Vertical Flame Retardancy Testing: Standards, Methods, and V-Rating Classifications

Vertical flame retardancy testing is a critical fire-safety evaluation that measures how polymeric materials, textiles, and components resist ignition, flame propagation, and flaming drips when a standard flame is applied to a vertically oriented specimen. Governed by internationally recognized standards such as UL 94, IEC 60695-11-10, GB/T 2408, and ASTM D6413, this test assigns widely cited classifications including V-0, V-1, and V-2 that directly determine material acceptability for electronics enclosures, wires, protective textiles, and countless safety-critical products. This article explains the test principles, specimen requirements, classification criteria, typical applications, and frequently asked questions to help manufacturers and engineers select and verify flame-retardant materials with confidence.

Technical Overview

Vertical flame retardancy testing—commonly known as the vertical burning test—is a small-scale laboratory procedure used to evaluate the relative burning characteristics of solid materials in a vertical orientation. A standardized gas flame is applied to the lower edge or surface of a suspended specimen, and parameters such as afterflame time, afterglow time, dripping behavior, and damaged length are recorded. Because the vertical orientation promotes upward flame spread and represents a more severe exposure than horizontal burning, vertical test results are generally required for materials used in the most demanding fire-safety applications.

For plastics, the most widely adopted vertical burning methods are specified in UL 94, which are harmonized with IEC 60695-11-10 (50 W vertical flame test), IEC 60695-11-20 (500 W flame test for 5VA and 5VB ratings), EN 60695-11-10, ASTM D3801, and the Chinese national standard GB/T 2408 (FV classifications). For textiles, vertical flammability is typically evaluated using GB/T 5455, ASTM D6413, and ISO 6941. Products tested in accredited third-party laboratories according to these standards are accepted by regulators, certification bodies, and OEM buyers worldwide.

• Evaluates ignition resistance, sustained burning, and flaming-drip behavior of materials
• Produces universally recognized classifications: V-0, V-1, V-2, 5VA, 5VB (plastics) and pass/fail or graded results (textiles)
• Forms a mandatory input for product safety certification of electrical and electronic equipment, cables, and protective clothing
• Supports material selection, quality control, incoming inspection, and regulatory compliance

Testing Samples

Plastic Bar Specimens

For UL 94, IEC 60695-11-10, and GB/T 2408 vertical tests, rigid plastic specimens are cut to 125 mm in length by 13 mm in width, with the thickness matching the intended end-use application (commonly 1.6 mm, 3.0 mm, or other relevant thicknesses up to 13 mm). Two sets of five bar specimens are prepared and conditioned at 23 °C ± 2 °C and 50% ± 10% relative humidity for at least 48 hours prior to testing to ensure reproducible results.

Thin and Flexible Materials

Films, sheets, and thin flexible plastics that deform or curl when suspended vertically are tested in accordance with ISO 9773, which uses a special mandrel support to hold the specimen in the correct orientation during flame application.

Cellular and Foam Materials

Rigid and flexible foams are evaluated with ISO 9772 and related methods, using larger specimens of approximately 150 mm by 50 mm to accommodate the faster burning and distortion typical of cellular plastics.

Textile Fabrics

For textile vertical flame tests under GB/T 5455, specimens of 300 mm by 80 mm are prepared in both warp and weft directions. ASTM D6413 specimens measure approximately 305 mm by 76 mm. Fabric specimens are conditioned to standard atmosphere before a 12-second ignition exposure, and afterflame time, afterglow time, and char length are measured.

Finished Components and Parts

Where standards permit, finished components such as connectors, relay housings, circuit board laminates, and equipment enclosures may be tested as received to verify that flame-retardant performance is retained in the actual manufactured part.

Testing Items

Vertical flame retardancy testing quantifies the following core parameters, which together determine the classification or pass/fail outcome:

• Afterflame time (t1 and t2): the duration of sustained flaming after the first and second flame applications
• Total afterflame time: the cumulative afterflame time across all five specimens in a set
• Afterglow time: the duration of glowing combustion without flame following cessation of flaming
• Flaming drips: whether burning particles fall and ignite the cotton indicator placed beneath the specimen
• Flame spread to the clamp: whether the specimen burns or glows up to the specimen holder
• Char length or damaged length: the extent of burned, charred, or weakened material on textile specimens
• Burn-through behavior: whether a 500 W flame perforates plaque specimens in the 5VA/5VB test
• Melting, shrinking, and deformation observations relevant to material behavior in fire scenarios

Testing Methods

UL 94 / IEC 60695-11-10 Vertical Burning Test (50 W Flame)

The bar specimen is clamped at its upper end so that it hangs vertically with its lower edge approximately 10 mm above the top of a Bunsen burner tube. A 20 mm ± 1 mm blue flame (nominal 50 W) is applied to the lower edge of the specimen for 10 seconds, then withdrawn while the afterflame time t1 is recorded. Once flaming stops, the flame is applied a second time for 10 seconds, and the second afterflame time t2 and afterglow time are recorded. A layer of dry surgical cotton is positioned 300 mm below the specimen to catch any flaming drips. The procedure is repeated on five specimens, and the results are evaluated against classification criteria.

Classification Criteria for V Ratings

• V-0: each afterflame time t1 or t2 does not exceed 10 seconds; total afterflame time for the set does not exceed 50 seconds; afterflame plus afterglow after the second application does not exceed 30 seconds; no flaming drips ignite the cotton; no flaming reaches the clamp
• V-1: each afterflame time does not exceed 30 seconds; total afterflame time does not exceed 250 seconds; afterflame plus afterglow does not exceed 60 seconds; no flaming drips ignite the cotton; no flaming reaches the clamp
• V-2: identical to V-1 except that brief flaming drips that ignite the cotton are permitted

UL 94 / IEC 60695-11-20 5V Test (500 W Flame)

The 5V method applies a more severe 500 W premixed flame with a 125 mm overall height to both bar and 150 mm by 150 mm plaque specimens. The flame is applied five times for 5 seconds each, with 5-second intervals. 5VB classification permits localized burn-through of plaques, while the more stringent 5VA classification requires no burn-through, no ignition of the cotton by flaming drips, and afterflame plus afterglow time not exceeding 60 seconds after the fifth application. 5VA is the highest small-scale flammability rating for plastics in the UL 94 system.

Vertical Flame Tests for Textiles

In GB/T 5455 and ASTM D6413, a fabric specimen is suspended vertically in a frame and a defined gas flame—typically applied for 12 seconds—impinges on the bottom edge or lower face of the fabric. Afterflame time, afterglow time, and char length are measured, and results are compared with product specification limits such as those for flame-retardant protective clothing, children's sleepwear under 16 CFR 1615 and 1616, and interior decoration fabrics. ISO 6941 additionally characterizes flame spread rate on vertically oriented textiles, measuring the time for surface flame propagation between reference gauges.

Reporting and Test Report Content

A complete test report from an accredited third-party laboratory identifies the standard applied, material identification and thickness, specimen conditioning, burner gas and flame calibration, individual and total afterflame and afterglow times, dripping and cotton-ignition observations, and the final classification, along as appropriate with photographs and diagrams of specimen behavior.

Applications

Vertical flame retardancy ratings are embedded in product safety requirements across nearly every industry in which fire hazard must be controlled:

• Consumer electronics and information technology equipment: fire enclosures of adapters, chargers, power supplies, and appliances frequently require V-1 or V-0 materials under product safety standards such as IEC 62368-1
• Electrical components: connectors, switches, terminal blocks, relay housings, and fuse holders are routinely specified in V-0 grade plastics
• Printed circuit boards: FR-4 laminates and other substrates are typically qualified to a V-0 rating
• Wire and cable: insulation and sheath compounds are verified with vertical flame methods including the VW-1 vertical wire flame test
• Battery and new energy products: module housings, structural parts, and thermal barrier materials for lithium-ion systems rely on V-0 and 5VA materials
• Protective textiles: flame-resistant workwear, firefighting garments, and industrial uniforms are qualified using vertical textile flame tests referenced in specifications such as NFPA 2112
• Children's sleepwear and home textiles: regulatory flammability requirements rely on vertical burning performance for fabrics and trim
• Automotive and rail interior parts, building decoration materials, and furniture components where upward flame spread must be limited

FAQ

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

All three classifications are produced by the same 50 W vertical burning procedure. They differ in the permitted afterflame times and dripping behavior: V-0 requires extinction within 10 seconds per application with no flaming drips, V-1 allows up to 30 seconds per application with no flaming drips, and V-2 allows up to 30 seconds with brief flaming drips permitted. V-0 represents the highest level of self-extinguishing performance among the three.

How does vertical testing differ from horizontal flammability testing?

In the horizontal test the specimen lies flat and flame spreads horizontally against buoyant forces, producing a milder exposure and slower burning suitable for less critical materials. In the vertical test the specimen hangs downward, so the flame front travels upward with the natural direction of flame propagation, making it a far more severe and discriminating test. Vertical ratings are therefore demanded for safety-critical applications.

Are GB/T 2408 and IEC 60695-11-10 equivalent to UL 94?

Yes, these standards are technically harmonized. GB/T 2408 designates the vertical results as FV-0, FV-1, and FV-2, corresponding directly to UL 94 V-0, V-1, and V-2. Manufacturers supplying global markets should nevertheless confirm which standard edition their target market or certification scheme requires, as minor procedural details and marking requirements can differ.

What specimen thickness should be tested?

Materials must be tested at the minimum thickness at which they will actually be used in the end product, because flammability ratings are thickness-dependent. A compound rated V-0 at 3.0 mm may only achieve V-1 or V-2 at 1.6 mm or thinner sections. Test reports always state the tested thickness, and design engineers must verify that the rated thickness matches the wall thickness of their part.

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

V-0 is obtained with a 50 W, 20 mm flame applied twice for 10 seconds to bar specimens. 5VA and 5VB use a much more severe 500 W flame applied five times to both bars and large plaques, with 5VA additionally prohibiting any burn-through of the plaque. 5VA therefore represents a significantly higher flame-retardance level than V-0 and is commonly specified for high-power equipment enclosures and critical fire-barrier parts.

Why do flaming drips matter so much in vertical testing?

Flaming molten droplets falling from a burning part can ignite secondary materials below the component, propagating fire well beyond the original source. This is why a cotton indicator pad is placed beneath the specimen and why the distinction between V-1 (no flaming drips) and V-2 (flaming drips permitted) is a decisive factor in material selection for enclosures and elevated installations.

Can finished products be tested, or only raw material plaques?

Both. Compounds are qualified on standardized bars, and many certification schemes permit or require additional verification on the actual molded part to confirm that processing, wall thickness, colorants, and additives have not degraded flame-retardant performance. Accredited third-party laboratories can conduct both material-level and component-level vertical burning assessments and support full compliance programs from material selection to final product certification.