Vertical Burning Duration Determination: Test Principles, Methods, and Flammability Classification

Vertical burning duration determination is a core flammability test that measures how long a vertically oriented polymeric specimen continues to burn after a standardized ignition flame is removed, expressed as afterflame time. The method, governed by internationally recognized standards such as UL 94, IEC 60695-11-10, ASTM D3801, and GB/T 2408, assigns widely used material ratings including V-0, V-1, and V-2. This article explains the test principles, sample requirements, measured parameters, operating procedures, and practical applications of vertical burning duration testing.

Technical Overview

Vertical burning duration determination quantifies the persistence of combustion in a plastic specimen held in a vertical orientation after exposure to a small, controlled ignition flame. A laboratory burner produces a nominal 50 W blue flame approximately 20 mm in height, which is applied to the lower edge of the specimen for two successive 10-second intervals. Precision timers record the afterflame time following each application, and the resulting data are compared against classification criteria to establish a flammability rating for the material.

The principal test methods are the vertical burning test described in UL 94, the equivalent procedure in IEC 60695-11-10 (also published as an ISO/IEC dual-logo standard), ASTM D3801 for solid plastics in a vertical position, and the vertical method in GB/T 2408. Related procedures cover thin flexible materials in rolled form (VTM-0, VTM-1, VTM-2 per ISO 9773) and more severe 500 W exposures for 5VA and 5VB classifications per IEC 60695-11-20. Because vertical orientation represents the most demanding common posture for flame spread, short burning durations and the resulting V-0 ratings are frequently required by product safety specifications worldwide.

The significance of this measurement extends across material development, quality control, and regulatory compliance. Flame retardant formulations, polymer selection for enclosures, and end-product certifications all rely on reproducible afterflame time data generated under strictly controlled laboratory conditions by accredited third-party laboratories.

Testing Samples

Rigid Plastic Compounds

Standard bar specimens measure 125 mm in length and 13.0 mm in width, with the thickness supplied or specified by the client, since flammability performance is thickness dependent and certification typically demands evaluation at the minimum and maximum production wall thickness. Typical materials include engineering thermoplastics such as polycarbonate, ABS, polyamide, PBT, and polypropylene, as well as thermosetting molding compounds. A minimum set of five specimens is required for each thickness and color.

Thin and Flexible Polymeric Materials

Films, membranes, and thin sheets that distort, shrink, or collapse when clamped as flat bars are evaluated using the rolled-specimen vertical procedure. Specimens are wrapped around a mandrel and secured to achieve a defined geometry, and the same dual 10-second flame application and timing sequence is applied to assign VTM-0, VTM-1, or VTM-2 classifications.

Electrical and Electronic Component Materials

Materials used in equipment enclosures, relay and connector bodies, switch housings, and printed circuit board laminates such as FR-4 copper-clad sheets are frequently tested. Specimens may be cut directly from molded parts or from representative plaques of the same compound and thickness, ensuring that the data reflect real production material.

Testing Items

The vertical burning duration determination captures the following measured and observed parameters for each specimen:

• Individual afterflame time t1 recorded after removal of the first 10-second flame application
• Individual afterflame time t2 recorded after removal of the second 10-second flame application
• Individual afterglow time, meaning the duration of glow without flame after the second application
• The combined afterflame time (t1 + t2) for each specimen and the total afterflame time summed across the full set of five specimens
• Whether afterflame or afterglow propagates upward and reaches the specimen holding clamp
• Flaming or incandescent drips, and whether falling particles ignite the indicator cotton placed beneath the specimen
• Qualitative burning behavior including melting, charring, shrinkage, curling, and structural deformation

Testing Methods

Apparatus and Test Environment

Testing is performed in a draft-free enclosure or fume hood that permits safe venting of combustion products while preventing air movement from destabilizing the flame. A laboratory burner of approximately 9.5 mm barrel diameter is fueled with methane of at least 98 percent purity, adjusted to produce a 20 mm high blue flame as verified with a calibrated ruler or flame gauge. Specimens are clamped by their upper 6 mm with the major axis vertical, and a pad of dry absorbent cotton is positioned 300 mm below the lower edge of the specimen to catch and reveal flaming drips. Timing is performed with an instrument of 0.1 second resolution or better.

Specimen Conditioning

Before testing, specimens are conditioned at 23 ± 2 °C and 50 ± 10 percent relative humidity for a minimum of 48 hours to normalize moisture content and ensure reproducible ignition behavior. Additional preconditioning, such as elevated-temperature aging followed by cooling in a desiccator, may be specified for certain end-use programs.

Test Procedure

The burner is inclined at approximately 20 degrees from the vertical and the flame is applied to the midpoint of the specimen lower edge for 10 ± 0.5 seconds. The burner is then withdrawn without disturbing the flame, and the afterflame time t1 is recorded. Immediately after flaming from the first application ceases, the flame is reapplied for a second 10 ± 0.5 second interval, withdrawn, and the afterflame time t2 and subsequent afterglow time are recorded. Throughout both applications, the operator observes whether flaming particles fall onto the cotton and whether burning reaches the clamp. The sequence is repeated for all five specimens, and where results are borderline or inconsistent, a second set of five specimens is tested in accordance with the decision rules of the applicable standard.

Classification Criteria

The measured burning durations and drip behavior are compared with the following classification requirements:

• V-0: each individual afterflame time does not exceed 10 s, the total afterflame time for the set does not exceed 50 s, afterglow does not persist beyond 30 s after the second application, burning does not reach the clamp, and no flaming drips ignite the cotton
• V-1: each individual afterflame time does not exceed 30 s, the total afterflame time for the set does not exceed 250 s, afterglow does not persist beyond 60 s after the second application, burning does not reach the clamp, and no flaming drips ignite the cotton
• V-2: identical duration criteria to V-1, but flaming drips that ignite the cotton indicator are permitted

Where applications demand higher ignition severity, such as large equipment enclosures, the 500 W procedures of IEC 60695-11-20 are used to pursue 5VA or 5VB classifications with repeated longer flame applications.

Applications

Vertical burning duration data support safety engineering and market access across many industries:

• Consumer electronics and information technology equipment, where enclosure plastics must satisfy flammability requirements referenced by product safety standards such as IEC 62368-1
• Household appliances and white goods, covering housings, internal brackets, fan impellers, and terminal blocks
• Automotive interior components, including trim panels, connectors, and battery system enclosures governed by OEM material specifications
• Electrical components such as relay bodies, switch housings, coil bobbins, and printed circuit board laminates
• Flame retardant research and development, where afterflame time trends quantify the effectiveness of additive and reactive systems
• Incoming material quality control and supplier verification within manufacturing supply chains
• Certification support, with accredited third-party laboratories issuing test reports accepted for regulatory submissions and global market access

FAQ

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 the ignition source is removed. Afterglow time is the period during which combustion continues as glowing without flame after the second application. Both values are recorded because glowing particles can still ignite adjacent materials even when open flame has ceased.

What does a V-0 rating mean?

A V-0 rating indicates the highest standard performance level in the 50 W vertical test: the material self-extinguishes within 10 seconds per ignition, the cumulative afterflame time for five specimens remains at or below 50 seconds, and no flaming drips ignite the cotton indicator. It is the rating most commonly specified for engineering plastics in electrical and electronic applications.

How many specimens are required for the test?

A minimum of five bar specimens is tested for each thickness and color. If the results of a set are borderline or one specimen fails the criteria, the standard permits or requires an additional set of five specimens to confirm the classification.

Why is methane specified as the burner fuel?

Methane of at least 98 percent purity provides a stable, reproducible 20 mm blue flame with consistent heat output, which is essential for comparable results between laboratories. The flame geometry is verified before testing using calibrated gauges as required by the referenced method.

How does the vertical test differ from the horizontal burning test?

In the horizontal configuration, the flame is applied to the end of a horizontally mounted specimen and a linear burning rate is measured, making it suitable for less demanding applications. In the vertical configuration, heat and flame naturally rise along the specimen surface, producing more severe exposure, so the evaluation criterion becomes afterflame duration rather than burning rate. Vertical ratings such as V-0 therefore demonstrate substantially greater flame resistance than horizontal HB classifications.

Can results vary between different laboratories?

Minor variations can arise from flame calibration, ambient conditions, and specimen conditioning, which is why testing is performed under the tight tolerances defined in UL 94, IEC 60695-11-10, ASTM D3801, and GB/T 2408. Accredited third-party laboratories with calibrated apparatus and trained operators deliver reproducible results suitable for certification and regulatory purposes.