Vertical Burning Characteristics Evaluation: Principles, Methods, and Classification of Flammability

Vertical burning characteristics evaluation is a core flammability test used to determine how polymeric materials resist ignition, flame propagation, and flaming drips when exposed to a small flame in a vertical orientation. Governed by internationally recognized standards such as UL 94, IEC 60695-11-10, and GB/T 2408, this evaluation assigns V-0, V-1, or V-2 ratings that are essential for material selection in electrical, electronic, and consumer product applications. This article provides a comprehensive technical overview of the testing principles, specimen requirements, classification criteria, and practical applications of vertical burning tests.

Technical Overview

Vertical burning characteristics evaluation is a small-scale laboratory flammability test in which a test specimen is mounted in a vertical orientation and exposed twice to a standardized low-energy flame source, typically a 50 W premixed methane-air burner flame. The test quantifies the afterflame time, afterglow time, and flaming drip behavior of the material, and assigns a classification grade of V-0, V-1, or V-2 based on defined performance criteria. Because vertical mounting represents the most severe orientation for flame propagation, upward flame spread along the specimen surface, this method is widely regarded as one of the most demanding ignition-resistance tests for polymeric materials.

The evaluation is primarily associated with the UL 94 vertical burning test, which has been harmonized at the international level. Accredited third-party laboratories routinely perform this evaluation for material qualification, product safety certification, and regulatory compliance purposes. The principal standards organizations and corresponding standards include:

• UL 94 (ANSI/UL 94): Tests for Flammability of Plastic Materials for Parts in Devices and Appliances, vertical burning tests V-0, V-1, and V-2
• IEC 60695-11-10: Fire hazard testing, 50 W horizontal and vertical flame test methods
• GB/T 2408: Chinese national standard for the determination of burning characteristics of plastics, horizontal and vertical methods
• ISO 9773: Burning behavior of plastics, vertical test for thin specimens in contact with a small-flame ignition source
• ISO 9772: Cellular plastics, small specimen horizontal burning test (foam classification reference)
• ISO 12992: Vertical burning behavior of films and sheets in a vertical configuration
• IEC 60695-11-4 and IEC 60695-11-3: Specifications for the 50 W reference flame sources used in the test

Testing Samples

Rigid Plastic Specimens

Standard bar specimens for the vertical burning test measure 125 mm in length by 13 mm in width, with the thickness corresponding to the intended end-use application. Common thicknesses include 1.5 mm, 3.0 mm, and other values up to approximately 13 mm. Because flammability performance is strongly thickness-dependent, each material is typically evaluated and classified at multiple nominal thicknesses, and the reported rating is only valid at or above the tested thickness.

Thin-Film and Flexible Materials

Flexible films and thin sheet materials that distort, shrink, or retract away from the flame in the standard V test are evaluated using alternative vertical configurations, such as the rolled or mandrel-supported specimen method described in ISO 9773 and the thin-film VTM procedures of UL 94. These methods allow materials thinner than the standard minimum thickness to be classified with equivalent V-0, V-1, or V-2 grades.

Specimen Conditioning

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. An alternative preconditioning sequence of 168 hours at 70 °C in an air-circulating oven, followed by cooling in a desiccator for at least 4 hours, may be used to represent the effects of long-term thermal aging. Ten specimens in total, tested in two sets of five, are generally required for each thickness to confirm a classification.

Testing Items

The evaluation measures and records the following parameters for each specimen and flame application:

• Afterflame time t1: the duration of sustained flaming after the first 10-second flame application
• Afterflame time t2: the duration of sustained flaming after the second 10-second flame application
• Afterglow time t3: the duration of glowing combustion after the cessation of flaming following the second application
• Total afterflame time for the set of specimens: the arithmetic sum of t1 and t2 values
• Occurrence of flaming drips and whether they ignite surgical cotton placed 300 mm below the specimen
• Whether afterflame or afterglow extends up to the specimen holding clamp
• Complete consumption or charring of the specimen beyond the clamped zone

Classification Criteria

Based on the recorded data, specimens are classified according to the following criteria:

• V-0: Individual afterflame time ≤ 10 s; total afterflame time for the set ≤ 50 s; no ignition of the cotton indicator by flaming drips; no afterflame or afterglow reaching the clamp; afterglow time after second application ≤ 30 s
• V-1: Individual afterflame time ≤ 30 s; total afterflame time for the set ≤ 250 s; no ignition of the cotton indicator by flaming drips; no afterflame or afterglow reaching the clamp; afterglow time after second application ≤ 60 s
• V-2: Individual afterflame time ≤ 30 s; total afterflame time for the set ≤ 250 s; ignition of the cotton indicator by brief flaming drips is permitted; no afterflame or afterglow reaching the clamp; afterglow time after second application ≤ 60 s

Testing Methods

Apparatus and Flame Source

The test employs a laboratory burner with a tube approximately 100 mm long and an internal diameter of 9.5 mm, fed with technically pure methane to produce a calibrated 20 mm high blue flame corresponding to a nominal 50 W heat output. Flame calibration in accordance with IEC 60695-11-4, including the confirmation of gas flow rates and a copper-block heat input verification, ensures reproducibility of the ignition source between laboratories. The test is conducted inside a fume hood or enclosure with controlled ventilation to maintain a stable flame while extracting combustion products.

Test Procedure

The specimen is secured vertically by a clamp gripping its upper 10 mm, so that the lower edge hangs freely above the burner. A layer of dry, absorbent surgical cotton is positioned horizontally 300 mm below the bottom edge of the specimen. The burner flame is applied to the center of the lower edge for 10 seconds and then withdrawn, while the afterflame time t1 is recorded. After flaming ceases, the flame is applied a second time for 10 seconds; for thin specimens that deflect away from the flame, the burner may be tilted approximately 20 degrees from vertical to maintain flame contact. The second afterflame time t2 and subsequent afterglow time t3 are recorded, along with any dripping and cotton ignition events.

Evaluation and Reporting

All ten specimens must satisfy the criteria for a given class for the material to receive that classification. If any specimen fails, the material is assigned the next lower class or retested with an additional set of specimens under the conditions specified by the standard. The test report documents the material identification, nominal thickness, conditioning method, all individual and total afterflame and afterglow times, dripping behavior, cotton ignition results, and the final V-0, V-1, or V-2 classification. It is important to note that the rating applies only to the specific thickness and color tested, as pigments and additives can significantly alter flammability behavior.

Applications

Vertical burning characteristics evaluation is a critical compliance and material qualification step across numerous industries:

• Electrical and electronic equipment: enclosures, connectors, switches, terminal blocks, and internal structural parts evaluated under product safety standards such as IEC 60950-1, IEC 62368-1, and IEC 60335 series requirements
• Printed circuit board substrates: rigid and flexible laminates, prepregs, and solder mask materials requiring minimum V-0 classification
• Household appliances: plastic housings and insulating components subjected to end-product flammability requirements
• Wire and cable: insulation, sheathing, and jacketing compounds for internal wiring and cord sets
• Automotive components: interior plastic parts, battery enclosures, and electronic module housings supporting OEM material specifications
• Consumer products: chargers, power adapters, luminaires, battery packs, and toys where flammability compliance is mandated
• Material research and development: screening flame-retardant formulations and comparing halogenated, halogen-free, and intumescent systems

FAQ

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

The primary differences lie in the permitted afterflame times and drip behavior. V-0 is the strictest grade, allowing a maximum individual afterflame time of 10 seconds and a total of 50 seconds with no flaming drips. V-1 allows up to 30 seconds per specimen and 250 seconds total, also without flaming drips. V-2 has the same timing limits as V-1 but permits brief flaming drips that may ignite the cotton indicator.

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

In the horizontal configuration (HB class per UL 94 and IEC 60695-11-10), flame propagation occurs along the specimen against gravity-driven buoyancy, making it a less severe test. The vertical configuration promotes upward flame spread and exposes the material surface directly to heated combustion gases, providing a far more rigorous assessment of self-extinguishing behavior.

Are UL 94 and IEC 60695-11-10 test results interchangeable?

The test principles, specimen dimensions, flame source, and V-class criteria are technically aligned, and results are generally accepted as equivalent in most certification schemes. However, subtle differences in flame calibration procedures and documentation requirements exist, so manufacturers should confirm acceptance with the relevant certification body or market regulator before relying on equivalence.

How many specimens and thicknesses are required for a full evaluation?

A complete classification normally requires ten bar specimens (two sets of five) for each nominal thickness. Materials are commonly tested at both the minimum intended end-use thickness and one or more thicker sections, since the reported rating is only valid for the thickness tested and above.

Does a V-0 rating mean the material is non-flammable?

No. A V-0 rating indicates that the material self-extinguishes rapidly after removal of a small ignition flame and does not produce flaming drips under the specified laboratory conditions. It does not characterize performance in large-scale fires or against higher-energy ignition sources such as those used in the 5VA and 5VB (500 W) tests.

How should a manufacturer prepare for vertical burning evaluation?

Manufacturers should submit molded or cut specimens at the exact end-use thickness, provide complete material and color identification, specify the target classification, and confirm whether standard or oven-aged preconditioning is required. Engaging accredited third-party laboratories with calibrated flame sources and ISO/IEC 17025-compliant quality systems ensures reliable, mutually recognized test results.