FR‑4: The Ultra‑Strong, Flame‑Retardant Composite of Choice for Modern Engineering

Why FR‑4 is the benchmark for high‑performance composites. Known for its exceptional strength, FR‑4 can match or surpass many metals. Its Ultimate Tensile Strength reaches 45 000 psi—outperforming cast iron (29 000 psi), copper (31 900 psi), aluminum (43 500 psi) and structural steel (40 000 psi).

Composition and properties. FR‑4 is engineered from woven glass fabric saturated with a difunctional epoxy resin, yielding an anisotropic, direction‑dependent material. Standards are set by the National Electrical Manufacturers Association (NEMA) and the UL 94 V‑0 flame‑retardancy rating.

Beyond strength, FR‑4 delivers a suite of engineering advantages. Here are six key reasons why it excels:

1. Flame‑retardant. With UL 94 V‑0 certification, a vertical specimen stops burning within 10 seconds after two blue‑flame exposures; dripping is allowed only if non‑igniting.
2. Chemical resistance. FR‑4 tolerates a wide range of solvents—alcohols, aromatic hydrocarbons such as benzene and toluene—, nickel plating baths, and aggressive acids including 100 % hydrofluoric and hydrochloric. For a full chemical resistance chart, see our High Performance Material Guide.
3. Water‑proof. Near‑zero water absorption renders FR‑4 inert in freshwater and seawater, making it ideal for marine structures, fish‑farm pods, boats, and undersea communication cables.
4. Excellent electrical insulation. Low water uptake yields superior dielectric loss characteristics and high dielectric strength in both wet and dry conditions. This makes FR‑4 a staple in PCB manufacturing and for insulating fasteners, spacers, and other electrical components.
5. Low density. With a specific gravity of 1.8, FR‑4 is lighter than aluminum (2.7) and far lighter than cast iron (7.2), copper (8.96), or steel (7.82).
6. UV resistance. Grades formulated with a tetrafunctional epoxy system withstand ultraviolet exposure, further enhancing suitability for outdoor and underwater applications.

[i] Dielectric loss is the amount of heat an insulating material dissipates when subjected to an electrical current.

[ii] Dielectric strength is the maximum electrical field a material can withstand without losing its insulating properties.

[iii] Tetrafunctional describes a compound having four bonding sites in a molecule.

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