Plastic Vertical Burning Testing: Standards, Test Methods, and Flammability Classifications

Plastic vertical burning testing is a fundamental flammability assessment that measures how quickly polymeric materials self-extinguish after exposure to a small ignition flame and whether they release flaming drips, forming the basis of the globally recognized UL 94 V-0, V-1, and V-2 classifications. This article explains the applicable standards, specimen requirements, testing procedures, acceptance criteria, and typical industrial applications of the vertical burning test for plastics, providing a practical reference for engineers, quality managers, and materials specialists.

Technical Overview

Plastic vertical burning testing is a laboratory-scale flammability test in which a vertically mounted plastic specimen is exposed to a controlled small flame, and its burning behavior is quantified through afterflame time, afterglow time, and the ignition of a cotton indicator by flaming drips. Because the vertical orientation represents the most severe common exposure condition for plastic parts—flame propagation is assisted by the upward flow of hot combustion gases—the vertical test is significantly more demanding than its horizontal counterpart and is required for materials used near potential ignition sources.

The most widely applied procedure is the 50 W vertical test described in UL 94 and its technically equivalent international editions, including IEC 60695-11-10, EN 60695-11-10, ISO 1210, ASTM D3801, and the Chinese national standards GB/T 2408 and GB/T 5169.16. The test assigns one of three classifications—V-0, V-1, or V-2—depending on how quickly the specimen stops burning after removal of the test flame and whether burning particles ignite cotton placed below the specimen. A more severe 500 W procedure, which assigns the 5VA and 5VB classifications, is covered by IEC 60695-11-20, ASTM D5048, GB/T 5169.17, and the corresponding section of UL 94.

Test results are expressed as a classification that is valid only for the tested material at the tested thickness and color. Accredited third-party laboratories routinely perform these tests to support product safety certification, regulatory compliance, material qualification, research and development, and incoming quality control.

Testing Samples

Bar Specimens for the 50 W (V-Classification) Test

Standard specimens are rigid bars measuring 125 mm in length and 13 mm in width, cut or molded at the thickness to be rated, typically between 0.1 mm and 13 mm. Two sets of five specimens are tested per thickness and condition. Each bar is clamped over approximately 6 mm at its upper end so that the longitudinal axis is vertical and the lower edge is freely exposed to the burner flame.

Bar and Plaque Specimens for the 500 W (5VA/5VB) Test

The 500 W procedure uses vertical bars of 125 mm by 13 mm to evaluate flame application to a bottom edge, together with square plaques, typically 150 mm by 150 mm, to evaluate flame application to a flat face. Plaque specimens are also used to assess burn-through, meaning the formation of a hole through the material. Both bar and plaque criteria must be satisfied to obtain a 5VA or 5VB classification.

Thin, Flexible, and Cellular Materials

Materials that distort, shrink, or curl away from the flame cannot always be evaluated with the standard bar configuration. Thin flexible films may be supported on a mandrel in accordance with ISO 9773, while foamed plastics are generally assessed using the dedicated horizontal burning method of ISO 9772 or the specimen-support provisions specified in the applicable standard.

Specimen Conditioning

Before testing, specimens are conditioned at 23 °C ± 2 °C and 50 % ± 5 % relative humidity for at least 48 hours. For full classification programs, an additional set of specimens is oven-aged at 70 °C ± 2 °C for 168 hours and then cooled in a desiccator for at least 4 hours before testing, in order to evaluate the effect of long-term thermal exposure on flammability performance.

Testing Items

The following parameters are measured and reported for each specimen and each set:

• 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 incandescent glow, without flame, after the second application.
• Cotton ignition: whether flaming or glowing particles dripped from the specimen ignite the cotton indicator.
• Flame spread to the clamp: whether the specimen burns up to or above the clamped region.
• Total afterflame time: the cumulative afterflame time for the five specimens of each set and for the combined sets.
• Final classification: V-0, V-1, or V-2 for the 50 W test, or 5VA or 5VB for the 500 W test, always reported together with the specimen thickness.

The test report documents the material designation, color and pigmentation, thickness, conditioning history, fuel gas purity, individual timing values, observations of dripping and afterglow, and the resulting classification, allowing results to be traced and compared across laboratories and production lots.

Testing Methods

Apparatus and Test Environment

Testing is performed inside a draft-free enclosure or laboratory fume hood, typically with an internal volume of at least 0.5 cubic meters, so that ambient air movement cannot influence flame behavior. A Bunsen or Terrill-type burner fueled by methane or natural gas of at least 98 percent purity produces a calibrated blue flame of 20 mm ± 1 mm in height, corresponding to approximately 50 W of thermal output. A pad of clean, dry surgical cotton approximately 50 mm by 50 mm by 6 mm is positioned 300 mm below the lower edge of the specimen to capture and reveal any flaming drips.

Procedure for the 50 W Vertical Test

• Mount the specimen vertically in the clamp with its lower edge fully exposed, and verify that the cotton pad is correctly positioned 300 mm below the specimen.
• Place the burner so that its tube top is 10 mm below the lower edge of the specimen, apply the flame for 10 seconds ± 0.5 seconds, then withdraw the burner and record afterflame time t1.
• After flaming from the first application has ceased, reapply the flame for a second 10-second period, withdraw the burner, and record afterflame time t2 and subsequent afterglow time t3.
• Throughout the test, observe whether flaming drips ignite the cotton and whether the flame front reaches the clamp.
• Repeat the procedure for the five specimens of each set, then evaluate the results against the classification criteria; the standard's retest rules apply if any individual specimen fails a criterion.

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

• V-0: afterflame time of 10 seconds or less for each application, total afterflame time of 50 seconds or less per set, no flaming drips that ignite the cotton, no burning to the clamp, and afterglow of 30 seconds or less after the second application.
• V-1: afterflame time of 30 seconds or less for each application, total afterflame time of 250 seconds or less per set, no flaming drips that ignite the cotton, no burning to the clamp, and afterglow of 60 seconds or less after the second application.
• V-2: identical timing limits to V-1, except that flaming drips that ignite the cotton are permitted.

All criteria must be met by every specimen in a set. If one or more specimens fail, additional sets must be tested in accordance with the retest provisions of the standard, and the most severe observed behavior governs the final classification.

Procedure for the 500 W Test (5VA and 5VB)

A 125 mm high flame with a 40 mm inner blue cone is applied to the lower edge of the vertical bar five times, each application lasting 5 seconds with a 5-second interval; plaque specimens receive the same five applications to a flat face. For a 5VA classification, the specimen must show no ignition of the cotton by drips, no burning to the clamp, a combined afterflame and afterglow time of 60 seconds or less after the fifth application, and no burn-through of the plaque. For 5VB, the same timing and drip criteria apply, but burn-through of the plaque is permitted. 5VA is the most demanding flammability classification for plastics under UL 94.

Applications

Vertical burning classifications underpin material selection and fire-safety compliance across a broad range of industries:

• Information technology and audio-video equipment: enclosures and internal parts of computers, power adapters, displays, and battery packs are typically required to achieve V-0, or 5VA/5VB for certain enclosures, under product safety standards such as IEC 62368-1.
• Household appliances: components located near live parts or potential ignition sources in appliances conforming to the IEC 60335 series must meet defined flammability levels.
• Electrical components: connectors, switches, terminal blocks, circuit-breaker housings, and printed-circuit-board laminates used in low-voltage switchgear and controlgear within the IEC 60947 series.
• Medical electrical equipment: polymeric parts of devices built to IEC 60601-1, where component-level flammability ratings feed into overall equipment fire-risk assessment.
• Wire and cable: insulation and sheathing compounds are frequently qualified with V-classifications in parallel with dedicated cable flame propagation tests.
• Automotive and electrified vehicles: original equipment manufacturer specifications commonly require V-0 materials for electronic control modules, charging system components, connectors, and interior parts positioned near electrical sources.
• Materials development and quality control: compounders, molders, and end-product manufacturers use vertical burn data to qualify flame-retardant formulations, monitor lot-to-lot consistency, and prepare regulatory and customer documentation.

Manufacturers frequently combine vertical burning testing with complementary fire-hazard assessments—such as the glow-wire tests of IEC 60695-2-10 to IEC 60695-2-13, the needle-flame test of IEC 60695-11-5, and the horizontal burning test of UL 94 HB—to build a complete flammability profile for end-product compliance.

FAQ

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

The classifications reflect how quickly a material self-extinguishes and whether it produces flaming drips. V-0 requires afterflaming of 10 seconds or less per application and 50 seconds or less total per set; V-1 allows up to 30 seconds per application and 250 seconds total; V-2 meets the same timing limits as V-1 but tolerates flaming drips that ignite the cotton indicator. V-0 is the most demanding of the three.

Which standards govern plastic vertical burning testing?

The 50 W vertical test is covered by UL 94, IEC 60695-11-10, EN 60695-11-10, ISO 1210, ASTM D3801, GB/T 2408, and GB/T 5169.16, which are technically aligned. The 500 W test for 5VA and 5VB classifications is covered by IEC 60695-11-20, ASTM D5048, GB/T 5169.17, and the corresponding part of UL 94.

Why must the thickness be reported together with the classification?

Flammability depends strongly on specimen geometry. A material rated V-0 at 3 mm may only achieve V-2, or fail entirely, at 1.5 mm. Classifications are valid only for the tested thickness and color, so purchase specifications and drawings should always state both the rating and the thickness.

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

In the horizontal test the specimen lies flat and the linear burning rate over a measured distance is recorded, making it a relatively mild assessment. In the vertical test the flame is applied to the bottom edge, hot gases assist upward flame propagation, and the evaluation focuses on self-extinguishment and dripping. Applications involving proximity to ignition sources almost always require vertical classifications.

How many specimens are required, and how long does testing take?

The 50 W test requires two sets of five bar specimens per thickness and conditioning state, with additional retest sets as needed; the 500 W test requires both bars and plaques with multiple flame applications per specimen. Including mandatory conditioning and oven aging, a full classification program at accredited third-party laboratories typically takes one to two weeks, while quality-control screening without full conditioning can be completed faster.

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

The V-classifications use a 20 mm, 50 W flame applied twice for 10 seconds each, whereas the 5V classifications use a 125 mm, 500 W flame applied five times for 5 seconds each and additionally evaluate burn-through on plaque specimens. 5VA, which prohibits burn-through, is the highest plastics flammability rating under UL 94 and is reserved for enclosure materials requiring maximum resistance to repeated flame contact.