The vertical burning test is a widely recognized flammability evaluation method used to assess how plastic materials react when exposed to a small open flame under controlled laboratory conditions. Governed by standards such as UL 94, IEC 60695-11-10, ASTM D3801, and GB/T 5169.16, this test classifies materials into flammability ratings such as V-0, V-1, and V-2, which are essential for product safety compliance in electronics, electrical equipment, and numerous other industries. This article provides a comprehensive overview of the test principle, sample requirements, test items, methodology, and practical applications of the vertical burning test.
Technical Overview
The vertical burning test is a small-scale laboratory flame test designed to measure the flammability characteristics of polymeric materials, particularly rigid plastics used in electrical and electronic products. In this test, a bar-shaped specimen is clamped in a vertical orientation, and a standardized gas flame is applied to its lower edge for a specified duration. The test evaluates how quickly the material self-extinguishes after the ignition source is removed, whether it produces flaming drips, and whether it continues to glow after flaming ceases.
The most commonly referenced standard is UL 94, developed by UL (Underwriters Laboratories), which defines the vertical burning classifications V-0, V-1, V-2, 5VA, and 5VB. Equivalent or harmonized methods are published by other standards organizations, including IEC 60695-11-10, ASTM D3801, GB/T 5169.16 (China), and ISO 10351. These standards are technically aligned in most respects, allowing manufacturers to demonstrate compliance across multiple markets with a single test program conducted at accredited third-party laboratories.
The vertical burning test is more demanding than the horizontal burning test because the specimen orientation promotes upward flame propagation through preheating of the material above the flame front. As a result, a V-0 rating is generally considered the benchmark requirement for critical components such as enclosures, circuit boards, and connectors in safety-critical applications.
Testing Samples
Rigid Plastic Specimens
Standard specimens for the UL 94 V-0/V-1/V-2 test are rectangular bars measuring 125 mm in length by 13 mm wide, cut from sheets or molded plaques of the material under evaluation. The thickness most frequently tested is 3.0 mm, although testing at the minimum and maximum thicknesses used in the end product is common practice, since flammability performance is strongly thickness-dependent. A minimum of five specimens per set is required, and two sets are recommended when results are near the classification boundary.
Thin and Flexible Materials
Films, flexible sheets, and other materials that distort or shrink away from the flame in the standard vertical configuration cannot be evaluated with the rigid-bar method. For these products, the vertical thin-material (VTM) variants described in UL 94 and related specifications are applied. Specimens are wrapped around a mandrel to maintain the required geometry, and classification criteria analogous to VTM-0, VTM-1, and VTM-2 are assigned.
Cellular and Foam Materials
Rigid and flexible foams are assessed using dedicated procedures such as ISO 9772 or the HF classification methods in UL 94, because their low density and cellular structure produce burning behavior that differs significantly from solid plastics. Where vertical orientation testing of foam is required, specimen geometry and flame application parameters are adapted according to the relevant product standard.
Specimen Conditioning
Before testing, specimens must be conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 48 hours, unless the governing specification requires alternative pre-treatment. Proper conditioning ensures reproducible ignition behavior and consistent afterflame measurements across test laboratories.
Testing Items
The vertical burning test measures a defined set of quantitative and observational parameters, each of which contributes to the final flammability classification:
- Afterflame time after the first flame application (t1): the duration in seconds that the specimen continues to flame after the burner flame is removed.
- Afterflame time after the second flame application (t2): the flaming duration following re-ignition of the specimen.
- Afterglow time (t3): the time the specimen continues to glow without flaming after the second application, measured after flaming ceases.
- Total afterflame time: the sum of afterflame times for all five specimens, compared against the maximum allowable total for each classification.
- Flaming drips: whether burning particles fall from the specimen and whether they ignite a layer of surgical cotton placed 300 mm below the specimen.
- Flame spread to the clamp: whether flaming or glowing reaches the holding fixture at the top of the specimen, indicating uncontrolled propagation.
- Specimen consumption: visual evidence of complete burn-up, which disqualifies the material from classification.
Testing Methods
Apparatus and Test Environment
Testing is performed inside a closed chamber or fume hood of at least 0.5 cubic meters to eliminate drafts while allowing natural exhaust of combustion products. A laboratory burner, typically a Bunsen or Terry-type burner with a 10 mm nominal barrel diameter, is supplied with methane gas of specified purity to produce a blue flame 20 mm in height. Each specimen is secured at its upper end by a clamp so that its long axis is vertical, with 10 mm of the lower end exposed above the burner tube, and dry absorbent cotton is positioned 300 mm beneath the specimen bottom edge.
Test Procedure
The burner is positioned so that the flame impinges on the bottom edge of the specimen for 10 seconds, then withdrawn, and the first afterflame time t1 is recorded along with any dripping that ignites the cotton. Once flaming stops, the burner is re-applied for a second 10-second interval, after which the second afterflame time t2 and the afterglow time t3 are recorded. This sequence is repeated for all five specimens in the set, and any flaming particles, glow persistence, or burn-up to the clamp is documented.
Classification Criteria
Based on the recorded data, the material is assigned one of the following classifications under UL 94 and its harmonized equivalents:
- V-0: each afterflame time no greater than 10 seconds; total afterflame time for the set no greater than 50 seconds; no flaming drips igniting the cotton; no afterglow exceeding 30 seconds; and no burning to the clamp.
- V-1: each afterflame time no greater than 30 seconds; total afterflame time no greater than 250 seconds; no flaming drips igniting the cotton; afterglow no greater than 60 seconds.
- V-2: identical timing limits to V-1, except flaming drips that ignite the cotton are permitted.
- 5VA and 5VB: more severe classifications using a higher-energy 500 W flame applied five times, with 5VA additionally requiring that the specimen not exhibit a burn-through hole.
Materials failing to meet the V-2 criteria are designated as non-classified for the vertical burning test and may need to be evaluated using the horizontal burning (HB) method or flame-retardant formulation improvements.
Applications
Vertical burning test results are embedded in product safety requirements across many industries, making the test a gateway to market access for polymeric components:
- Consumer and IT electronics: enclosures for laptops, monitors, chargers, power adapters, and battery packs commonly require V-0 materials.
- Electrical installation equipment: breakers, distribution boards, terminal blocks, sockets, and switches rely on V-0 rated housings to limit fire propagation.
- Printed circuit boards: FR-4 and other laminate substrates are certified at defined thicknesses to satisfy V-0 requirements in end-product standards.
- Wire and cable: insulation and sheathing compounds for internal wiring are validated through vertical or VTM configurations.
- Household appliances and lighting: components near live parts or heat sources must meet flammability ratings referenced in IEC 60335 and IEC 60598 series standards.
- Automotive electronics: interior electronic control units, connectors, and charging components reference UL 94 or equivalent ISO and GB requirements.
- Medical devices: equipment housings and power components are specified with high flame ratings to satisfy IEC 60601-based safety assessments.
Manufacturers typically commission vertical burning tests at accredited third-party laboratories to support product certification submissions, supplier material approvals, and internal quality control programs.
FAQ
What is the difference between V-0, V-1, and V-2 ratings? The three ratings differ mainly in allowable afterflame duration and dripping behavior. V-0 is the most stringent, permitting individual afterflame times up to 10 seconds and no flaming drips; V-1 allows up to 30 seconds per specimen with no flaming drips; and V-2 matches the V-1 timing limits but tolerates flaming drips that may ignite the indicator cotton.
How many specimens are required for the test? A standard set consists of five conditioned specimens. If any single specimen fails a criterion, a second set of five specimens is tested, and the second set must fully satisfy the classification requirements for the material to pass.
Why is the vertical test stricter than the horizontal test? In the vertical orientation, hot combustion gases rise along the specimen and preheat material above the flame zone, accelerating flame spread. This makes vertical burning a conservative and safety-relevant measure of self-extinguishing performance.
Are UL 94 and IEC 60695-11-10 results interchangeable? The two standards are technically harmonized for the V-0/V-1/V-2 method, and results are generally accepted as equivalent. However, certification bodies may require testing to the specific standard cited in the applicable end-product specification.
Does specimen thickness affect the result? Yes. Thicker sections generally burn more slowly and self-extinguish more readily, so flammability ratings are always reported with the tested thickness. End products must not use the material below the minimum certified thickness.
What do flaming drips indicate? Flaming droplets signal that burning material can fall away and ignite secondary fuel, such as the cotton indicator, which represents nearby combustible material in real installations. This behavior is prohibited for V-0 and V-1 and only accepted under V-2.
