Plastic Vertical Burning Test: Standards, Procedures, and Flame Ratings Explained

The plastic vertical burning test is a fundamental flammability assessment that evaluates how plastic materials react when exposed to a small open flame in a vertical orientation, forming the basis of the widely recognized UL 94 V-0, V-1, and V-2 classifications. This article explains the testing principle, applicable standards such as UL 94, IEC 60695-11-10, ASTM D3801, and GB/T 2408, specimen requirements, test procedures, and rating criteria, helping manufacturers select compliant materials for electrical, electronic, and consumer products.

Technical Overview

The plastic vertical burning test measures the comparative flammability characteristics of polymeric materials when a test specimen held in a vertical position is exposed to a standardized low-energy flame. It is one of the most widely performed flammability tests in the plastics industry and serves as a key acceptance criterion for materials used in electronic enclosures, electrical components, and consumer appliances. The test evaluates afterflame time, afterglow time, and the tendency of flaming drips to ignite cotton positioned beneath the specimen.

The primary reference standards include UL 94 (Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances), IEC 60695-11-10 (50 W horizontal and vertical flame test methods), ASTM D3801 (Standard Test Method for Measuring the Comparative Burning Characteristics of Solid Plastics in a Vertical Position), and GB/T 2408, the Chinese national standard technically aligned with the IEC method. These standards define the burner, flame energy, specimen geometry, conditioning requirements, and classification criteria used to assign vertical burning grades.

Based on test results, materials are classified into progressively less stringent categories: V-0, V-1, and V-2, with V-0 representing the highest resistance to ignition among the 50 W vertical classes. Two additional, more severe vertical classifications, 5VA and 5VB, employ a 500 W flame and are used for applications requiring enhanced fire performance.

Testing Samples

Solid Rigid Plastic Specimens

For the standard vertical burning test (V classification), specimens are cut to 125 mm in length by 13 mm wide, with the thickness matching the intended end-use application, typically up to 13 mm. Each classification test requires two sets of five bar specimens, and materials are expected to be evaluated at the minimum and maximum thicknesses at which they will be supplied. Edges should be smooth, and specimens must be free of burrs, flash, or surface contamination that could influence burning behavior.

Thin Flexible Film and Sheet Materials

Thin films that distort, shrink, or curl when tested as free-hanging bars may be evaluated using the VTM (vertical thin material) series of methods defined in UL 94, in which the specimen is wrapped around a mandrel. Specimen dimensions and mandrel geometry follow the specific VTM-0, VTM-1, and VTM-2 procedures.

Specimen Conditioning

Prior to testing, specimens are conditioned in a controlled atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 48 hours. For certain materials, an additional conditioning sequence at elevated temperature (for example, 70 °C for 168 hours followed by cooling in a desiccator) may be specified before the flammability test is performed.

Testing Items

The vertical burning test records the following key observations for each specimen:

• Afterflame time t1: the duration of continued flaming after the first 10-second flame application is removed.
• Afterflame time t2: the duration of continued flaming after the second 10-second flame application is removed.
• Afterglow time t3: the duration of glowing combustion, without flame, after the second application ceases.
• Whether flaming or flaming drips ignite the surgical cotton placed 300 mm below the specimen.
• Whether afterflame or afterglow propagates up to the specimen clamp holding the sample.
• For 5VA/5VB testing, whether the flame burns through the plaque or creates a hole.

These observations are aggregated across all specimens and compared against the classification criteria to determine the final material rating.

Testing Methods

Apparatus and Test Conditions

Testing is conducted in a laboratory fume hood or enclosed combustion cabinet with the airflow quiescent during the test. A Bunsen or Tirrill-type burner with a nominal 9.5 mm inner diameter tube is used, fueled by technical-grade methane or natural gas delivering a 20 ± 2 mm blue flame corresponding to approximately 50 W. The specimen is clamped vertically with its long axis parallel to the burner tube, and the flame is applied to the lower edge of the bar at a defined distance below the specimen tip.

Procedure for the V-Series Test

The flame is applied to the bottom edge of the specimen for 10 seconds and then withdrawn, while the operator records the afterflame time t1. After flaming stops, the flame is applied a second time for another 10 seconds and withdrawn, and the operator records the afterflame time t2, the afterglow time t3, and whether burning drips ignite the cotton indicator. This sequence is repeated for all ten specimens across both conditioning states, and the individual and total times are then evaluated against the classification limits.

Classification Criteria

• V-0: each afterflame time does not exceed 10 seconds; the sum of afterflame times for all ten specimens does not exceed 50 seconds; no ignition of the cotton indicator; no flaming or glowing reaching the clamp.
• V-1: each afterflame time does not exceed 30 seconds; the total afterflame time for ten specimens does not exceed 250 seconds; no ignition of the cotton indicator; no flaming or glowing reaching the clamp.
• V-2: identical timing limits to V-1, except that flaming drips are permitted to ignite the cotton indicator.
• 5VA: the most stringent vertical rating, using a 500 W flame applied five times for 5 seconds each to both bar and plaque specimens; no flaming drips that ignite cotton, and no burn-through holes in plaques.
• 5VB: same flame severity as 5VA; flaming drips that do not ignite cotton are acceptable and plaques may develop burn-through holes.

Extended Vertical Test (5VA / 5VB)

For applications demanding higher fire performance, the 500 W flame test applies a 125 mm premixed flame to 125 mm × 125 mm plaques and 125 mm × 13 mm bars in five 5-second cycles. Materials such as flame-retardant polycarbonate, modified PPO, and certain glass-filled engineering thermoplastics are frequently qualified to these levels for use in critical electrical enclosures.

Applications

Vertical burning ratings are specified across many industries wherever polymeric parts may be exposed to potential ignition sources:

• Enclosures and structural parts of information technology equipment, audio-video apparatus, and household appliances.
• Internal components of electronic devices, including connectors, coil bobbins, terminal blocks, and circuit board substrates.
• Insulating systems, switchgear components, and wiring accessories in low-voltage electrical installations.
• Automotive interior trim and electrical modules, where manufacturers often adopt vertical burning grades as part of their material specifications.
• Consumer goods, power tools, lighting fixtures, and chargers requiring evidence of flame-retardant compliance for product safety certification.
• Research and development of flame-retardant compounds, incoming material inspection, and quality control during production.

Because a vertical orientation exposes the specimen to upward flame spread and the hottest combustion gases, the vertical test is considerably more severe than the horizontal burning test, and vertical ratings are generally required for parts located near live electrical circuits or unattended energy sources.

FAQ

What is the difference between the horizontal and vertical burning tests for plastics?

The horizontal burning test (HB rating, covered in IEC 60695-11-10 and GB/T 2408) applies the flame to a horizontally mounted specimen and measures the burning rate or extinguishing distance, making it suitable for less critical applications. The vertical burning test applies the flame to the lower edge of a vertically mounted specimen, evaluates afterflame time, afterglow, and dripping behavior, and provides the more demanding V-0, V-1, V-2, 5VA, and 5VB classifications required for most electrical and electronic applications.

What does a UL 94 V-0 rating mean?

A V-0 rating indicates that when the specimen is subjected to two 10-second applications of a 50 W test flame, flaming stops within 10 seconds after each application, the combined afterflame time for all ten specimens is no greater than 50 seconds, no flaming drips ignite the cotton indicator, and combustion does not reach the holding clamp. It is the most common specification for engineering plastics used in electronic enclosures.

Which standards are equivalent to one another?

UL 94, IEC 60695-11-10, ASTM D3801, and GB/T 2408 specify technically aligned 50 W vertical flame procedures, and test data generated under one method is frequently accepted under the others, although each standard must be verified for scope and details before claiming equivalence. ISO 9773 covers vertical testing of thin flexible specimens.

Why is the thickness of the test specimen important?

Flammability performance depends strongly on material thickness: thicker sections generally extinguish faster than thin ones. Ratings are therefore assigned at specific thicknesses, and a material qualified at 3.0 mm cannot automatically be assumed to meet the same rating at 1.5 mm or thinner without additional testing at the reduced thickness.

Can a vertical burning test be performed at an independent laboratory?

Yes. Accredited third-party laboratories equipped with standardized combustion cabinets, calibrated burners, and conditioned specimen storage can perform vertical burning tests in accordance with UL 94, IEC 60695-11-10, ASTM D3801, or GB/T 2408 and issue test reports that support material datasheets, regulatory compliance, and product safety certifications.