Flame retardancy rating determination is a critical safety evaluation for plastic materials used in electrical, electronic, automotive, and construction applications. This article explains the main flammability classification systems, including UL 94, IEC 60695, ISO 4589, and the corresponding GB standards, along with sample requirements, testing items, methods, and practical applications of the results.
Technical Overview
Plastic materials are organic polymers that are inherently combustible, which makes flammability performance a fundamental safety consideration whenever they are used in electrical devices, appliances, vehicles, or buildings. Flame retardancy rating determination refers to a set of standardized laboratory tests that measure how a plastic ignites, burns, drips, and self-extinguishes under controlled flame, heat, or glowing-source exposure. The results are expressed as standardized classification grades that engineers, regulators, and buyers can use to compare materials and demonstrate compliance with product safety requirements.
The most widely recognized classification framework is the UL 94 flame test series, developed under the UL standards system and technically aligned with IEC 60695-11-10 and IEC 60695-11-20. These standards define horizontal burning grades (HB) and vertical burning grades (V-2, V-1, V-0, 5VA, 5VB), where higher grades indicate faster self-extinction and reduced afterflame and afterglow times. Complementary methods include the limiting oxygen index (LOI) test per ISO 4589-2 and ASTM D2863, the glow wire tests per IEC 60695-2-11/-12/-13, and the needle flame test per IEC 60695-11-5. In China, the equivalent national standards GB/T 2408, GB/T 2406.2, and GB/T 5169 series mirror these international methods and are commonly specified in domestic regulatory and procurement documents.
Testing is typically performed by accredited third-party laboratories under controlled temperature and humidity conditions, using precisely dimensioned specimens prepared from the material in its final commercial form. Correct classification depends not only on the polymer itself but also on flame retardant additives, fillers, thickness, colorants, and processing history, which is why ratings are always reported together with the specimen thickness.
Testing Samples
Flammability behavior varies significantly with geometry, thickness, and processing method, so specimens must be prepared to match the end-use condition of the material as closely as possible. Accredited third-party laboratories generally accept the following sample forms.
Molded or Cut Plaques and Sheets
Flat specimens with standard dimensions (for example, 125 mm × 13 mm bars for UL 94 HB and V tests, or 150 mm × 150 mm panels for 5VA/5VB tests) are prepared by injection molding, compression molding, or cutting from sheet stock. Each reported grade must be supported by testing at the minimum wall thickness at which the rating is claimed, because thin sections ignite and burn more readily than thick sections.
Injection-Molded Components
Finished parts such as connectors, enclosures, switch bodies, and structural housings can be tested directly when the standard specimen geometry cannot be produced. Testing on actual components verifies that gate location, fiber orientation, sink marks, and wall thickness variation do not degrade the classified performance of the compound.
Films and Thin Materials
Flexible films, tapes, and thin membranes are evaluated with adapted clamping frames that prevent specimen movement during burning. Materials thinner than the scope of the bar tests may be assessed under the thin-film provisions of the applicable standard or by alternative methods such as LOI, depending on the specification invoked by the end product standard.
Colored and Filled Compounds
When a material family is offered in multiple colors or filler loadings, representative formulations of each variant should be submitted, since pigments and additives can measurably influence ignition time, dripping behavior, and self-extinguishing performance.
Testing Items
A comprehensive flame retardancy evaluation typically combines several of the following test items, selected according to the product standard, market regulation, or customer specification that applies to the end use.
- Horizontal burning test, UL 94 HB (equivalent to IEC 60695-11-10 and GB/T 2408 horizontal method): determines the linear burning rate or whether the specimen burns to a defined mark.
- Vertical burning test, UL 94 V-0 / V-1 / V-2 (equivalent to IEC 60695-11-10 and GB/T 2408 vertical method): measures afterflame time, afterglow time, and ignition of cotton by flaming drips after two 10-second flame applications.
- Vertical burning test, UL 94 5VA / 5VB (equivalent to IEC 60695-11-20 and GB/T 5169.16): a more severe five-application test that also evaluates whether the flame burns through a plaque specimen.
- Limiting oxygen index, LOI (ISO 4589-2, ASTM D2863, GB/T 2406.2): determines the minimum oxygen concentration in an oxygen/nitrogen mixture required to support candle-like burning, reported as a percentage.
- Glow wire flammability index, GWFI (IEC 60695-2-12, GB/T 5169.12): evaluates ignition and self-extinguishing behavior against a heated glow wire at temperatures up to 960 °C.
- Glow wire ignition temperature, GWIT (IEC 60695-2-13, GB/T 5169.13): determines the temperature 25 K above which the glow wire does not cause ignition of the specimen within a defined time.
- Needle flame test (IEC 60695-11-5, GB/T 5169.5): applies a small 12 mm flame to assess fire hazard from localized fault conditions in small components.
- Comparative tracking and hot-wire ignition screening tests where specified by the relevant material or product standards.
Testing Methods
Each test follows a tightly controlled procedure so that ratings are reproducible between laboratories. Specimens are conditioned, typically for at least 48 hours at 23 °C and 50% relative humidity, before testing in a draft-free enclosure.
Horizontal Burning Method (HB)
A 25 mm and 100 mm reference mark is scribed on the bar specimen, which is clamped horizontally. A 20 mm blue flame from a specified burner is applied for 30 seconds to the free end. The laboratory records whether the flame front passes the 25 mm mark, the time to travel between the 25 mm and 100 mm marks, and the calculated burning rate in millimeters per minute. Materials that do not burn to the 100 mm mark, or that burn slower than the specified rate limits for the given thickness, satisfy the HB classification.
Vertical Burning Method (V-0, V-1, V-2)
The bar specimen is clamped vertically with a layer of surgical cotton placed 300 mm below its lower edge. A 20 mm burner flame is applied to the center of the lower edge for 10 seconds, removed, and reapplied for another 10 seconds after the specimen stops burning or glowing. The laboratory records the individual afterflame times after each application, the total afterflame plus afterglow time for all specimens, whether flaming drips ignite the cotton, and whether the specimen burns up to the clamp. Classification depends on meeting the time and dripping criteria for the respective grade.
500 W Vertical Method (5VA, 5VB)
Both bar specimens and separate square plaques are tested against a higher-energy 125 mm flame applied five times for 5 seconds each. The 5VA grade, the most demanding UL 94 classification, additionally requires that the plaque not exhibit burn-through, while 5VB permits a hole in the plaque but not sustained flaming or flaming drips.
Limiting Oxygen Index Method
The specimen is mounted vertically in a glass chimney through which a metered oxygen/nitrogen mixture flows. The oxygen concentration is adjusted in a bracketing sequence until the minimum concentration that just supports burning over a defined distance or time is found. A higher LOI value indicates a material that requires a more oxygen-rich atmosphere to keep burning; engineering thermoplastics intended for electrical enclosures commonly target LOI values of 28% or above.
Glow Wire Methods
A nickel-chromium glow wire tip is heated by electric current to a specified temperature, verified by a calibrated thermocouple, and then pressed against the specimen surface with a defined force for 30 seconds. For GWFI, the laboratory observes whether the specimen ignites and extinguishes within 30 seconds after removal without ignition of the wrapping tissue placed beneath. For GWIT, the temperature is stepped to find the highest value at which no ignition occurs within the specified exposure window.
Needle Flame Method
A small burner producing a 12 mm high flame is applied to the specimen for a specified duration, typically 5 to 30 seconds depending on the standard invoked. The test assesses whether the part ignites, how long flames persist, whether burning drips ignite the underlying tissue layer, and is frequently applied to completed sub-assemblies rather than raw material bars.
Applications
Flame retardancy ratings are embedded in the safety requirements of nearly every industry that plastics serve, and test data are used at multiple stages of the product lifecycle.
- Electrical and electronic equipment: enclosures, PCBs, connectors, and internal barriers for IT equipment, chargers, and power supplies are classified under UL 94 or IEC 60695 to meet the fire hazard requirements of product safety standards.
- Household appliances: plastic parts positioned near live parts or heating elements, such as switch housings, fan blades, and control panels, are graded by glow wire and needle flame methods in accordance with IEC 60335-based requirements.
- Automotive: interior trim, battery enclosures, charging connectors, and electronic control units require documented flammability compliance under OEM specifications and regulations such as FMVSS 302 for interior materials.
- Building and construction: wall linings, roofing membranes, pipes, and insulation boards are evaluated for flame spread, drip behavior, and oxygen index under ISO, EN, ASTM, and GB frameworks supporting national building codes.
- Wire and cable: insulation and sheathing compounds are screened with LOI and vertical flame tests before full-scale cable flame tests are undertaken.
- Material selection and procurement: design engineers and purchasing teams use published ratings to compare candidate compounds, while regulators and certification bodies use laboratory reports as evidence of conformity.
- Research and development: formulation developers iterate flame retardant packages and verify improvements through HB, V, and LOI testing before commercializing new grades.
FAQ
What is the difference between UL 94 V-0, V-1, and V-2?
All three are vertical burning classifications based on afterflame time, afterglow time, and dripping behavior. V-0 requires the shortest combined afterflame times and no flaming drips igniting cotton; V-1 allows longer afterflame times with no igniting drips; V-2 has timing limits similar to V-1 but permits flaming drips provided they do not ignite the cotton indicator placed below the specimen.
Why is specimen thickness always stated with the rating?
Flammability depends strongly on geometry. A compound may achieve V-0 at 3.0 mm but only V-2 or HB at 1.5 mm. Ratings are therefore valid only at or above the tested thickness, and thinner walls in a final part require separate verification.
Are UL 94, IEC 60695, and GB/T ratings interchangeable?
The UL 94 vertical and horizontal methods are technically aligned with IEC 60695-11-10 and IEC 60695-11-20, and the corresponding GB/T standards adopt the same procedures. In practice, the acceptance of results across schemes depends on the specific certification or regulatory requirement, so the applicable standard should be confirmed before testing begins.
How is the limiting oxygen index different from the UL 94 tests?
LOI is a quantitative measure of the minimum oxygen concentration needed to sustain burning, expressed as a percentage, and is useful for comparing formulations. UL 94 is a pass/fail classification based on exposure to a defined flame. The two methods are complementary, and many specifications require both.
What do GWFI and GWIT temperatures tell a designer?
GWFI indicates the highest glow wire temperature at which the material self-extinguishes without igniting surroundings, while GWIT indicates the temperature below which the glow wire will not ignite the material at all. Appliance safety standards prescribe minimum required values, such as 750 °C or 850 °C, depending on the part location and the nature of nearby live or unattended parts.
How should samples be submitted for testing?
Submit specimens in the color, thickness, and processing condition representative of the end use, in sufficient quantity for the required set of specimens and any repeat runs, together with material identification, grade designation, and intended thickness classification. Accredited third-party laboratories can also assist with specimen preparation from molded plaques when raw material is supplied.