Vertical Burning Performance Testing: Standards, Methods, and Classification Criteria

Vertical burning performance testing is a fundamental flammability evaluation in which a controlled flame is applied to vertically oriented specimens to measure after-flame time, afterglow behavior, and flaming drips. It is essential for qualifying plastic materials, textiles, foams, and cable products against internationally recognized standards such as UL 94, IEC 60695-11-10, ASTM D3801, and GB/T 2408. This article explains the technical principles, sample requirements, test procedures, classification criteria, and typical applications of vertical burning performance determination.

Technical Overview

Vertical burning performance determination is a laboratory method used to assess the relative flammability and self-extinguishing capability of polymeric and textile materials. In this test, a standardized small flame (typically a 20 mm blue methane flame for plastics, or a 40 mm flame for textiles) is applied to the lower end of a vertically clamped specimen for a defined duration. The laboratory then records key parameters such as after-flame time, afterglow time, flaming drips, and whether burning particles ignite an indicator material placed below the specimen.

Because gravity pulls the flame upward along the specimen, the vertical orientation represents a significantly more severe exposure condition than horizontal burning methods. As a result, vertical test classifications such as UL 94 V-0, V-1, and V-2 are widely regarded as benchmark requirements for materials used in electrical enclosures, electronic components, and safety-critical applications.

• Plastics and engineering polymers: UL 94, IEC 60695-11-10, ISO 1210, ASTM D3801, GB/T 2408 (vertical method)
• Textiles and flexible materials: GB/T 5455, ASTM D6413, 16 CFR 1615/1616 (children's sleepwear)
• Wires and cables: VW-1 under UL 2556, IEC 60332 series (single vertical burning test)
• Higher-severity assessments: IEC 60695-11-4 (500 W vertical test) and 5VA/5VB under UL 94

Testing Samples

Plastic and Rubber Materials

Rigid and flexible plastics, thermoplastics, thermosets, engineering compounds, and flame-retardant modified materials submitted in the form of molded or cut plaques. Standard UL 94 V specimens measure 125 mm in length and 13 mm in width, with thicknesses typically ranging from 0.8 mm up to 13 mm. Testing is normally performed on at least two sets of five specimens each.

Textile Fabrics

Woven, knitted, coated, laminated, and nonwoven fabrics used in protective clothing, curtains, upholstery, tents, and bedding. For the vertical method described in GB/T 5455, specimens of 300 mm by 80 mm are prepared, with the fabric tested in both lengthwise and widthwise directions where required.

Flexible Cellular Materials

Flexible polyurethane foams and other cellular materials used in seating, cushioning, and padding, evaluated for their burning behavior when oriented vertically.

Wires, Cables, and Finished Components

Insulated wires and cables evaluated by the VW-1 vertical flame test, as well as finished parts, connectors, enclosures, and insulating parts that cannot be cut into standard bar specimens and may instead be assessed as finished components under applicable clauses of the vertical burning standards.

Testing Items

The vertical burning test quantifies the following measurable characteristics, which together determine the flammability classification of the material:

• After-flame time after the first flame application (t1)
• After-flame time after the second flame application (t2)
• Afterglow time following the second application (t3)
• Total after-flame time for the complete set of specimens
• Presence of flaming drips or particles and whether they ignite the cotton indicator
• Whether flaming reaches the holding clamp
• Char length (damaged length) and flame spread behavior for textile specimens
• Smoldering or afterglow duration for textile materials

Testing Methods

Specimen Conditioning

Prior to testing, specimens are conditioned to ensure reproducible results. One set is typically conditioned for at least 48 hours at 23 ± 2 °C and 50 ± 5% relative humidity, while a second set receives additional preconditioning at 70 °C for 168 hours followed by cooling in a desiccator. Testing is conducted inside a ventilated chamber or fume hood with the gas burner adjusted to produce the specified blue flame height using methane gas of defined calorific value.

Vertical Burning Procedure for Plastics (UL 94 / IEC 60695-11-10 / GB/T 2408)

The specimen is clamped vertically with its long axis perpendicular to the bench, and a layer of dry surgical cotton is placed 300 mm below the lower edge of the specimen to catch flaming drips. The 20 mm blue flame is applied to the center of the lower edge of the specimen for 10 ± 0.5 seconds and then withdrawn while the timer records the after-flame duration t1. Once flaming stops, the flame is applied a second time for 10 seconds, and the after-flame time t2 and afterglow time t3 are recorded along with any dripping behavior. The procedure is repeated for all specimens in both conditioned sets.

Classification Criteria

• V-0: after-flame time of each specimen does not exceed 10 seconds; total after-flame time for the set does not exceed 50 seconds; no flaming drips ignite the cotton; after-flame plus afterglow after the second application does not exceed 30 seconds
• V-1: after-flame time of each specimen does not exceed 30 seconds; total after-flame time does not exceed 250 seconds; no flaming drips ignite the cotton; after-flame plus afterglow does not exceed 60 seconds
• V-2: identical timing requirements to V-1, except that flaming drips are permitted if they briefly ignite the cotton indicator
• Failed: specimens whose burning exceeds the above limits, burn up to the clamp, or produce prolonged ignition of the indicator are not classified under the V-scale

Vertical Burning Procedure for Textiles (GB/T 5455 / ASTM D6413)

The fabric specimen is suspended vertically in a frame inside an enclosed cabinet. A defined gas flame (typically 40 mm in height) is applied to the lower edge of the specimen for a fixed period, commonly 12 seconds. After flame removal, the laboratory records the after-flame time, smoldering or afterglow time, and damaged (char) length. Multiple specimens are tested in each principal direction, and the results are compared against the acceptance limits of the applicable product standard or specification.

Vertical Flame Test for Wires and Cables (VW-1)

For insulated wires and cables, the vertical wire flame test applies the burner flame to the specimen at an angle for a repeated sequence of five 15-second applications. The specimen must stop burning within 60 seconds after each application, the Kraft paper flag must not burn more than 25% of its surface, and the cotton indicator must not be ignited.

Applications

Vertical burning performance data supports material selection, product safety certification, and regulatory compliance across many industries:

• Consumer electronics and IT equipment, where enclosure materials must meet minimum flammability ratings required by product safety standards
• Household appliances and electrical accessories such as switches, sockets, plugs, and circuit breaker housings
• Wires, cables, and wiring harnesses for buildings, machinery, vehicles, and rolling stock
• Automotive and rail interior components and battery system materials
• Protective clothing, workwear, and flame-resistant garments evaluated by vertical textile methods
• Children's sleepwear, curtains, drapery, upholstery, tents, and other textile products with regulatory flammability requirements
• Aerospace and defense applications demanding rigorous flame-resistance qualification of polymers and composites

Manufacturers, material suppliers, and importers frequently engage accredited third-party laboratories to perform vertical burning tests when preparing compliance documentation for market access, supplier approvals, or research and development of flame-retardant formulations.

FAQ

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

The main differences lie in the permitted after-flame times and dripping behavior. V-0 is the most stringent rating, allowing a maximum individual after-flame time of 10 seconds and a total of 50 seconds per set with no flaming drips. V-1 permits longer burning (30 seconds individual, 250 seconds total) but still prohibits drips that ignite cotton. V-2 has the same timing limits as V-1 but tolerates flaming drips that ignite the cotton indicator.

Why is the vertical test considered more severe than the horizontal test?

In the vertical orientation, the flame is applied to the bottom edge and hot gases rise along the specimen surface, preheating the material above the flame front and promoting continued burning. This makes self-extinguishing performance much harder to achieve, which is why vertical ratings are typically required for high-risk applications such as electrical enclosures.

What sample information should be provided before testing?

The laboratory should receive the material name, nominal thickness, color and filler content where applicable, the target standard and classification, the desired specimen orientation for textiles, and sufficient material to prepare the required number of specimens, usually two sets of five bars for plastics or several fabric pieces cut in both directions.

How should results be reported?

A professional test report states the applied standard, specimen dimensions and thickness, conditioning history, flame application details, individual and total after-flame times, afterglow times, dripping and cotton ignition observations, and the final classification, for example UL 94 V-0 at the tested thickness. Because flammability ratings are thickness-dependent, the report must always specify the thickness at which the rating was achieved.

How long does vertical burning testing take?

Conditioning dominates the schedule: 48 hours of standard conditioning is routine, and additional preconditioning at elevated temperature may require up to one week. The actual burning sequence for a set of specimens is typically completed within a few hours, so most projects are finalized within one to two weeks depending on conditioning requirements and the number of thicknesses evaluated.