E-51 FR (Flame-retardant grade)
Flame-retardant grade
Required for electrical enclosures, PCB laminates and rail/transport parts needing V-0 and high LOI.
Values for the FR condition
Other E-51 grades and fills
E-51 is listed in 9 conditions; see the grade overview at E-51.
Compare all 9 grades and fillsHide
| Grade · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness |
|---|---|---|---|---|---|
Flexibilized / elastic potting grade | Use where thermal-cycling stress on delicate components must be absorbed; trades strength and Tg for compliance. | 18 | 22 | 45 | 55 Shore D |
Alumina-filled thermally conductive potting compound | Use when heat must be conducted out of potted power electronics while keeping full electrical insulation. | — | 45 | 0.8 | 92 Shore D |
Rubber-toughened | Choose for structural adhesives and peel/impact-loaded joints where brittle unmodified epoxy cracks. | — | 55 | 8 | 80 Shore D |
FRThis page Flame-retardant grade | Required for electrical enclosures, PCB laminates and rail/transport parts needing V-0 and high LOI. | — | 58 | 2.5 | 87 Shore D |
Silica/quartz-filled casting | Standard electrical encapsulation compound: low CTE, low shrinkage and low cost for transformers, insulators and IC potting. | — | 60 | 0.8 | — |
AMINEDefault Amine-cured unfilled casting | Default general-purpose room- or mild-heat-cured clear casting for adhesives, potting and tooling. | — | 65 | 4 | 86 Shore D |
Anhydride-cured casting | Choose for electrical casting/potting needing higher Tg, low dielectric loss and long pot life at elevated cure. | — | 72 | 3 | — |
Glass-cloth laminate / GFRP | Structural and electrical laminates (insulation board, filament-wound pipe, blade spar) where high specific strength is needed. | — | 350 | 2 | — |
Carbon-fibre reinforced | Pick for maximum stiffness-to-weight in the fibre direction (tubes, plates, drone/robot arms); properties are strongly anisotropic. | — | 1,600 | 1.4 | — |
What is E-51 FR?
E-51 FR is E-51 in the FR grade — required for electrical enclosures, PCB laminates and rail/transport parts needing V-0 and high LOI. E-51 FR has a tensile strength of 58 MPa (8.41 ksi), stronger than 66% of polymer grades. Elongation at break is 2.5% and the elastic modulus is 3.2 GPa (0.464 Msi). E-51 FR has a density of 1.25 g/cm³ (0.0452 lb/in³), heavier than 64% of polymer grades. Its glass-transition temperature is 115°C (239 °F). AMINE is the default condition FabDigit quotes; FR is available on request or by drawing note.
Advantages
- Stiff — 3.2 GPa (0.464 Msi), better than 77% of polymer grades
- Polishes to a fine finish — 70/100, better than 77% of polymer grades
Limitations
- Low fracture toughness — 0.6 MPa·√m (0.546 ksi·√in), worse than 96% of polymer grades
- Limited ductility — 2.5%, worse than 89% of polymer grades
- Difficult to machine — 45/100, worse than 78% of polymer grades
- Degrades in UV without stabilisers — 25/100, worse than 76% of polymer grades
E-51 FR properties
Typical room-temperature values for E-51 FR; 24 are specific to this condition. Other listed values are E-51 reference data and may not represent this condition. Bars are the position among polymer grades.
Physical6
E-51 FR has a density of 1.25 g/cm³ (0.0452 lb/in³), heavier than 64% of polymer grades. Its glass-transition temperature is 115°C (239 °F).
Mechanical15
E-51 FR has a tensile strength of 58 MPa (8.41 ksi), stronger than 66% of polymer grades. Elongation at break is 2.5% and the elastic modulus is 3.2 GPa (0.464 Msi).
Thermal10
E-51 FR is rated for continuous service to 110°C (230 °F). It conducts heat at 0.2 W/m·K (0.116 BTU/hr·ft·°F), worse than 56% of polymer grades. Thermal expansion is 62 µm/m·K (34.4 µin/in·°F).
Electrical7
E-51 FR is an electrical insulator with a dielectric strength of 20 kV/mm (508 V/mil) and a resistivity of 1×10¹³ Ω·m.
Chemical & Environmental3
Corrosion resistance is very good (80/100), worse than 52% of polymer grades. UV resistance is poor.
Sustainability4
Producing a kilogram of E-51 FR takes about 137 MJ of energy and emits 6.5 kg of CO₂ — more than 80% of polymer grades. Typical recycled content is 0%.
Manufacturability4
E-51 FR's machinability is fair (45/100, worse than 78% of polymer grades).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with E-51 FR
Is E-51 easy to machine?
Fully cured castings turn, mill and drill easily with sharp carbide and light feeds; keep tools cool to avoid softening near Tg, and use dust extraction (filled/GF grades are highly abrasive — PCD or diamond tooling for laminates).
Can E-51 be welded?
Thermoset — cannot be welded or remelted; join by adhesive bonding with the same epoxy after abrading and solvent-cleaning the surfaces.
Can E-51 be formed, bent or molded?
Processed as a two-part liquid: degas, mix stoichiometrically with amine/anhydride hardener, cast/RTM/wind or prepreg, then cure RT–150 °C with post-cure to reach full Tg; exotherm limits practical section thickness of unfilled castings.
What surface finishes work on E-51?
Sands and polishes to a high gloss; needs a pigmented or UV-blocking topcoat outdoors to prevent yellowing/chalking, and cured surfaces accept paint/plating primers after abrasion or plasma treatment.
E-51 chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| volatilesGB/T 13657 volatiles ≤2 % | 0 – 2 | 0.5 |
| inorganic chlorineinorganic chlorine ≤0.002 % | 0 | 0 |
| hydrolyzable/organic chlorinelower limits for electronic grades | 0 – 0.2 | 0.05 |
| bisphenol A diglycidyl ether (DGEBA oligepoxy value 0.48–0.54 eq/100 g, EEW 185–208 g/eq, 11–14 Pa·s at 25 °C | ≥ 98 (balance) | — |
E-51 FR — frequently asked
What is E-51 FR used for?
E-51 FR is typically used for potting and encapsulation, composite laminates, structural adhesives and tooling and molds. In short: baseline general-purpose epoxy; widest hardener choice; castable and laminating grade.
What is the tensile strength of E-51 FR?
E-51 FR has a typical tensile strength of 58 MPa (8.41 ksi) — stronger than 66% of polymer grades. Strength varies by condition: see the 9 listed tempers.
Is E-51 FR easy to machine?
Reasonably — machinability is rated fair (45/100, worse than 78% of polymer grades). Fully cured castings turn, mill and drill easily with sharp carbide and light feeds; keep tools cool to avoid softening near Tg, and use dust extraction (filled/GF grades are highly abrasive — PCD or diamond tooling for laminates).
Can E-51 FR be welded?
Thermoset — cannot be welded or remelted; join by adhesive bonding with the same epoxy after abrading and solvent-cleaning the surfaces.
What surface finishes work on E-51 FR?
Sands and polishes to a high gloss; needs a pigmented or UV-blocking topcoat outdoors to prevent yellowing/chalking, and cured surfaces accept paint/plating primers after abrasion or plasma treatment.
Can E-51 FR be formed, bent or molded?
Processed as a two-part liquid: degas, mix stoichiometrically with amine/anhydride hardener, cast/RTM/wind or prepreg, then cure RT–150 °C with post-cure to reach full Tg; exotherm limits practical section thickness of unfilled castings.
What is the maximum service temperature of E-51 FR?
E-51 FR is rated for continuous use to about 110°C (230 °F), and briefly to 150°C. Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between E-51 AMINE and E-51 Flex?
AMINE is the default condition — default general-purpose room- or mild-heat-cured clear casting for adhesives, potting and tooling. Flex: use where thermal-cycling stress on delicate components must be absorbed; trades strength and Tg for compliance.
Sources
- Engineered Materials Handbook Vol.3: Adhesives and Sealants — ASM International (1990)
- Engineered Materials Handbook Vol.2: Engineering Plastics — ASM International (1988)
- Araldite / Aradur liquid bisphenol-A epoxy systems technical data — Huntsman Advanced Materials (2022)
Property data is compiled from published supplier and standards handbooks and normalised for comparison. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.