Plastics for Electrical Insulation — G10, PTFE, Ultem & More

Choosing the right plastic for an electrical insulation application requires balancing dielectric strength, volume resistivity, arc resistance, flammability rating, and mechanical requirements at the operating temperature. The principal candidates span a wide performance and cost range: paper-based phenolics for general low-voltage work, glass-filled epoxy laminates (G10 and FR4) for structural PCB and busbar work, glass-silicone laminates (G7/G9) for high-temperature service, PTFE for RF and microwave components, and Ultem/PEI for demanding structural-insulation parts. This guide compares them across the dimensions that determine which material belongs in your assembly.

TL;DR

Dielectric strength ranges from ~300 V/mil (glass phenolic G7) to ~500 V/mil (PTFE), but mechanical and thermal requirements usually dictate the selection before dielectric strength does.
UL94 V-0 is the minimum flame rating for most enclosed-equipment insulation; all materials in this guide carry V-0 in at least some thicknesses.
G10 and FR4 is the standard for structural PCB substrates and high-voltage busbar supports—glass/epoxy laminate with broad availability.
Paper phenolics (XX, XXX, XXXP) are cost-effective for low-voltage, low-frequency applications; avoid in wet or high-humidity environments.
PTFE offers the lowest dielectric constant (Dk ≈ 2.1) and dissipation factor across all frequencies—indispensable for RF and microwave work.
Ultem/PEI combines structural strength, UL94 V-0, and service to 340°F (170°C)—the top choice when mechanical load and insulation must coexist.
Volume resistivity is a stronger long-term insulation predictor than dielectric strength alone; confirm both values for critical designs.

10¹⁸", "1×10¹⁷"], ["Dielectric Constant (1 MHz)", "4.8", "5.0–6.0", "3.7–4.2", "5.0–6.5", "4.8", "2.1", "3.15"], ["Max Continuous Temp", "250°F (121°C)", "250°F (121°C)", "428°F (220°C)", "300°F (149°C)", "300°F (149°C)", "500°F (260°C)", "340°F (171°C)"], ["UL94 Rating", "V-0", "V-1 / HB", "V-0", "V-0", "V-0", "V-0", "V-0"], ["Arc Resistance (sec)", "60–120", "45–125", "120–180", "100–150", "60–120", "300+", "128"], ["Moisture Absorption (24hr)", "0.10%", "0.75–1.5%", "0.08%", "0.05%", "0.10%", "<0.01%", "0.25%"], ["Relative Cost", "$$", "$", "$$$", "$$", "$$$", "$$$$", "$$$$"], ]} />

G10 and FR4 — Structural Glass-Epoxy Laminate

G10 is woven glass fabric bonded with epoxy resin, laminated under heat and pressure. FR4 is the flame-retardant version of G10 and the dominant PCB substrate material in the electronics industry. Both carry UL94 V-0 and provide excellent balance of dielectric properties (Dk ≈ 4.8 at 1 MHz), mechanical strength, and machinability. FR4 laminate sheet is available in thicknesses from 0.005" to 4" and is routinely CNC-machined into bus bar supports, switchgear spacers, terminal blocks, and relay bases.

Machining and Fabrication Notes

G10 and FR4 is abrasive to cutting tools. Use sharp carbide end mills and drills, run at moderate speeds with high feed rates to minimize heat. The cut surface generates glass dust—wear respiratory protection and use dust collection. Laser cutting is viable for sheets under 0.250"; waterjet is preferred for thicker stock.

G10 vs. FR4 — When It Matters

G10 and FR4 have nearly identical electrical and mechanical properties. The distinction is the flame-retardant additive in FR4 (brominated epoxy system), which may affect certain chemical resistance scenarios. For most electrical insulation work, FR4 is specified by default because it satisfies UL94 V-0. See the full G10 and FR4 material hub for laminate grades, tolerances, and NEMA LI-1 designations.

Paper Phenolic (XX, XXX, XXXP) — Economy Electrical Insulation

Paper phenolic laminates use kraft paper or cotton-rag paper as the base fabric bonded with phenolic resin. NEMA grades XX, XXX, and XXXP are punching grades intended for die-stamped terminal boards, spacers, and small insulating components at power-line frequencies (50/60 Hz). They are the lowest-cost option in this group and are widely stocked in sheet and rod form.

When Paper Phenolic Is Appropriate

Paper phenolics are suitable for dry indoor applications at voltages below 600 V, in temperatures to 250°F (121°C). Their dielectric strength (300–400 V/mil) is adequate for many distribution and relay applications, and their arc resistance (45–125 sec) is acceptable for applications without sustained arcing.

Moisture Limitations

The key limitation is moisture absorption: paper phenolics absorb 0.75–1.5% water in 24 hours, causing measurable dimensional swelling and a significant drop in volume resistivity under humid conditions. For outdoor, wet-location, or high-humidity indoor applications, substitute G10 and FR4, G9, or another glass-reinforced laminate.

Grade Differences

XX — general-purpose paper/phenolic, NEMA grade.
XXX — improved punchability; preferred for stamped parts.
XXXP — paper base with reduced resin flow; best punchability.

For more detail, see the paper phenolic material hub.

Glass Phenolic Laminates — G7, G9, G11

Glass-fabric reinforced phenolic and other resin systems extend service temperature and moisture resistance far beyond paper-base grades. Three grades are most relevant for electrical insulation work:

G7 — Glass/Silicone for High Temperature

G7 uses a silicone resin binder with woven glass fabric. It has the highest continuous service temperature of any standard laminate: 428°F (220°C) continuous, with short-term excursions higher. Dielectric properties remain stable at elevated temperatures, and its arc resistance (120–180 sec) is excellent. G7 is specified for motor slot liners, transformer coil supports, and oven-side insulation where G10 would soften or delaminate.

G9 — Glass/Melamine for Arc-Resistant Applications

G9 uses melamine resin with woven glass. It provides outstanding arc resistance (100–150 sec) and tracking resistance, making it preferred for contactors, switchgear barriers, and arc chutes where sustained arcing may occur. Maximum service temperature is 300°F (149°C)—lower than G7 but higher than G10.

G11 — High-Temperature Glass/Epoxy

G11 is the high-temperature variant of G10 and FR4, using a higher-Tg epoxy system. It retains strength and electrical properties to 300°F (149°C), bridging the gap between standard FR4 and G7. G11 is often the choice for high-temperature PCB assemblies and structural insulation in motor drives.

Full data for each: glass phenolic (G9/G11) hub, glass-silicone G7 hub, glass-melamine hub.

PTFE — RF/Microwave and High-Purity Insulation

Polytetrafluoroethylene (PTFE) is the dielectric material of choice for RF connectors, coaxial cables, waveguide components, and any circuit where signal loss must be minimized. Its dielectric constant (Dk ≈ 2.1) is the lowest of any solid engineering plastic, and its dissipation factor (Df ≈ 0.0002–0.0005) is orders of magnitude lower than phenolics or glass-epoxy. PTFE maintains these properties from cryogenic temperatures to 500°F (260°C) and exhibits essentially no moisture absorption.

UL94 and Arc Resistance

PTFE is inherently flame-retardant (UL94 V-0) without additives. It does not sustain combustion and produces no burning droplets. Its arc resistance exceeds 300 seconds, superior to all competing materials.

Mechanical Limitations

PTFE is soft (Shore D 55–60) and cold-flows under sustained compressive loads. In structural insulation applications where load-bearing is required, use filled PTFE (glass-filled or carbon-filled grades) or substitute Ultem/PEI for a stronger alternative. PTFE is typically used in non-structural roles: bushings, washers, tape, standoffs, and coaxial components.

Common Forms

PTFE sheet is available in skived (0.001"–0.250"), molded compression, and extruded rod form. Tube is standard for insulation sleeves and heat-shrinkable applications. Review the PTFE material hub for full grade and form information.

Ultem/PEI — Structural High-Performance Insulation

Ultem (polyetherimide, PEI) combines the best attributes of high-temperature thermoplastics with outstanding electrical insulation. Dielectric strength of ~830 V/mil is the highest in this group. Volume resistivity (1×10¹⁷ Ω·cm) is among the highest available in a melt-processable thermoplastic. UL94 V-0 is achieved in thicknesses as thin as 0.010". Continuous service temperature reaches 340°F (171°C)—suitable for motor end bells, coil bobbins, and relay housings that see elevated ambient temperatures.

Key Advantages Over Phenolics and G10

Unlike thermoset laminates, Ultem can be injection-molded into complex insulation shapes, which eliminates secondary machining costs for high-volume parts. As a thermoplastic, it can also be machined from rod and sheet for prototype and low-volume work. Dimensional stability is excellent, and it resists the mild acids and bases commonly used in PCB cleaning and defluxing.

Limitations

Ultem is not recommended in contact with strong solvents (chlorinated solvents, MEK, ketones) or in applications where its Dk (3.15) causes excessive signal loss at high frequencies—use PTFE for RF work above ~500 MHz. See the Ultem/PEI vs. PEEK comparison when choosing between the two for demanding structural-insulation parts.

How to Choose — Decision Matrix

Operating frequency:

DC through 60 Hz power → G10 and FR4, paper phenolic, G7, G9 all acceptable; choose on temperature and cost.
RF and microwave (>1 MHz) → PTFE or PTFE-composite laminates; avoid all other materials in the signal path.

Temperature:

≤250°F (121°C) → Paper phenolic (dry indoor only), G10 and FR4, Ultem.
≤300°F (149°C) → G9, G11, Ultem (upper range).
≤340°F (171°C) → Ultem.
≤428°F (220°C) → G7.
≤500°F (260°C) → PTFE.

Arc and tracking resistance:

High-arc environments (contactors, switchgear) → G9 or G7; do not use FR4 or paper phenolics.
Low arc-probability environments → G10 and FR4 or Ultem sufficient.

Humidity/moisture exposure:

Wet or high-humidity → eliminate paper phenolics; use glass-reinforced grades.
Immersion or outdoor → PTFE, G10 and FR4, or G7.

Structural load:

Significant mechanical load + insulation → Ultem/PEI or G10 and FR4.
Non-structural seals, gaskets, tape → PTFE.

Cost priority:

Lowest cost → Paper phenolic (XX, XXX) for dry low-voltage indoor service.
Mid-tier → G10 and FR4 (best value for structural insulation).
High performance → G7, Ultem, PTFE.

Sizes & Forms Commonly Stocked

Material	Sheet / Laminate	Rod / Tube	Notes
G10 and FR4	24×48", 36×48", 0.005"–4" thick	1/4"–4" dia	Sheet per NEMA LI-1 spec
Paper Phenolic (XX/XXX)	24×48", 1/8"–3" thick	1/4"–4" dia	Punching grade in thin sheet
G7 (Glass-Silicone)	24×48", 1/8"–2" thick	1/4"–3" dia	Tan/yellow color
G9 (Glass-Melamine)	24×48", 1/8"–2" thick	1/4"–3" dia	Off-white
PTFE	12×12" to 24×48", 0.010"–2" thick	1/4"–6" dia, tube	Skived and molded grades
Ultem/PEI	12×24", 24×48", 1/8"–3" thick	1/4"–5" dia	Amber/translucent color

Spec Sheet & Test Data

Full electrical, mechanical, and flammability data:

Need electrical insulation material with UL94 certification documentation? Contact FedMat for certified laminate sheet, rod, and tube with full traceability across all grades.

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