The application requires Class H (180°C) insulation rating.
• Coil supports, bobbin flanges, or bus-bar insulators operate in high-temperature motors, transformers, or furnace equipment.
• Thermal cycling resistance is a design concern.
• Flexibility at low temperature is required (cryogenic applications).

More related guides

Applications

• Electrical Insulation
• Motor and Transformer Components

Industries

• Electronics
• Aerospace

Other comparisons

• G10 vs FR4
• FR4 vs G7
• G10 vs G9

Spec sheets

• G10 hub
• Phenolic Glass (G-Series) hub