Plastic flame retardancy rating testing evaluates how polymer materials resist ignition, flame spread, and burning drips when exposed to a standardized ignition source. Classified under internationally recognized systems such as UL 94, IEC 60695-11-10, and GB/T 2408, these ratings are mandatory acceptance criteria for plastic components used in electronics, household appliances, automotive interiors, and cable products. This guide explains the main rating classifications, sample requirements, test procedures, and typical applications across regulated industries.
Technical Overview
Plastic flame retardancy rating testing is a standardized evaluation of how polymeric materials behave when exposed to a controlled flame or hot-surface ignition source. The tests measure ignitability, flame spread rate, afterflame time, afterglow persistence, the tendency to produce flaming droplets or particles, and, in the most severe procedures, burn-through of the specimen. The resulting rating determines whether a material is acceptable for a given application in electrical, electronic, automotive, and construction products.
The dominant classification framework is the UL 94 standard, published by UL, which ranks materials from HB (horizontal burning, the least demanding) through V-2, V-1, and V-0 (50 W vertical burning) up to 5VA and 5VB (500 W vertical burning, the most severe level for enclosures). Technically equivalent or closely aligned methods are published by IEC (IEC 60695-11-10 and IEC 60695-11-20), ISO (ISO 1210, ISO 9772, ISO 9773), ASTM International (ASTM D635, D3801, D5048), and China (GB/T 2408, GB/T 5169.16, GB/T 5169.17). Because product safety standards worldwide reference these ratings, test reports from accredited third-party laboratories are widely used to demonstrate regulatory compliance and market access.
Rating Hierarchy at a Glance
- HB (Horizontal Burning): the lowest rating; suitable for parts with limited fire exposure.
- V-2 (Vertical Burning): afterflame no longer than 30 seconds per specimen; flaming drips that ignite cotton are permitted.
- V-1 (Vertical Burning): afterflame no longer than 30 seconds per specimen; no flaming drips.
- V-0 (Vertical Burning): afterflame no longer than 10 seconds per specimen; no flaming drips; the most commonly specified rating for electronic enclosures.
- 5VA and 5VB (500 W Vertical Burning): the most stringent UL 94 ratings; 5VA prohibits burn-through of a plaque, while 5VB allows a hole but requires all other criteria to be met.
- VTM-0, VTM-1, VTM-2: vertical ratings for thin, flexible films tested per ISO 9773.
- HF-1, HF-2, HBF: ratings for cellular (foam) materials tested per ISO 9772.
Testing Samples
Rigid Thermoplastic Bars and Plaques
For the UL 94 V and HB tests, injection-molded or cut bars of 125 mm × 13 mm are prepared at the thickness of the end product, with 1.6 mm and 3.2 mm being the most common reference thicknesses. The V test normally requires two sets of five bars, while the HB test uses three bars. The 5VA and 5VB tests additionally require 150 mm × 150 mm plaques. Since colorants, fillers, and glass fibers can affect flammability, specimens representing the minimum and maximum thicknesses and the most and least flammable colors should be included.
Foam Materials
Cellular materials such as polyurethane, polyethylene, and polystyrene foams are tested as 150 mm × 50 mm specimens at the end-use thickness in accordance with ISO 9772, resulting in HBF, HF-2, or HF-1 classifications.
Thin and Flexible Films
Films thinner than approximately 0.1 mm, or specimens that distort, shrink, or flex during a standard vertical test, are evaluated using the VTM procedure of ISO 9773, in which a 200 mm × 50 mm strip is rolled into a cone shape around a mandrel before ignition.
Wire, Cable, and Finished Components
Cable specimens are typically 600 mm lengths tested per IEC 60332-1-2 for single vertical flame propagation or IEC 60332-3 for bunched-cable installation conditions. For finished components that cannot be cut into standard bars, glow wire testing per IEC 60695-2-11 and the needle flame test per IEC 60695-11-5 are applied directly to the complete part.
Specimen Conditioning
Most standards require two conditioning states before testing: at least 48 hours at 23 ± 2 °C and 50 ± 5% relative humidity, and 168 hours at 70 °C in an air-circulating oven followed by cooling in a desiccator. The surgical cotton used to catch flaming drips is oven-dried before the test. Proper conditioning is essential because moisture content and thermal history measurably influence afterflame behavior.
Testing Items
Accredited third-party laboratories typically offer the following flammability test items for plastic materials:
- Horizontal burning test (HB rating): ASTM D635, IEC 60695-11-10, GB/T 2408.
- 50 W vertical burning test (V-0, V-1, V-2 ratings): ASTM D3801, IEC 60695-11-10, GB/T 5169.16.
- 500 W vertical burning test (5VA, 5VB ratings): ASTM D5048, IEC 60695-11-20, GB/T 5169.17.
- Thin-material vertical burning test (VTM ratings): ISO 9773.
- Foam horizontal burning test (HF-1, HF-2, HBF ratings): ISO 9772.
- Glow wire flammability index (GWFI): IEC 60695-2-12.
- Glow wire ignitability temperature (GWIT): IEC 60695-2-13.
- Glow wire test on end products: IEC 60695-2-11.
- Needle flame test: IEC 60695-11-5.
- Limiting oxygen index (LOI): ASTM D2863, ISO 4589-2, GB/T 2406.2.
- Smoke density: ISO 5659-2, ASTM E662.
- Heat release rate (cone calorimeter): ISO 5660-1, ASTM E1354.
- Automotive interior horizontal burn test: FMVSS 302, ISO 3795, GB 8410.
- Flame propagation tests for cables: IEC 60332-1-2, IEC 60332-3.
Testing Methods
Vertical Burning Test (V-0, V-1, V-2)
A bar specimen is clamped vertically with its long axis horizontal... rather, with the long axis vertical and its lower edge exposed to a 20 mm (approximately 50 W) methane flame applied for 10 seconds. After the flame is removed, the afterflame time t1 is recorded; the flame is then reapplied for another 10 seconds, and the second afterflame time t2, the afterglow time, and any ignition of the cotton pad placed 300 mm below the specimen are recorded. For V-0, each afterflame time must not exceed 10 seconds and the total for a five-specimen set must not exceed 50 seconds; for V-1 and V-2, the limits are 30 seconds per specimen and 250 seconds per set. V-0 and V-1 prohibit flaming drips, while V-2 permits drips that ignite the cotton. No specimen may burn or glow up to the clamp.
Horizontal Burning Test (HB)
The specimen is mounted horizontally with one end inclined at 45 degrees and exposed to a 25 mm blue flame for 30 seconds or until the flame front reaches a reference mark 25 mm from the free end. The time for the flame front to travel from the 25 mm mark to the 100 mm mark is measured to calculate the burn rate. An HB rating is granted if the specimen extinguishes before the 100 mm mark, or if the burn rate does not exceed 40 mm/min for thicknesses of 3.0 to 13 mm, or 75 mm/min for thicknesses below 3.0 mm.
500 W Vertical Burning Test (5VA, 5VB)
Bar specimens and 150 mm × 150 mm plaques are exposed to a 125 mm (approximately 500 W) flame applied to the lower corner in five cycles of 5 seconds on and 5 seconds off. The criteria limit afterflame plus afterglow after the fifth application, prohibit flaming drips that ignite cotton, and, in the plaque test, distinguish between the two grades: a 5VA material must not exhibit burn-through, whereas a 5VB material may develop a hole but must still satisfy all other criteria. These ratings are required for enclosures exposed to higher-energy ignition sources.
Glow Wire Flammability Tests
A nickel-chromium heating element is electrically heated to a specified temperature between 550 °C and 960 °C and pressed against the specimen for 30 seconds. The glow wire flammability index (GWFI) is the highest temperature at which the specimen self-extinguishes within 30 seconds after removal of the glow wire and does not ignite the tissue paper placed beneath it. The glow wire ignitability temperature (GWIT) is defined as 25 K above the highest temperature at which the specimen does not ignite or self-extinguishes within 5 seconds. Glow wire tests are central to compliance with IEC 60335-1 for household appliances, where required temperatures of 550 °C, 650 °C, 750 °C, or 850 °C depend on the function of the part and the current it carries.
Limiting Oxygen Index (LOI)
In the LOI test per ASTM D2863, ISO 4589-2, or GB/T 2406.2, a vertically mounted specimen is ignited from the top in a controlled mixture of oxygen and nitrogen, and the minimum oxygen concentration required to sustain burning for 3 minutes or over a 50 mm length is determined. Since ambient air contains approximately 21% oxygen, plastics with an LOI above 26% are generally considered self-extinguishing; typical values range from about 18% for ordinary ABS to above 30% for flame-retardant engineering grades and above 45% for PVC.
Applications
Electronics and IT Equipment
Standards such as IEC 62368-1 for audio/video, information, and communication technology equipment require enclosure and support materials to meet minimum vertical burning ratings, with V-1 being a common baseline and V-0 the prevailing industry specification for chargers, adapters, connectors, circuit board spacers, and battery compartments.
Household Appliances
IEC 60335-1 relies on a combination of glow wire temperatures, needle flame testing, and UL 94 ratings for internal components, terminal blocks, and unattended appliances, ensuring that live connections and high-current parts do not propagate fire.
Automotive and Electric Vehicles
Interior trim materials must achieve a horizontal burn rate not exceeding 102 mm/min under FMVSS 302, ISO 3795, and GB 8410. Electric vehicle applications add further requirements for battery enclosures, high-voltage connectors, and charging components, where V-0 and comparative tracking performance are routinely specified.
Rail and Mass Transit
EN 45545-2 defines fire safety requirement sets for railway materials that combine LOI, smoke density per ISO 5659-2, and heat release data from cone calorimeter testing, making comprehensive flammability characterization essential for interior panels, seat shells, and cable systems.
Building Products and Cables
Construction plastics are classified under national reaction-to-fire frameworks, while power and data cables are evaluated for single and bunched flame propagation under the IEC 60332 series, driving demand for halogen-free flame-retardant compounds verified by laboratory testing.
FAQ
What is the difference between UL 94 V-0, V-1, and V-2?
The three vertical ratings differ in permitted afterflame time and dripping behavior. V-0 allows a maximum afterflame time of 10 seconds per specimen and 50 seconds total per set with no flaming drips; V-1 allows 30 seconds per specimen and 250 seconds total with no flaming drips; V-2 allows the same times as V-1 but tolerates flaming drips that ignite cotton. All three prohibit burning up to the clamp.
Is 5VA a higher rating than V-0?
Yes. The 5VA and 5VB tests use a roughly ten times more energetic flame, apply it five times instead of twice, and include plaque burn-through criteria. 5VA is the most demanding UL 94 rating and is typically reserved for enclosures near potential ignition sources.
Is GB/T 2408 equivalent to UL 94?
GB/T 2408 and GB/T 5169.16 are harmonized with IEC 60695-11-10, which is technically equivalent to the UL 94 horizontal and 50 W vertical tests, so HB and V-0/V-1/V-2 results are directly comparable. For the 500 W test, GB/T 5169.17 corresponds to IEC 60695-11-20.
Does the rating depend on wall thickness?
Yes. A flame retardancy rating is only valid at the tested thickness, and thinner walls burn more readily. A material rated V-0 at 3.2 mm may only achieve V-1 or fail at 1.6 mm, so testing should always be performed at the minimum production thickness.
When is glow wire testing required instead of UL 94?
Glow wire testing is required when standards such as IEC 60335-1 evaluate finished parts, live electrical connections, or materials near heat sources. It simulates ignition from a hot or overloaded component rather than an open flame, and GWIT and GWFI values are assigned at discrete temperatures from 550 °C to 960 °C.
What LOI value qualifies a plastic as flame retardant?
Because ambient air contains about 21% oxygen, a plastic with an LOI above 26% will generally not continue burning in air. Commercial flame-retardant grades of ABS, PC, PA, and PBT typically reach LOI values of 28% to 35% or higher.
How long does plastic flame retardancy testing take?
Including specimen conditioning, a single UL 94 rating or glow wire series typically takes one to two weeks at accredited third-party laboratories. Programs covering multiple thicknesses, colors, and standards should be scheduled with additional time for report review and, where needed, inclusion in supplier certification dossiers.