Vertical Burning Afterflame Time Testing: Standards, Procedures, and Classification Criteria

Vertical burning afterflame time testing is a critical flammability evaluation that measures how long a specimen continues to burn with flame after the ignition source is removed in a vertical orientation. This test is fundamental for classifying plastic materials and textiles according to internationally recognized standards such as UL 94, IEC 60695-11-10, ISO 1210, GB/T 2408, and ASTM D3801. Understanding the test methodology, specimen requirements, and acceptance criteria helps manufacturers ensure product safety and regulatory compliance across electronics, automotive, and textile industries.

Vertical burning afterflame time testing is a critical flammability evaluation that measures how long a specimen continues to burn with flame after the ignition source is removed in a vertical orientation. This test is fundamental for classifying plastic materials and textiles according to internationally recognized standards such as UL 94, IEC 60695-11-10, ISO 1210, GB/T 2408, and ASTM D3801. Understanding the test methodology, specimen requirements, and acceptance criteria helps manufacturers ensure product safety and regulatory compliance across electronics, automotive, and textile industries.

Technical Overview

Afterflame time, also known as flame continuation time, is defined as the duration for which a specimen continues to burn with a visible flame after the ignition source has been removed during a vertical burning test. It is one of the most important quantitative parameters in flammability assessment, directly reflecting the self-extinguishing capability of a material when exposed to a small open flame.

In the vertical burning configuration, the specimen is clamped with its longitudinal axis oriented vertically, and a standardized burner flame is applied to the lower edge or surface of the specimen for a specified duration. Once the flame is withdrawn, the technician records the afterflame time, the afterglow time (continued glowing combustion without flame), whether flaming drips ignite a cotton indicator placed beneath the specimen, and whether the specimen burns completely to the holding clamp.

The vertical orientation is deliberately more severe than the horizontal configuration because the upward heat and flame propagation naturally assists combustion along the specimen. Consequently, materials that pass vertical burning tests—such as those classified as V-0 under UL 94—demonstrate superior flame-retardant performance and are widely specified for safety-critical applications.

• Primary standards: UL 94, IEC 60695-11-10, ISO 1210, GB/T 2408, GB/T 5169.16, ASTM D3801
• Textile-related standards: GB/T 5455, ASTM D6413, ISO 15025
• Key measured parameters: single afterflame time, total afterflame time, afterglow time, flaming drips, cotton ignition
• Typical classification grades: V-0, V-1, V-2 (plastics); flame spread index and damage length (textiles)

Testing Samples

Rigid and Flexible Plastic Materials

For tests conducted per UL 94, IEC 60695-11-10, ISO 1210, and GB/T 2408, standard bar specimens measure 125 mm in length by 13 mm in width, with thicknesses typically of 3.0 mm, 1.6 mm, or the minimum thickness at which compliance is claimed. A set of five specimens is required for the basic procedure, and an additional set may be needed when retesting or when evaluating multiple thicknesses. Specimens must be conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 48 hours prior to testing.

Textile Fabrics

For vertical flame tests on textiles per GB/T 5455 and ISO 15025, rectangular strips are cut with dimensions commonly of 300 mm × 80 mm. Fabrics are conditioned to equilibrium in a standard atmosphere before testing, and testing is usually performed both in the original state and after a defined number of laundering cycles when required by product specifications.

Foam Materials and Components

Flexible polyurethane foams, acoustic insulation, and finished components such as connector housings, cable insulation samples, and enclosures may also be evaluated. Component testing often follows the relevant end-product standard, with specimens cut or mounted to represent the actual installation condition as closely as possible.

Testing Items

The vertical burning test generates several measurable outcomes that together determine the flammability classification of the material:

• Individual afterflame time (t1 and t2): the duration of flaming combustion after the first and second flame applications, recorded separately for each specimen
• Total afterflame time: the sum of all afterflame times for the complete set of five specimens across both ignition applications
• Afterglow time: the time glowing combustion persists after flaming ceases
• Flaming drips: whether burning particles fall from the specimen and whether they ignite surgical cotton placed 300 mm below the specimen
• Afterflame plus afterglow time: a combined metric used in certain classification schemes
• Damage length / burn distance: the extent of specimen consumed or damaged, particularly relevant for textile evaluations
• Afterflame of cotton indicator: a pass/fail observation critical to distinguishing V-2 from V-1 ratings

Testing Methods

Apparatus and Test Conditions

Testing is performed inside a ventilated fume chamber or enclosure of at least 0.5 m³ internal volume, large enough to permit natural air circulation while shielding the specimen from drafts. A laboratory burner—typically a Bunsen or Tirrill type with a nominal 9.5 mm barrel diameter—is fueled by methane or natural gas with controlled calorific value. The burner is adjusted to produce a blue flame of 20 mm height, verified with an appropriate ruler or gauge.

Specimen Mounting and Flame Application

The plastic bar specimen is clamped at its upper end with the longitudinal axis vertical, so that the lower end is positioned 10 mm above the top of the burner tube. A layer of dry, absorbent surgical cotton is placed 300 mm below the point where the specimen's lower edge is clamped. The flame is applied to the center of the lower edge of the specimen for 10 seconds, then withdrawn while the timer is started. After flaming stops, the flame is applied a second time for another 10 seconds, and timing measurements are repeated.

Measurement and Observation

For each application, the laboratory records: the afterflame time t1 after the first application and t2 after the second application, the afterglow time, whether flaming drips ignite the cotton, and whether the specimen burns up to the clamp. Observations must be made without excessive ambient light interference, and the chamber is purged between tests to remove combustion products.

Classification Criteria per UL 94 / IEC 60695-11-10

• V-0: afterflame time for each specimen ≤ 10 s after each application; total afterflame time for the set ≤ 50 s; no flaming drips igniting cotton; specimen does not burn to the clamp; afterflame or afterglow does not persist beyond 30 s after the second application
• V-1: afterflame time for each specimen ≤ 30 s; total afterflame time ≤ 250 s; no flaming drips igniting cotton; specimen does not burn to the clamp; afterflame or afterglow does not persist beyond 60 s after the second application
• V-2: same timing limits as V-1, except flaming drips that ignite the cotton are permitted

Textile Vertical Burning Procedure

For GB/T 5455 and similar textile methods, the fabric strip is suspended vertically in the cabinet, a standardized flame (commonly 40 mm high from a specified burner) is applied to the bottom edge for 12 seconds, and after flame application ceases, the technician records the afterflame time, afterglow time, and damaged length. Results are typically reported as the maximum or average values across the required number of replicate specimens, according to the governing product specification.

Applications

Vertical burning afterflame time testing serves a broad range of industries and regulatory contexts:

• Electrical and electronic products: verifying V-0 ratings for enclosures, PCB laminates, connectors, switches, and insulating parts required by end-product safety standards
• Wire and cable: evaluating insulation and sheathing compounds for flame propagation resistance
• Automotive interiors: qualification of plastic trim, headliners, and seat components against burn-rate requirements
• Protective textiles and workwear: confirming flame-retardant performance of flame-resistant fabrics used in firefighting garments, industrial uniforms, and welding apparel
• Building and construction materials: supporting reaction-to-fire evaluations for decorative and insulating products
• Research and development: comparing flame-retardant formulations, optimizing additive loading, and validating material substitution decisions
• Quality control: routine incoming inspection and batch conformity verification of flame-retardant compounds

Manufacturers frequently engage accredited third-party laboratories to perform these tests for certification purposes, while in-house laboratories conduct the same procedures during development and production monitoring.

FAQ

Q: What is the difference between afterflame time and afterglow time?

Afterflame time is the period during which the specimen continues to burn with a visible flame after removal of the ignition source. Afterglow time is the period during which glowing combustion without flame persists after flaming stops. Both parameters are recorded because afterglow can present an ongoing ignition hazard even after visible flaming has ceased.

Q: Why is the vertical test considered more severe than the horizontal test?

In the vertical orientation, heated gases and the flame front travel upward along the specimen, preheating the material ahead of the flame and promoting sustained combustion. This makes the vertical method a more discriminating test of a material's self-extinguishing properties, which is why the highest common ratings, such as V-0, are based on vertical performance.

Q: How many specimens are required?

The standard UL 94 vertical procedure requires a set of five bar specimens, each subjected to two 10-second flame applications. If any single specimen's results fall outside the classification limits, a second set of five specimens from a freshly prepared sample may be tested to confirm the classification.

Q: What gas and flame height are used?

The burner is fueled with methane or natural gas of specified calorific content, adjusted to produce a 20 mm blue flame for plastic specimen testing. Textile methods may specify different burner geometry and flame heights, so the applicable standard must always be followed precisely.

Q: What does the cotton indicator test reveal?

Dry surgical cotton placed 300 mm beneath the specimen detects flaming droplets or particles. If these falling particles ignite the cotton, the material is assigned V-2 rather than V-1, because flaming drips can propagate fire to adjacent materials in real-world installations.

Q: Can a V-0 rating obtained at one thickness apply to thinner parts?

No. Flame retardancy is thickness-dependent; thinner sections generally burn more readily. Ratings are therefore established at discrete thicknesses, and compliance for a given part requires testing at or below the part's minimum wall thickness according to the relevant standard's rules.