Flame retardancy ratings explained

Flame retardancy is the most over-specified and most misread property on any datasheet. A V-0 requirement looks simple until you discover the rating is thickness-dependent, that UL94 and glow wire measure completely different things, and that FR chemistry quietly costs you impact strength and flow. Here is how the ratings work.

UL94: the small-flame test

UL94 exposes a vertical or horizontal test bar to a small flame and measures whether it self-extinguishes, how fast, and whether it drips burning particles. From lowest to highest requirement:

• HB: slow burn on a horizontal bar; the minimum rating for most commodity resins.
• V-2: self-extinguishes within 30 s per flame application, but burning drips are allowed.
• V-1: same extinction time as V-2, no burning drips.
• V-0: extinguishes within 10 s, no burning drips — the most commonly demanded rating.
• 5VB / 5VA: a larger flame plus a direct burn-through test on plaques; 5VA allows no hole. Required for larger enclosures in many appliance and IT standards.

Critical detail: every UL94 rating is tied to a minimum thickness. A datasheet line reading 'V-0 @ 1.5 mm' says nothing about 0.8 mm walls. Always spec the rating at your actual minimum wall thickness, and ask for the full thickness series when designing thin enclosures.

Glow wire: the appliance standard

IEC 60335 (household appliances) does not care about open flames — it simulates an overheating element pressing against plastic. Two numbers matter:

• GWFI (Glow Wire Flammability Index): the highest temperature at which a material self-extinguishes within 30 s after the wire is removed.
• GWIT (Glow Wire Ignition Temperature): the temperature at which the material ignites during contact.

Unattended appliances typically need GWFI ≥ 850°C for parts supporting live connections. Note that UL94 V-0 does not automatically deliver 850°C glow wire — they are separate tests, and you must check both values independently.

LOI and CTI

Limiting Oxygen Index (LOI) measures the minimum oxygen concentration that sustains burning; air is 21%, so LOI above ~28% indicates self-extinguishing behavior. It is useful for comparing FR efficiency between grades but is not an acceptance criterion in most product standards. Comparative Tracking Index (CTI) rates resistance to electrical tracking along the surface and determines material group (I, II, IIIa, IIIb) for creepage distance calculations in connectors and switchgear.

Halogenated vs halogen-free FR

• Halogenated (bromine/chlorine + antimony): efficient, low loading, best property retention — but restricted in many electronics and rail programs.
• Phosphorus-based (e.g. in PC and PC/ABS): common halogen-free route for housings; works in the condensed phase.
• Nitrogen/phosphorus intumescent systems (e.g. in PA and PP): used where glow wire performance and halogen-free compliance are both required.

Halogen-free grades typically sacrifice some flow, impact strength, and colorability, and often cost 15–30% more. Verify the full mechanical set, not just the FR line.

Spec checklist

• Name the standard your product must pass (UL/IEC/GB), not just 'V-0'.
• State the minimum wall thickness the rating must hold at.
• If an appliance, add GWFI/GWIT targets for parts near live connections.
• Ask for UL Yellow Card / file number to confirm third-party certification rather than internal test claims.