Last updated: 2026-09-05 · 5 min read
UL94 Flame Rating — What Does V-0 Actually Mean, Which Resins Achieve It Without Additives and How Does Wall Thickness Change the Rating?
UL94 is a flammability standard that measures how a plastic self-extinguishes after flame exposure. Ratings from lowest to highest: HB (slow burning), V-2 (self-extinguishes in 30s, drips allowed), V-1 (10s, no drips) and V-0 (10s, no drips, no glowing). Only three engineering resins achieve inherent V-0 at 0.8mm without additives: PPS, PEI and PEEK. All others — PC, ABS, nylon, PBT — require halogenated, phosphorus or mineral additives to reach V-0.
Key Facts
- The UL94 vertical burn test (the most common variant) holds a 125mm × 13mm × thickness bar vertically and applies two 10-second flame exposures from a methane burner (flame temperature 954°C). V-0 requires: each sample self-extinguishes within 10 seconds after either flame application, total flame time for all 10 samples is under 50 seconds, no sample drips flaming particles that ignite cotton below, and no sample burns completely to the holding clamp. The test is repeated on conditioned (70°C/7 days) and as-molded samples.
- Wall thickness determines the UL94 rating — the same resin can be V-0 at 3.0mm but only HB at 0.8mm. Thicker sections have more thermal mass and the flame is absorbed by a larger volume of material, which dissipates heat faster than the surface can pyrolyze. A PC grade rated V-0 at 3.0mm may be V-1 at 1.5mm and HB at 0.8mm. Always check the UL94 rating at the actual part wall thickness — the datasheet value at 3.0mm is irrelevant if your part is 1.2mm. See the wall thickness design rules for how wall choice affects every other molding parameter.
- Flame retardant additives reduce the UL94 rating requirement but also degrade mechanical properties. Halogenated FR (brominated, chlorinated) at 10–20% loading achieves V-0 at 1.5mm but reduces impact strength by 20–40% and increases density by 5–10%. Phosphorus FR at 8–15% loading achieves V-0 but reduces tensile strength by 10–20%. Mineral FR (aluminum hydroxide, magnesium hydroxide) at 40–60% loading achieves V-0 but makes the resin brittle and increases density by 20–30%. The choice depends on which mechanical property you can afford to lose.
How Do Common Engineering Resins Compare on UL94 at Standard Wall Thickness?
| Resin | UL94 at 1.5mm | UL94 at 3.0mm | Inherent or Additive? |
|---|---|---|---|
| PPS GF40 | V-0 | V-0 | Inherent (no additive) |
| PEI (ULTEM) | V-0 | V-0 | Inherent (no additive) |
| PEEK | V-0 | V-0 | Inherent (no additive) |
| PC (unfilled) | HB | V-2 | Needs FR additive |
| PC FR grade | V-0 | V-0 | Phosphorus FR |
| ABS (unfilled) | HB | HB | Needs FR additive |
| ABS FR grade | V-1 | V-0 | Brominated FR |
| Nylon 6/6 GF30 | V-1 | V-0 | Mineral FR |
| PBT GF30 | V-1 | V-0 | Mineral FR |
UL94 vertical burn test — the most common flammability test for plastics (ASTM E162 / IEC 60695-11-10). A 125mm × 13mm bar is held vertically, exposed to two 10-second methane flames (954°C). Ratings: HB (slow burn), V-2 (≤30s after-flame, drips OK), V-1 (≤10s, no drips), V-0 (≤10s, no drips, no glowing). The rating is valid only at the tested thickness.
PPS, PEI and PEEK are the only engineering resins that achieve V-0 inherently at thin walls — their aromatic backbones char instead of burning, forming a protective carbon layer. All other resins require additive packages that trade mechanical performance for flame resistance. For electronics enclosures, connectors and automotive components where V-0 is required at walls below 1.5mm, these three resins are the only options without FR additives. See the PEI processing guide and PPS processing guide for inherent V-0 resin parameters.
What Three Design Rules Govern UL94 Compliance for Injection Molded Parts?
- Specify the UL94 rating at the actual part wall thickness — not the datasheet value. The manufacturer’s Yellow Card lists the thinnest wall at which the resin achieves each rating. If the card says V-0 at 1.5mm and your part is 1.0mm, the part is not V-0 — it may be V-1 or HB. Request the Yellow Card from the resin supplier and check the rating at your specific wall thickness. If the resin does not achieve V-0 at your wall thickness, switch to an FR grade or increase the wall thickness to meet the rating.
- Color concentrates can degrade the UL94 rating — verify with the actual color package. The UL94 rating is tested on the natural (uncolored) resin. Adding color concentrate (2–5% loading) can change the burning behavior — titanium dioxide (white) can improve the rating by reflecting heat, while carbon black (black) can degrade it by absorbing heat and promoting pyrolysis. If the part must be colored, request the UL94 rating for the specific color system — some color packages are flame-rated (tested and listed on the Yellow Card), while others void the rating.
- Glass fiber improves UL94 by 1 rating level — unfilled V-2 becomes GF V-0 at the same thickness. Glass fiber does not burn and acts as a heat sink in the polymer matrix. Unfilled PC is HB at 1.5mm; PC GF30 is V-1 at 1.5mm. Unfilled nylon 6/6 is HB at 1.5mm; nylon 6/6 GF30 is V-1 at 1.5mm. The glass fiber also reduces dripping because it reinforces the char layer. For parts that require V-0 at thin walls, glass-filled grades are the first upgrade before adding FR additives.
Intumescence (in phosphorus FR systems) — the swelling of the char layer when exposed to flame, forming an insulating barrier 10–50× the original coating thickness. Phosphorus FR (ammonium polyphosphate, melamine polyphosphate) triggers intumescence at 200–300°C, shielding the substrate from heat. This is how halogen-free FR achieves V-0 at 1.5mm with only 8–15% additive loading.
UL94 vertical burn: HB (slow burn), V-2 (30s, drips OK), V-1 (10s, no drips), V-0 (10s, no drips, no glowing). Inherent V-0 at 0.8mm: PPS, PEI, PEEK only. Wall thickness matters: same resin V-0 at 3.0mm, HB at 0.8mm. Glass fiber improves 1 rating level. Color can change rating.
- UL94 rating confirmed at actual wall thickness — check the resin Yellow Card; datasheet value at 3.0mm is irrelevant if part is 1.0mm.
- Color package flame-rated — uncolored resin rating may not apply; TiO₂ can improve, carbon black can degrade. Request rated color system.
- GF upgrade considered first — glass fiber improves 1 rating level (HB→V-2, V-2→V-1) before adding FR additives.
- Inherent V-0 resins evaluated — PPS, PEI, PEEK at thin walls without additives; higher resin cost but no mechanical property loss.
- Halogen-free requirement confirmed — EU RoHS / IEC 61249-2-21: Cl <900 ppm, Br <900 ppm. Phosphorus and mineral FR comply; brominated FR does not.
Frequently Asked Questions
What is the difference between UL94 V-0 and 5V?
UL94 5V is a more stringent rating than V-0 — it uses a larger flame (500W vs. 50W for V-series) applied for 5 × 5-second intervals (vs. 2 × 10-second for V-series). 5V has two sub-ratings: 5VA (self-extinguishes, no drips, no through-burning) and 5VB (self-extinguishes, no drips, but through-burning allowed). The test bar is 150mm × 13mm × thickness — same as V-series but longer.
Fewer resins achieve 5VA — PEEK, PEI and PPS GF40 achieve 5VA at 2.0mm. PC FR grades typically achieve 5VA at 3.0mm but only V-0 at 1.5mm. If your application requires 5VA (e.g., electrical enclosures for outdoor use, transformer housings), the resin options are more limited and the wall thickness must be thicker than for V-0.
Does UL94 V-0 mean the part cannot burn in a real fire?
No — UL94 measures small-scale flammability under controlled laboratory conditions (13mm bar, methane flame). It does not predict behavior in a real fire (which involves higher temperatures, oxygen-enriched atmosphere, adjacent combustible materials and sustained exposure). V-0 means the material self-extinguishes within 10 seconds after the small flame is removed — it does not mean the material is fireproof or will not contribute to a large fire.
For real-fire performance, additional tests are needed: cone calorimeter (heat release rate, total heat released), smoke density (NBS smoke chamber), and toxicity of combustion gases (FED — fractional effective dose). These tests are not part of UL94 but are required by building codes (ASTM E84, EN 13501) and transportation standards (FAR 25.853 for aircraft, FMVSS 302 for automotive).
Can a resin achieve V-0 without halogenated flame retardants?
Yes — three approaches: inherent flame resistance (PPS, PEI, PEEK), phosphorus-based FR additives, and mineral fillers. Phosphorus FR (red phosphorus, ammonium polyphosphate, melamine polyphosphate) works by promoting char formation and intumescence (swelling of the char layer to insulate the substrate). These are halogen-free and achieve V-0 at 1.5mm for PC, ABS and PBT at 8–15% loading. Mineral FR (aluminum hydroxide, magnesium hydroxide) works by endothermic decomposition — releasing water vapor to cool the surface. These require 40–60% loading and are used primarily in wire and cable applications.
The trend in electronics and automotive is toward halogen-free FR — EU RoHS and IEC 61249-2-21 define “halogen-free” as chlorine <900 ppm and bromine <900 ppm. Phosphorus and mineral FR systems meet these limits; brominated FR systems do not.
Key Takeaways
- UL94 V-0: self-extinguishes in 10s, no drips, no glowing.
- Inherent V-0 at thin walls: PPS, PEI, PEEK only.
- Wall thickness matters: check rating at actual part wall, not datasheet.
- Glass fiber improves 1 level. Halogen-free FR: phosphorus or mineral.
Send the part drawing with the required UL94 rating at the actual wall thickness and the color requirement — the quotation specifies whether the resin achieves the rating inherently (PPS, PEI, PEEK) or requires an FR grade, and whether the color package is flame-rated.
Sources: Underwriters Laboratories — UL94 Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances · SABIC — UL94 Flame Retardancy Guide and Resin Selection by Rating · Plastics Technology — UL94 Testing and Flame Retardant Additive Comparison · Society of Plastics Engineers (SPE) — Flame Retardancy Technical Papers · MatWeb — Material Property Database with UL94 Ratings by Wall Thickness
— Arthur, Tooling Engineer at aCatMould