A vertical burning comparison test is a standardized flammability evaluation in which plastic and polymeric materials are exposed to a 50 W premixed flame in a vertical orientation, allowing their ignition, afterflame, dripping, and self-extinguishing behavior to be compared against established ratings such as UL 94 V-0, V-1, and V-2. Performed in accordance with standards including UL 94, IEC 60695-11-10, and GB/T 5169.16, these comparative experiments support flame-retardant material development, quality control, and interlaboratory validation in accredited third-party laboratories. This article explains the technical principles, sample requirements, test items, methodology, and typical applications of vertical burning comparison experiments.
Technical Overview
A vertical burning comparison experiment is a controlled laboratory procedure in which the flammability characteristics of polymeric materials are evaluated and compared by applying a small premixed gas flame to the lower edge of vertically mounted specimens. Because the flame propagates upward through the specimen in the same direction as the hot combustion gases, the vertical orientation represents the most severe of the common small-scale burning configurations. This makes it a key screening and classification tool for flame-retardant plastics used in electrical and electronic products.
The term "comparison experiment" refers to two related practices. The first is the side-by-side comparison of different materials, formulations, or thicknesses under identical test conditions to rank their burning behavior. The second is the interlaboratory comparison, in which a homogeneous sample batch is distributed among several accredited third-party laboratories and tested under the same protocol to verify the consistency, repeatability, and reproducibility of the vertical burning method itself.
- UL 94 — the primary standard for vertical burning classifications V-0, V-1, and V-2
- IEC 60695-11-10 — the internationally harmonized 50 W horizontal and vertical flame test method for plastic materials
- GB/T 5169.16 — the Chinese national adoption of IEC 60695-11-10
- ASTM D3801 — comparative burning characteristics of solid plastics in a vertical position
- ISO 1210 — comparative determination of the flammability of plastics using a small flame source
- ISO 9773 — vertical burning test for thin, flexible specimens that distort when mounted flat
Vertical burning comparison experiments are frequently complemented by other fire-hazard small-scale tests, such as the needle-flame test (IEC 60695-11-5), the glow-wire ignition series (IEC 60695-2-10 to IEC 60695-2-13), and the limiting oxygen index method (ISO 4589-2), to build a complete picture of a material's fire performance.
Testing Samples
Standard Bar Specimens
The standard specimen for the 50 W vertical burning test is a rectangular bar measuring 125 ± 5 mm in length and 13.0 ± 0.5 mm in width, with the thickness supplied or agreed upon by the parties, typically in the range of 0.4 mm to 3.0 mm and generally not exceeding 13 mm. The flammability rating is thickness-specific, meaning that a V-0 result obtained at 1.6 mm cannot automatically be claimed at 0.8 mm. For each condition, two sets of five bar specimens are prepared so that a second set can be tested when the first set yields marginal or non-conforming results.
Thin and Flexible Materials
Films, tapes, and thin non-rigid sheets that twist, curl, or contract when exposed to flame cannot be evaluated reliably as flat bars. For these products, the rolled-specimen method of ISO 9773 is applied, in which a 200 mm × 50 mm strip is wound into a conical form on a mandrel and ignited at the lower edge in a vertical orientation, producing the corresponding thin-material classifications.
Specimen Conditioning and Preparation
Before testing, specimens are conditioned in one of the standard atmospheres specified by the test method, most commonly 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 48 hours. An alternative preconditioning of 70 °C for 168 hours followed by cooling in a desiccator for at least 4 hours may be used, for example when hygroscopic materials are involved. Specimens must be free of burrs, flash, and sharp edges, must not be cleaned with solvents that alter the surface, and must be documented with their thickness, color, density, and filler or pigment content, since these parameters measurably influence burning behavior.
Design of Comparison Sets
In a material-comparison experiment, all candidate materials should be molded or cut to the same nominal thickness and conditioned identically so that the only variable is the material itself. In an interlaboratory comparison, a single homogeneous batch is split among participating laboratories, and each laboratory tests the same specimen geometry from the same lot, enabling meaningful statistical evaluation of method consistency.
Testing Items
A vertical burning comparison experiment measures and records the following parameters for every specimen:
- Afterflame time t1 — the duration of flaming after the first 10-second flame application
- Afterflame time t2 — the duration of flaming after the second 10-second flame application
- Afterglow time — the duration of glowing combustion after flaming ceases following the second application
- Total afterflame time — the sum of t1 and t2 for all five specimens in a set
- Cotton ignition — whether flaming drips or particles ignite the indicator cotton placed beneath the specimen
- Burning to the clamp — whether afterflame or afterglow propagates along the full specimen length to the holding clamp
- Classification result — the assignment of V-0, V-1, V-2, or "not classified" based on the combined criteria
- Comparative statistics — mean values, ranges, and dispersion measures used to rank materials or to quantify interlaboratory agreement
For comparison purposes, the most informative outputs are usually the individual and total afterflame times, the presence or absence of flaming drips, and the consistency of results across the specimen set, since these directly determine both the rating and the confidence that can be placed in it.
Testing Methods
Apparatus and Flame Conditions
Testing is conducted inside a closed, draft-free enclosure or fume hood that prevents ambient air movement from disturbing the flame. The ignition source is a laboratory burner with an approximately 9.5 mm inner-diameter tube, supplied with technical-grade methane of at least 98 % purity (or natural gas of equivalent heat input), adjusted to produce a blue flame with a height of 20 ± 1 mm. The specimen is held vertically in a clamp that grips it at its top edge, and a pad of dry, untreated surgical cotton approximately 50 mm × 50 mm × 6 mm is positioned 300 mm below the lower edge of the specimen to catch and indicate flaming drips. Timing devices with a resolution of at least 0.1 s are used for all interval measurements.
Test Procedure
- Mount the specimen vertically with its long axis aligned to gravity, and verify the cotton pad is in place 300 mm below the bottom edge.
- Apply the calibrated 20 mm blue flame to the midpoint of the specimen's lower edge for 10 ± 0.5 seconds, then withdraw the burner.
- Record t1, the afterflame time following the first application, and observe whether flaming drips fall onto the cotton.
- As soon as flaming stops, reapply the flame for a further 10 ± 0.5 seconds, then withdraw again.
- Record t2, the afterglow time, cotton ignition, and whether burning reaches the clamp.
- Repeat the sequence for the remaining specimens in the set, then test the second set of five if the results are marginal or a criterion is not met.
Rating Criteria
- V-0: afterflame time for each individual application does not exceed 10 s; total afterflame time for the set of five specimens does not exceed 50 s; no ignition of the indicator cotton by flaming drips or particles; no afterflame or afterglow persisting longer than 30 s after the second flame removal; and no burning up to the specimen clamp.
- V-1: afterflame time for each individual application does not exceed 30 s; total afterflame time for the set does not exceed 250 s; indicator cotton is not ignited; the remaining criteria are identical to V-0.
- V-2: the timing criteria match V-1, but flaming drips or particles that ignite the indicator cotton are permitted.
- Not classified: specimens that burn to the clamp, exceed the applicable time limits, or otherwise fail the criteria of the attempted class.
The standards include defined provisions for retesting a second set of specimens when individual results fall marginally outside a limit, and the final classification is assigned only after the specified evaluation rules are satisfied in full.
Comparison Experiment Protocol
In a rigorous comparison experiment, all controllable variables are fixed before testing begins: gas purity and flow rate, flame height calibration, conditioning atmosphere, specimen geometry, operator technique, and enclosure conditions. For interlaboratory comparisons, results from each laboratory are compiled and analyzed statistically in accordance with ISO 5725 or ASTM E691 to determine the repeatability limit r within a laboratory and the reproducibility limit R between laboratories. Formal proficiency testing schemes organized under the principles of ISO/IEC 17043 extend this approach, allowing accredited third-party laboratories to demonstrate the reliability of their vertical burning test results against peer laboratories and to identify systematic deviations in equipment or procedure.
Applications
Vertical burning comparison experiments are applied across the entire life cycle of plastic materials in the electrical, electronics, appliance, and automotive industries:
- Flame-retardant formulation development, enabling chemists to rank candidate resins, synergists, and loading levels by afterflame behavior and drip performance
- Material selection and design verification for enclosures, connectors, switches, relay bodies, and printed-circuit-board laminates required by product safety standards such as IEC 62368-1 and IEC 60335-1
- Incoming-material inspection and supplier comparison, confirming that production lots of compounded plastics retain the certified flammability rating
- Batch-level quality control at extrusion and injection molding plants, where periodic vertical burning checks detect process drift
- Interlaboratory validation and proficiency testing, demonstrating measurement consistency among accredited third-party laboratories
- Academic research and failure analysis, providing quantitative evidence when investigating fire incidents or evaluating novel polymer systems
FAQ
What is the difference between vertical and horizontal burning tests?
In a horizontal burning test, such as those yielding the HB classification, the specimen lies flat and flame spread along its length is timed to determine a burning rate. In the vertical test, the specimen stands upright and the flame attacks the bottom edge, so combustion gases travel upward with the flame, producing a far more severe condition. Vertical testing therefore provides the stricter V-0, V-1, and V-2 classifications used for the majority of electrical safety requirements.
What does a V-0 rating actually mean?
A V-0 rating means that the material self-extinguishes within 10 seconds after each of the two flame applications for every specimen, that the total afterflame time for the five-specimen set does not exceed 50 seconds, that no flaming drips ignite the indicator cotton, and that no specimen burns up to the clamp. It is the most demanding of the three vertical classifications.
Why can two laboratories obtain different results for the same material?
Differences typically arise from specimen thickness or conditioning deviations, gas composition and flame height calibration, operator timing technique, enclosure drafts, or the natural variability of the material itself. Interlaboratory comparison experiments are specifically designed to detect and quantify these differences, and they often reveal which procedural parameter is responsible.
Is a vertical burning rating transferable between thicknesses?
No. The classification applies only to the thickness tested. A material rated V-0 at 3.0 mm may perform significantly worse at 1.5 mm, so products must be verified with specimens molded at the actual minimum wall thickness used in the design.
How many specimens are required for one classification?
Two sets of five bar specimens are prepared for each thickness and condition. The second set is tested when the first set produces marginal or non-conforming results, and the standard's evaluation rules determine the final outcome based on the combined evidence.
How does the vertical burning test differ from glow-wire or needle-flame tests?
The vertical burning test characterizes the intrinsic flammability of a material specimen using a 50 W premixed burner flame and assigns a material classification. Glow-wire tests use an electrically heated wire to simulate thermal stress from faulty components on finished parts, while the needle-flame test uses a tiny diffusion flame to simulate a small ignition source within equipment. The three methods are complementary and are often specified together in product safety standards.
