Vertical burning specimen testing is a laboratory flammability test used to evaluate how plastic materials react when exposed to a small open flame in a vertical orientation, classifying them as V-0, V-1, or V-2 according to UL 94, IEC 60695-11-10, GB/T 2408, and ASTM D3801. This test is essential for product safety compliance in electronics, appliances, and automotive applications, measuring afterflame time, afterglow, and flaming drip behavior. This article explains the sample requirements, test methods, classification criteria, and practical applications of vertical burning specimen testing.
Technical Overview
Vertical burning specimen testing is one of the most widely used small-scale flammability tests for polymeric materials. In this method, a test specimen is mounted vertically and exposed twice to a standardized 20 mm (50 W) gas flame, after which the burning behavior of the material is evaluated. Because the vertical orientation allows the flame to remain in contact with the specimen surface and promotes upward flame propagation, the vertical test is significantly more severe than its horizontal counterpart and is reserved for materials used in higher-risk applications.
The test determines whether a material self-extinguishes after removal of the ignition source, how long it continues to burn (afterflame time), whether it glows after flaming ceases (afterglow time), and whether burning droplets ignite a cotton indicator placed below the specimen. Based on the results, materials are classified as V-0, V-1, or V-2, with V-0 representing the highest level of flame retardancy among the 50 W vertical classifications.
The primary governing standards include UL 94 (Tests for Flammability of Plastic Materials for Parts in Devices and Appliances), IEC 60695-11-10 and its European adoption EN 60695-11-10, GB/T 2408 and GB/T 5169.16 in the Chinese national standard system, ASTM D3801 in the American standard system, and ISO 1210 for the international community. These standards are technically harmonized, meaning results obtained under one scheme are generally accepted under the equivalent schemes.
Testing Samples
Rigid Plastic Specimens
Standard bar specimens for the 50 W vertical test measure 125 mm in length and 13 mm wide, cut from sheets or molded directly at the intended end-use thickness. Because flammability classification is thickness-dependent, materials are typically tested at both the minimum and maximum thickness used in the actual product, with 3.0 mm being a common reference thickness. Two sets of five specimens each are generally required so that both the as-received condition and any additional conditioning state can be evaluated.
Thin Film and Flexible Materials
Very thin films that would distort, shrink, or collapse during a standard vertical test are evaluated using the variant described in ISO 9773 and the corresponding VTM (vertical thin material) procedures. These specimens, typically 200 mm × 50 mm, are rolled onto a mandrel and secured to create a self-supporting cylindrical form before testing, producing classifications of VTM-0, VTM-1, or VTM-2 analogous to the rigid-material ratings.
Specimen Conditioning
Prior to testing, specimens must be conditioned in an atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 48 hours. For full material qualification programs, an additional set of specimens is aged in a circulating air oven at 70 °C for 168 hours and then cooled at room temperature in a desiccator for at least 4 hours before testing, ensuring the classification remains valid after long-term thermal exposure. The cotton indicator pads are dried in an oven at 100 °C for a minimum of 30 minutes and cooled in a desiccator before each test.
Testing Items
The vertical burning test evaluates the following measurable parameters for each specimen:
- Afterflame time t1: the duration the specimen continues to flame after the first 10-second flame application is removed.
- Afterflame time t2: the duration of flaming after the second 10-second flame application is removed.
- Afterglow time t3: the duration the specimen continues to glow without flaming after the second application.
- Total afterflame time: the sum of t1 and t2 values across all five specimens.
- Flaming drips: whether incandescent or flaming particles fall from the specimen and ignite the dry cotton indicator placed 300 mm below the specimen.
- Burn-through behavior: whether afterflame or afterglow propagates up to the holding clamp.
These observations are combined to assign the final classification of V-0, V-1, or V-2 for the tested thickness.
Testing Methods
Apparatus and Setup
Testing is performed in a fume chamber or draft-free enclosure. A laboratory burner, typically a Tirril or Bunsen type with a 10 mm nominal bore, is fueled by methane gas of specified purity to produce a blue flame with a height of 20 ± 1 mm. The specimen is clamped at its upper 6 mm so that its longitudinal axis is vertical, with the lower edge positioned 10 mm above the top of the burner tube. A layer of dry surgical absorbent cotton, approximately 50 mm × 50 mm with a maximum uncompressed thickness of 6 mm, is placed 300 mm below the point where the specimen's lower edge is located.
Test Procedure
- Apply the burner flame to the center of the specimen's lower edge for 10 ± 0.5 seconds, ensuring the burner is tilted at approximately 45 degrees or withdrawn without disturbing the flame.
- Withdraw the flame and immediately record t1, the afterflame time, using a stopwatch accurate to 0.1 seconds.
- Once flaming ceases (or after a maximum wait of 30 seconds), apply the flame a second time for 10 ± 0.5 seconds and record t2 and any afterglow time.
- Observe whether flaming drips ignite the cotton indicator and whether burning reaches the clamp.
- Repeat the sequence for all five specimens in the set, then calculate the total afterflame time.
Classification Criteria
A material is assigned a classification only if every specimen in the set meets all criteria for that class:
- V-0: each afterflame time ≤ 10 s; total afterflame time for the set ≤ 50 s; no ignition of cotton by flaming drips; no afterflame or afterglow reaching the clamp; afterglow persists no longer than 30 s after the second ignition.
- V-1: each afterflame time ≤ 30 s; total afterflame time ≤ 250 s; no ignition of cotton by flaming drips; no afterflame or afterglow reaching the clamp; afterglow persists no longer than 60 s.
- V-2: each afterflame time ≤ 30 s; total afterflame time ≤ 250 s; flaming drips that ignite the cotton are permitted; no afterflame or afterglow reaching the clamp; afterglow persists no longer than 60 s.
For more demanding applications, the 500 W vertical test (5VA and 5VB classifications) applies a 125 mm flame to the specimen five times for 5 seconds each, using larger plaque specimens in addition to bars, and 5VA additionally prohibits the formation of a burn-through hole.
Applications
Vertical burning test results are a foundational safety requirement across many industries:
- Consumer electronics and IT equipment: enclosures, internal structural parts, and PCB support materials must meet minimum V-class requirements under IEC 62368-1 audio/video and ICT safety requirements.
- Household appliances: insulating parts, switch housings, and decorative components are specified by flammability class in IEC 60335 series requirements.
- Electrical components: connectors, terminal blocks, coil bobbins, and relay bodies commonly require V-0 material at the relevant minimum wall thickness.
- Automotive interior components: materials adjacent to electrical sources are often qualified using vertical burning data alongside FMVSS 302 horizontal testing.
- Lighting equipment, batteries, cables, and medical device housings: flammability classification supports risk-based material selection and regulatory documentation.
Designers should note that classification is thickness-specific: a material rated V-0 at 3.0 mm may only achieve V-1 or fail entirely at 1.5 mm, so the certified rating must always be matched to the minimum wall thickness of the actual part.
FAQ
What is the difference between V-0, V-1, and V-2?
The three classes differ mainly in permitted burning time and dripping behavior. V-0 requires the fastest self-extinguishing (individual afterflame ≤ 10 s, total ≤ 50 s) with no flaming drips. V-1 allows longer burning (≤ 30 s individual, ≤ 250 s total) but still forbids drips that ignite cotton. V-2 matches the V-1 timing limits but tolerates flaming drips that ignite the cotton indicator.
How many specimens are required?
A standard determination uses five specimens per set. For material qualification, two sets of five specimens are tested to cover different thicknesses or orientations, and additional aged sets may be required depending on the certification program.
Are UL 94, IEC 60695-11-10, and GB/T 2408 results interchangeable?
The 50 W vertical test methods in UL 94, IEC 60695-11-10, EN 60695-11-10, ASTM D3801, ISO 1210, and GB/T standards are technically aligned, and accredited third-party laboratories typically report results against multiple equivalents. Minor procedural details and certification scope can differ, so the specific end-product standard should always be consulted.
What does a flaming drip indicate?
A flaming drip occurs when molten or burning material falls from the specimen and ignites the dry cotton below it. This behavior is penalized in V-0 and V-1 because falling flaming particles can propagate fire to adjacent materials or furnishings beneath the product.
What is typically included in a test report?
A compliant report identifies the test standard, material designation and color, specimen thickness, conditioning history, individual t1, t2, and afterglow values, cotton ignition observations, total afterflame time, and the resulting classification, together with laboratory accreditation information and specimen photographs where required.