Vertical Burning Test Result Analysis: Standards, Criteria, and Practical Interpretation

Vertical burning result analysis is the systematic interpretation of data from small-scale flame tests such as UL 94 and IEC 60695-11-10, which classify polymeric materials as V-0, V-1, or V-2 based on afterflame time, afterglow duration, and dripping behavior. Accurate analysis of these results enables manufacturers to select compliant flame-retardant materials, diagnose test failures, and demonstrate regulatory conformity for electronics, appliances, and other products. This guide explains the sample requirements, measured parameters, test methodology, classification criteria, and practical interpretation of vertical burning test results.

Technical Overview

The vertical burning test is one of the most widely used laboratory methods for evaluating the relative flammability of plastic and other polymeric materials. A small premixed gas flame is applied to the lower end of a vertically oriented bar specimen, and the material's ability to self-extinguish, resist flame propagation, and avoid producing flaming droplets is assessed. The principal standards governing this method include UL 94 (V and 5V methods), IEC 60695-11-10 (50 W vertical flame test), IEC 60695-11-20 (500 W flame test), ASTM D3801, GB/T 5169.16, and JIS C 60695-11-10. These methods are substantially harmonized, allowing results to be compared across certification schemes.

Vertical burning result analysis goes beyond simply recording observations. It converts raw timing data and visual phenomena into a formal classification, such as V-0, V-1, V-2, 5VA, or 5VB, and then draws engineering conclusions about material suitability, formulation robustness, and compliance with product safety standards. The key data points recorded and analyzed include:

• Individual afterflame time after the first flame application (t1)
• Individual afterflame time after the second flame application (t2)
• Total afterflame time for a complete set of five specimens
• Afterglow time following the second application (t3)
• Ignition of the cotton indicator by flaming or glowing particles
• Whether flaming or glowing reaches the specimen clamp
• Burn-through of plaque specimens, applicable to the 5VA and 5VB methods
• The final classification assigned under the relevant standard

It should be emphasized that vertical burning classifications describe material behavior under a specific, controlled laboratory condition. They are intended for quality control, material preselection, and regulatory compliance purposes, and they do not by themselves predict the behavior of a material in a full-scale fire.

Testing Samples

Bar Specimens for V-0, V-1, and V-2 Testing

Standard bar specimens measure 125 mm plus or minus 5 mm in length and 13.0 mm plus or minus 0.5 mm in width, with the thickness corresponding to the intended application, commonly 3.0 mm or 3.2 mm, 1.5 mm or 1.6 mm, and thinner gauges where required. A classification is only valid for the thickness actually tested, so evaluation programs often include the minimum wall thickness of the final part. One set consists of five specimens, and additional sets are prepared when retesting provisions are invoked. Bars must be representative of production material, free from flash, voids, and contamination, and marked so each specimen can be tracked individually.

Plaque Specimens for 5VA and 5VB Testing

The more severe 5VA and 5VB methods require flat plaques, typically 150 mm by 150 mm at the relevant thickness, in addition to bar specimens. The plaques are subjected to repeated flame applications to determine whether the flame burns through the material and creates a hole. This burn-through assessment is the principal distinction between the 5VA classification, where no hole is permitted, and 5VB, where a hole is acceptable provided other criteria are met.

Specimen Conditioning and Preparation

Unless otherwise specified, specimens are conditioned for at least 48 hours at 23 plus or minus 2 degrees Celsius and 50 plus or minus 5 percent relative humidity before testing. For the 5V methods, an additional conditioning sequence involving extended oven aging, for example 168 hours at 70 degrees Celsius, is applied to one group of specimens. Injection molding parameters, drying of hygroscopic resins, pigment loading, regrind content, and surface cleanliness can all influence burning behavior, so sample preparation must be documented as part of a defensible result analysis.

Testing Items

The vertical burning test measures a defined set of quantitative and observational parameters. Each parameter directly feeds into the classification decision:

• Afterflame time t1: the duration of continued flaming after removal of the burner following the first 10-second application
• Afterflame time t2: the duration of continued flaming after removal of the burner following the second 10-second application
• Total afterflame time: the sum of all afterflame times for a set of five specimens, compared against the class limit
• Afterglow time t3: the duration of glowing combustion without flame after the second application
• Dripping behavior: whether particles or drops fall during the test and whether they ignite the surgical cotton placed approximately 300 mm below the specimen
• Flame propagation: whether flaming or glowing advances as far as the clamp holding the specimen
• Plaque burn-through: whether the repeated 500 W flame perforates the plaque in the 5V methods
• Classification outcome: V-0, V-1, V-2, 5VA, 5VB, or failure to meet any class requirements

Testing Methods

Apparatus and Test Conditions

The 50 W vertical test uses a burner with a nominal 100 mm barrel producing a blue cone flame of 20 mm plus or minus 1 mm in height, fueled by methane of at least 98 percent purity or a gas of equivalent heat content. Testing is performed in a closed enclosure or fume hood, with the exhaust turned off during the actual burn so that drafts do not influence the results. A layer of dry surgical cotton is positioned 300 mm plus or minus 10 mm below the lower edge of the specimen to catch and reveal ignition by falling drops. The 5V methods employ a larger, approximately 500 W flame for the more severe evaluation.

Test Procedure

Each bar specimen is clamped with its long axis vertical and its lower edge 10 mm above the top of the burner barrel. The flame is applied for 10 plus or minus 0.5 seconds, then removed, and the first afterflame time t1 is recorded. Immediately after the afterflame from the first application extinguishes, the flame is reapplied for another 10 plus or minus 0.5 seconds and removed, after which the second afterflame time t2 and the subsequent afterglow time t3 are recorded. Throughout both applications, the operator observes whether flaming or glowing particles fall, whether the cotton ignites, and whether combustion reaches the clamp. The sequence is repeated for all five specimens in the set, and the results are compiled for classification.

Classification Criteria for V-0, V-1, and V-2

The classification is determined by comparing the recorded data against the criteria of the applicable standard. The core requirements are summarized as follows:

• V-0: each individual afterflame time is 10 seconds or less; the total afterflame time for the set is 50 seconds or less; afterglow does not exceed 30 seconds after the second application; no specimen burns or glows up to the clamp; no flaming drips ignite the cotton.
• V-1: each individual afterflame time is 30 seconds or less; the total afterflame time for the set is 250 seconds or less; afterglow does not exceed 60 seconds; no specimen burns or glows up to the clamp; no flaming drips ignite the cotton.
• V-2: identical timing and propagation limits as V-1, except that flaming drips that ignite the cotton indicator are permitted.

Classification Criteria for 5VA and 5VB

For the 500 W test methods, the flame is applied five times to each specimen. To achieve a 5VA or 5VB classification, no specimen may exhibit afterflame or afterglow lasting more than 60 seconds after the fifth application, and no flaming drips may ignite the cotton. The 5VA classification additionally requires that the plaque specimens do not exhibit burn-through, whereas 5VB permits a hole through the plaque as long as all other criteria are satisfied. Because of the higher flame energy and plaque requirement, 5VA represents the most demanding classification in this family of tests.

Retest Rules and Statistical Evaluation

Standards include defined retest provisions to account for borderline results. If only one specimen from a set of five fails to comply, or if the total afterflame time only marginally exceeds the class limit, for example 51 to 55 seconds in the case of V-0, a second set of five specimens is tested. The classification is granted only if every specimen in the second set complies with the individual criteria and the combined total afterflame time for all ten specimens does not exceed the retest limit, typically 55 seconds for V-0 and 255 seconds for V-1 and V-2. If two or more specimens fail in the original set, the material is not classified in that category without corrective action and a new evaluation.

Interpreting Results and Troubleshooting Failures

Sound result analysis distinguishes between the characteristic failure modes, because each points to different formulation or design remedies:

• Excessive afterflame time indicates insufficient gas-phase flame inhibition; increasing flame retardant loading, adjusting synergist ratios, or switching the retardant chemistry may be required.
• Flaming drips that ignite the cotton often result from low melt viscosity in the burning zone; anti-drip additives such as PTFE, glass fiber reinforcement, or intumescent char-forming systems are common solutions.
• Prolonged afterglow suggests smoldering char oxidation; mineral fillers such as aluminum hydroxide or magnesium hydroxide and optimized char promoters can reduce glowing.
• Burning up to the clamp signals gross flame propagation and typically demands a fundamental change in material class rather than minor formulation tuning.
• Results that pass with a narrow margin, for example a total afterflame time close to 50 seconds, indicate a marginal formulation with little robustness against lot-to-lot variation, processing shifts, or aging.
• Thickness sensitivity is critical: classification applies only at the tested thickness, and thin-wall sections generally show longer afterflame times; designs should be verified at the minimum production wall thickness.

Analysts should also verify that specimen conditioning, gas purity, flame calibration, and draft-free conditions were satisfied before drawing conclusions, because deviations in any of these factors can produce misleading failures or falsely favorable results.

Applications

Vertical burning test results are used throughout the plastics, electronics, and electrical equipment industries for material qualification, regulatory compliance, and product development. Typical applications include:

• Enclosure and structural materials for consumer electronics, information technology, and audio-video equipment, supporting compliance with product safety standards such as IEC 62368-1
• Connectors, relays, switches, terminal blocks, and other current-carrying or live parts
• Printed circuit board laminates, prepregs, and insulating films
• Home appliance housings and internal components
• Battery packs, power adapters, chargers, and power supply units
• Lighting products, including LED module holders and driver enclosures
• Wire and cable accessories such as markers, spacers, and compound evaluations
• Automotive interior and electronic polymer components, alongside regulatory horizontal burn and OEM specifications
• Flame-retardant resin development, competitive benchmarking, incoming quality control, and certification support at accredited third-party laboratories

FAQ

What does a V-0 result mean?

A V-0 classification means that every specimen in the set self-extinguished within 10 seconds after each of the two 10-second flame applications, the total afterflame time for five specimens did not exceed 50 seconds, afterglow lasted no more than 30 seconds, no flame or glow reached the clamp, and no flaming drips ignited the cotton indicator.

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

The three classes differ in extinguishing speed and dripping behavior. V-0 requires the fastest self-extinguishing and no flaming drips igniting cotton. V-1 allows longer afterflame times, up to 30 seconds per application and 250 seconds per set, while still prohibiting flaming drips that ignite cotton. V-2 shares the timing limits of V-1 but permits flaming drips that ignite the cotton indicator.

What is the difference between V-0 and 5VA?

5VA and 5VB are evaluated with a substantially more energetic 500 W flame applied five times, and they additionally require plaque specimens. 5VA is the most severe classification because the plaque must not burn through, while 5VB tolerates a hole. V-0, by contrast, is obtained with the 50 W flame on bar specimens only.

Why do flaming drips receive so much attention?

Burning droplets can fall onto materials below the test specimen, ignite them, and propagate fire outside the original part. Many product safety standards therefore restrict enclosures and critical parts to V-0 or V-1 materials, while V-2 is acceptable only where dropping behavior has been addressed at the product level.

Does a V-0 rating apply to all thicknesses and colors?

No. A vertical burning classification is valid only for the thickness range and the color or pigment system actually tested. Thinner sections typically burn longer, and some pigments act as synergists or antagonists with flame retardant packages, so qualification programs should always include the minimum production wall thickness and all relevant color variants.

What are the most common causes of failed vertical burning results?

Frequent causes include insufficient flame retardant loading, poor dispersion of additives, excessive regrind content, molding defects, testing at an unqualified thin thickness, contamination of the specimen surface, and conditioning or calibration errors in the laboratory. Corrective actions range from formulation adjustments and anti-drip agents to redesigning wall thickness or selecting a higher-rated material grade.

When is a retest allowed?

If only one specimen out of five fails, or the total afterflame time only slightly exceeds the limit, standards permit a second set of five specimens. The classification stands only if all second-set specimens comply and the combined ten-specimen afterflame time remains within the retest limit. Multiple failures require material correction and a fresh evaluation.

Are results from UL 94, IEC 60695-11-10, ASTM D3801, and GB/T 5169.16 interchangeable?

These standards describe essentially the same 50 W vertical flame method and yield comparable classifications in most cases. However, minor differences in equipment, gas specification, conditioning, and acceptance wording can exist, so reports should always state the exact standard applied, the specimen thickness, and the color, and certification programs should be executed under the specific standard named in the applicable scheme.

How does vertical burning testing relate to glow-wire and needle-flame tests?

Vertical burning tests assess the intrinsic response of a material to a small open flame, while glow-wire tests under the IEC 60695-2 series evaluate ignition resistance against a hot element and the needle-flame test under IEC 60695-11-5 simulates a small fault-induced flame. Product standards typically combine several of these methods, so material selection and result analysis should consider the full set of fire hazard tests required for the end application.