Determination of Combustion Performance of Plastics: Flammability Testing Methods, Standards, and Ap

Plastic combustion performance testing measures how polymeric materials ignite, propagate flame, and self-extinguish, providing essential fire-safety data for material selection, quality control, and regulatory compliance. Using internationally recognized methods such as UL 94, IEC 60695, ISO 4589-2 (oxygen index), and ISO 5660-1 (cone calorimetry), accredited third-party laboratories classify the burning behavior of plastics under controlled small-flame, glow-wire, and radiant-heat conditions. This article explains the technical principles, sample requirements, testing items, methods, and typical applications of plastic flammability determination.

Technical Overview

Plastics are organic polymeric materials that, unlike metals or ceramics, are combustible and can contribute to fire growth through ignition, flame spread, heat release, smoke generation, and flaming drips. The combustion performance of a plastic therefore describes a set of quantifiable characteristics: ignitability, burning rate, afterflame and afterglow duration, tendency to produce flaming droplets, limiting oxygen concentration for sustained burning, and heat and smoke release under radiant heating.

Determination of combustion performance is performed by exposing standardized specimens to controlled ignition sources, such as a small gas burner flame, a heated glow wire, a needle flame, or a radiant cone heater. The resulting behavior is timed, measured, and compared against classification criteria defined in international and national standards. The resulting ratings, such as UL 94 V-0, HB, 5VA, GWFI 960 °C, or a limiting oxygen index (LOI) value, are widely used by manufacturers, designers, certification bodies, and regulatory authorities to verify that materials meet fire-safety requirements in electrical equipment, appliances, vehicles, and buildings.

Testing Samples

Molded and Extruded Materials

Most flammability tests are performed on standard specimens injection-molded or machined from resin grades of thermoplastics and thermosets, including ABS, PC, PP, PE, PVC, PA (nylon), PBT, PET, POM, PSU, PPS, epoxy, and phenolic compounds. For UL 94 horizontal and vertical burning tests, typical bar dimensions are 125 mm × 13 mm, with thicknesses commonly evaluated at 1.5 mm and 3.0 mm, since flame retardancy is thickness-dependent.

Sheets, Films, and Thin Flexible Materials

Thin plastic films, laminates, and flexible materials that distort or shrink under a standard vertical flame are tested using a dedicated fixture in accordance with ISO 9773 or IEC 60695-11-10 Annex A, which holds specimens on a mandrel or frame to ensure repeatable flame contact.

Cellular Plastics and Foams

Rigid and flexible foams of density below approximately 250 kg/m³, such as EPS, XPS, PU, and PE foams, are tested in the horizontal burning configuration specified in ISO 9772 / UL 94 HBF, using specimens supported on a wire gauze to reproduce end-use support conditions.

Finished Parts and Components

End-product items such as enclosures, connectors, terminal blocks, switches, coil formers, and printed circuit boards can be evaluated directly using glow-wire, needle-flame, or 500 W flame methods, because component geometry, wall thickness, and proximity to potential ignition sources influence real fire behavior.

Testing Items

Depending on the selected method and product application, the determination of plastic combustion performance typically covers the following measurable items:

• Afterflame time and afterglow time after each flame application
• Vertical burning classification (V-0, V-1, V-2) and 500 W classifications (5VA, 5VB)
• Horizontal burning rate (mm/min) and HB classification
• Ignition of cotton indicator by flaming droplets or particles
• Limiting oxygen index (LOI, volume %)
• Glow-wire flammability index (GWFI) and glow-wire ignition temperature (GWIT)
• Needle-flame test result and spread of flame to adjacent parts or tissue paper
• Time to ignition, peak heat release rate, and total heat release (cone calorimeter)
• Smoke production rate, total smoke release, and smoke density
• Mass loss rate and linear burning distance

Testing Methods

Horizontal and Vertical Burning Test (UL 94 / IEC 60695-11-10 / ISO 1210 / GB/T 2408)

A 50 W premixed methane or natural gas flame (approximately 20 mm high) is applied to a vertically or horizontally clamped specimen. For vertical tests, the flame contacts the lower edge of the bar twice for 10 s each; for the HB test, the flame is applied for 30 s to the free end of a horizontal bar and the burning rate between reference marks is recorded. Classification depends on afterflame times, cotton ignition by drips, and whether the specimen burns to the clamp or the 100 mm mark.

• V-0: afterflame time ≤ 10 s per application, total ≤ 50 s for five specimens, no flaming drips igniting cotton.
• V-1: afterflame time ≤ 30 s per application, total ≤ 250 s, no flaming drips igniting cotton.
• V-2: timing criteria as V-1, but flaming drips that ignite cotton are permitted.
• HB: burning rate ≤ 75 mm/min for thicknesses below 3 mm, or ≤ 40 mm/min for 3–13 mm, or self-extinguishing before the 100 mm mark.

500 W Vertical Burning Test (UL 94 5VA / 5VB / IEC 60695-11-20 / GB/T 5169.16)

A more severe 500 W flame (approximately 125 mm high) is applied five times for 5 s to bar specimens and to horizontal plaques. The 5VA classification, the most stringent UL 94 level, requires no burn-through hole in the plaque, while 5VB permits a hole but forbids flaming drips and excessive afterflame. These ratings are commonly specified for enclosures of unattended electrical equipment.

Limiting Oxygen Index (ISO 4589-2 / ASTM D2863 / GB/T 2406.2)

A small specimen is ignited at its top in a vertical glass chimney through which a mixture of oxygen and nitrogen flows. The minimum oxygen concentration, expressed as volume percentage, that just supports candle-like burning for 3 minutes or over 50 mm is determined. Because ambient air contains about 21 % oxygen, materials with an LOI above 27 % are generally regarded as self-extinguishing, while LOI values below 21 % indicate easily ignitable materials. LOI is a quantitative, highly reproducible metric widely used for research, quality control, and comparing flame-retardant formulations.

Glow-Wire Testing (IEC 60695-2-10/-11/-12/-13 / GB/T 5169 series)

A glow-wire heated to a specified temperature between 550 °C and 960 °C is pressed against the specimen for 30 s, simulating thermal stress from overheated or glowing components. The glow-wire flammability index (GWFI) records the highest temperature at which the specimen either does not ignite or self-extinguishes within 30 s without igniting underlying tissue paper. The glow-wire ignition temperature (GWIT) is the temperature 25 K above the lowest temperature at which the specimen ignites. These indexes are core safety requirements for household appliances, connectors, and electrical components.

Needle-Flame Test (IEC 60695-11-5 / GB/T 5169.5)

A small 12 mm-high needle-burner flame, representing a fault-induced flame, is applied to the specimen for a defined duration (commonly 5 s, 10 s, 20 s, 30 s, or 60 s). The test assesses whether the part ignites, how long it burns (pass criteria typically require extinction within 30 s), and whether flame or burning drops ignite tissue paper or spread to adjacent components.

Cone Calorimeter Test (ISO 5660-1 / ASTM E1354 / GB/T 16172)

A 100 mm × 100 mm horizontal specimen is exposed to radiant fluxes of 25, 35, 50, or 75 kW/m² in a cone-shaped heater while oxygen consumption analysis quantifies heat release. Key outputs include time to ignition, peak heat release rate, total heat release, effective heat of combustion, smoke production, and CO/CO₂ yields. Cone calorimetry provides engineering-grade data for fire modeling and comparative material hazard assessment.

Related and Application-Specific Methods

• Smoke density of materials using a single-chamber test (ISO 5659-2 / GB/T 8323).
• Horizontal burning of vehicle interior materials (FMVSS 302 / ISO 3795), pass limit typically 100 mm/min.
• Flame propagation on single insulated wires and cables (IEC 60332-1-2) and bunched-cable tests (IEC 60332-3).
• Railway material fire performance requirements (EN 45545-2) and building product reaction-to-fire classification (EN 13501-1).

Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 48 hours before testing, and each classification normally requires a set of five (bars) or three to four (plaques) specimens.

Applications

Combustion performance data for plastics are applied across industries where fire safety, product certification, and regulatory compliance are mandatory:

• Electrical and electronic products: enclosure and internal part ratings (UL 94 V-0, 5VA) supporting safety of IT equipment, chargers, power supplies, and consumer electronics.
• Household appliances: glow-wire GWFI and GWIT verification for parts near live connections in refrigerators, washing machines, cookers, and heaters.
• Wire and cable: flame retardancy of insulation and sheathing compounds and finished cable flame tests.
• Automotive: interior trim, dashboards, and connectors evaluated by FMVSS 302 / ISO 3795 and component-level glow-wire tests.
• Building and construction: reaction-to-fire classification of insulation foams, panels, pipes, and flooring.
• Railway and public transport: materials meeting EN 45545-2 hazard classes for rolling stock interiors.
• Toys and childcare articles: flammability requirements for plastic toys under EN 71-2 and similar regulations.
• Research and development: LOI and cone calorimeter data for screening flame-retardant additives and optimizing polymer formulations.

FAQ

What is the difference between UL 94 V-0 and V-2?

Both ratings apply to 50 W vertical burning tests, but V-0 requires short afterflame times (≤ 10 s per ignition) and absolutely no flaming drips that ignite cotton, whereas V-2 allows longer afterflame times (≤ 30 s) and permits flaming drips. V-0 therefore represents a higher level of flame retardancy.

Why are specimens tested at several thicknesses?

Flame retardancy improves with thickness because thicker sections conduct heat away more slowly relative to their mass. A material may achieve V-0 at 3.0 mm but only V-1 or HB at 1.5 mm. Ratings are always reported together with the tested thickness, and end-use wall thickness must be equal to or greater than the certified thickness.

What LOI value indicates a flame-retardant plastic?

An LOI of 21 % corresponds roughly to burning in normal air. In engineering practice, materials with LOI ≥ 27 % are commonly considered self-extinguishing, while values above 30 % indicate highly flame-retardant compounds such as PVC, PPS, or filled engineering thermoplastics.

How do GWFI and GWIT differ?

GWFI describes the ability of a material to extinguish after glow-wire contact without igniting surroundings (a flammability index), while GWIT identifies the temperature at which the material itself ignites, reported 25 K higher than the measured ignition threshold. Appliance standards specify minimum required values for both, often GWFI 960 °C and GWIT 750 °C or 850 °C depending on part location.

Can finished products be tested directly instead of material plaques?

Yes. Glow-wire and needle-flame tests are routinely performed on finished components in their installed configuration, which better reflects real thermal stress. For UL 94 classifications, however, standardized molded bars and plaques of the specific material and color are normally required, since pigments and additives can significantly alter burning behavior.

How should specimens be prepared and conditioned?

Specimens must be clean, free of burrs and stress defects, cut or molded to the dimensional tolerances of the applicable standard, and conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 48 hours to ensure reproducible results.