Determination of Combustion Characteristics of Plastics: Test Methods, Standards, and Applications

The determination of combustion characteristics of plastics is a critical safety evaluation that measures how polymeric materials ignite, propagate flame, drip, and self-extinguish under controlled laboratory conditions. Governed by internationally recognized standards such as UL 94, IEC 60695-11-10, ISO 1210, ASTM D635, and ISO 4589, these tests provide the flammability ratings required for regulatory compliance in electrical, automotive, appliance, and construction applications. This article presents a comprehensive technical overview of specimen requirements, test items, methodologies, and practical applications of plastic flammability testing.

Technical Overview

Plastics are organic polymeric materials that, by nature, are combustible when exposed to sufficient heat and oxygen. Because they are widely used in electrical enclosures, connectors, cable insulation, automotive interiors, and building components, evaluating their fire behavior is essential for product safety, regulatory compliance, and material selection. The determination of combustion characteristics quantifies parameters such as ignition tendency, flame propagation speed, afterflame and afterglow duration, flaming drippings, and the minimum oxygen concentration required to sustain combustion.

The most widely applied test frameworks are published by leading standards organizations, including UL (UL 94), IEC (IEC 60695-11-10, IEC 60695-2 series, IEC 60695-11-5), ISO (ISO 1210, ISO 9772, ISO 9773, ISO 4589-2, ISO 5660-1), ASTM International (ASTM D635, ASTM D3801, ASTM D5048, ASTM D2863, ASTM E1354), and the Chinese national standards system (GB/T 2408, GB/T 2406.2, GB/T 5169 series). These methods are largely harmonized; for example, UL 94 vertical and horizontal procedures correspond technically to IEC 60695-11-10, ISO 1210, and GB/T 2408. Accredited third-party laboratories routinely perform these tests to support certification, quality control, and research and development programs.

Testing Samples

Standard Rigid Specimens

For small-flame tests such as UL 94, IEC 60695-11-10, and GB/T 2408, test bars are typically cut or injection-molded to dimensions of 125 mm × 13 mm, with the thickness reflecting the minimum wall thickness of the end product (commonly 0.8 mm, 1.5 mm, or 3.0 mm). A vertical burning classification generally requires five specimens per condition, while horizontal testing typically requires three. Specimens must be free of flash, bubbles, and surface contamination, as these defects can significantly influence burning behavior.

Flexible and Thin-Film Materials

Thin or flexible plastics that distort or shrink away from a burner flame cannot be evaluated with the standard vertical method. Instead, a mandrel-supported vertical test (ISO 9773, ASTM D4804, or UL 94 VTM) is applied, in which the film is wrapped around a metal rod to maintain specimen geometry during flame exposure.

Cellular and Foamed Plastics

Rigid and flexible cellular materials are tested in the horizontal position using ISO 9772 or ASTM D4986, which assess the extent and rate of burning for foams used in insulation, packaging, and cushioning applications.

Finished Parts and Components

When the flammability of an actual device component must be verified—such as an enclosure, terminal block, or bracket—complete parts may be subjected to needle-flame or glow-wire testing (IEC 60695-11-5 and IEC 60695-2-11/-2-12) to evaluate behavior under simulated fault conditions within the final product environment.

Prior to testing, specimens are conditioned in a controlled atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 48 hours. Many standards, including UL 94, additionally require a second set of specimens aged at 70 °C for 168 hours followed by cooling in a desiccator, ensuring the classification is valid for the material in both states.

Testing Items

Depending on the selected method, the combustion characterization program may include the following measured parameters:

• Afterflame time after the first and second flame applications (t1 and t2)
• Total afterflame time across all specimens
• Afterglow time and whether afterglow persists after flame extinction
• Linear burning rate (mm/min) determined from the time to burn between reference marks
• Damaged length or extent of combustion along the specimen
• Occurrence of flaming or non-flaming drippings and ignition of the surgical cotton indicator
• Absence or presence of burn-through holes in plaque specimens (5VA/5VB criteria)
• Limiting Oxygen Index (LOI), expressed as the minimum volumetric oxygen concentration that sustains candle-like burning
• Glow Wire Flammability Index (GWFI) and Glow Wire Ignition Temperature (GWIT)
• Heat release rate, total heat release, mass loss rate, and smoke production derived from cone calorimetry

Testing Methods

Horizontal Burning Test (HB)

Conducted per UL 94 HB, IEC 60695-11-10, ASTM D635, and GB/T 2408, a 20–25 mm blue flame from an inclined burner is applied to the free end of a horizontally clamped bar for 30 seconds. A material classified HB must either extinguish before the 100 mm mark or exhibit a burning rate not exceeding 40 mm/min for thicknesses of 3–13 mm, or 75 mm/min for thicknesses below 3 mm. The HB level is the lowest flammability rating and is generally accepted only for less critical, low-risk applications. Automotive interior materials are similarly evaluated in the horizontal orientation under ISO 3795, FMVSS 302, and GB 8410.

Vertical Burning Test (V-0, V-1, V-2)

The 50 W vertical test (UL 94, IEC 60695-11-10, ISO 1210, ASTM D3801, GB/T 2408) applies a 20 mm flame to the lower end of a vertically suspended bar for 10 seconds, twice, with cotton placed beneath the specimen. Classifications are assigned based on afterflame times, aggregate burning time, and dripping behavior:

• V-0: individual afterflame ≤ 10 s; total afterflame for both applications across all specimens ≤ 50 s; no ignition of the cotton indicator
• V-1: individual afterflame ≤ 30 s; total ≤ 250 s; no ignition of the cotton indicator
• V-2: identical timing criteria to V-1, but brief flaming drips that ignite the cotton are permitted

500 W Flame Test (5VA, 5VB)

The 5V apparatus test (UL 94, IEC 60695-11-20, ISO 10351, ASTM D5048) subjects both bar and plaque specimens to a higher-energy 125 mm flame applied five times for 5 seconds each. The 5VA classification—the most severe small-flame rating—requires no burn-through hole in the plaque, while 5VB permits a hole but prohibits flaming drips. These ratings are commonly mandated for enclosure materials in unattended equipment and for parts near high-energy sources.

Limiting Oxygen Index (LOI)

Per ISO 4589-2, ASTM D2863, and GB/T 2406.2, a vertically mounted specimen is ignited at its top and allowed to burn in a mixture of oxygen and nitrogen flowing through a glass chimney. The oxygen concentration is adjusted to determine the minimum level that supports burning for 3 minutes or over 50 mm. LOI provides a quantitative, repeatable measure of intrinsic flame retardancy: common engineering plastics such as polypropylene exhibit LOI values near 18 %, while flame-retarded or inherently resistant materials such as PVC and PTFE reach 45 % and above.

Glow Wire Tests

Simulating thermal stress from glowing electrical components, the glow wire methods (IEC 60695-2-10, -2-11, -2-12, -2-13; GB/T 5169 series) press a heated nickel-chromium wire tip against the specimen. GWFI testing determines whether the part self-extinguishes without igniting surrounding tissue paper, while GWIT establishes the temperature 25 K (or 30 K in some product standards) above the highest temperature at which ignition occurs. Common requirement levels are 550 °C, 650 °C, 750 °C, and up to 960 °C.

Needle Flame Test

Described in IEC 60695-11-5 and GB/T 5169.5, this test applies a small 12 mm butane flame to verify that a part does not propagate fire originating from a minor fault condition, such as a localized short circuit.

Cone Calorimetry

For advanced fire performance engineering, ISO 5660-1 and ASTM E1354 expose 100 mm × 100 mm specimens to a controlled radiant heat flux (typically 35 or 50 kW/m²). The technique yields heat release rate, peak heat release, effective heat of combustion, and smoke yield—key inputs for fire modeling and material comparison beyond pass/fail classifications.

Applications

Combustion characteristic data directly support safety design and market access across multiple industries:

• Electrical and electronic products: enclosure and PCB materials for IT and audio/video equipment must satisfy flammability requirements referenced in IEC 62368-1, typically V-1 or higher for fire enclosures
• Household appliances: IEC 60335-1 mandates glow-wire and needle-flame verification for live parts, uninsulated connections, and supports carrying live parts
• Automotive engineering: interior trim materials are controlled by horizontal burning rate limits under FMVSS 302, ISO 3795, and GB 8410, while battery housings increasingly demand 5VA performance
• Wires and cables: insulation and sheathing compounds are qualified through LOI and vertical flame testing to ensure circuit integrity and fire safety
• Building and rail transport: wall coverings, seating foams, and structural components are evaluated against reaction-to-fire regimes supported by LOI, calorimetry, and smoke density data under EN 45545-2 and related building codes
• Material development: flame-retardant formulators use LOI and UL 94 results to benchmark additive performance and optimize halogenated, phosphorus-based, and mineral-filled systems

FAQ

What is the difference between UL 94 V-0, V-1, and V-2?

The classifications differ in permitted afterflame duration and dripping behavior. V-0 allows individual afterflame times up to 10 seconds and a 50-second cumulative maximum without igniting cotton; V-1 extends these limits to 30 and 250 seconds; V-2 shares the V-1 timing but tolerates flaming drips that briefly ignite the cotton indicator.

Does specimen thickness affect the rating?

Yes. Flammability classifications are thickness-specific because thicker sections dissipate heat and resist ignition more effectively. A material rated V-0 at 3.0 mm may only achieve V-2 or fail at 1.5 mm; the reported rating must always state the tested minimum thickness.

Why do standards require testing both conditioned and aged specimens?

Aging at elevated temperature can alter polymer morphology, plasticizer distribution, or flame-retardant migration. Evaluating specimens after both standard conditioning and thermal aging confirms that the classification remains representative throughout the product service life.

Is UL 94 equivalent to IEC 60695-11-10 and GB/T 2408?

The 50 W horizontal and vertical flame procedures are technically aligned among these standards, allowing broadly comparable results. However, certification requirements, report formats, and specific acceptance criteria are defined by each end-product standard, so the applicable scheme should be confirmed before testing.

How should a laboratory be selected for testing?

Manufacturers should engage accredited third-party laboratories with ISO/IEC 17025-accredited flammability capabilities, calibrated burner and timing apparatus, controlled conditioning facilities, and experience with the target end-product standards to ensure results are accepted during certification audits.