Vertical burning rate testing is a fundamental flammability assessment in which a vertically oriented specimen is exposed to a standardized ignition source and parameters such as afterflame time, char length and flame propagation speed are quantified. Governed by standards including GB/T 5455, ASTM D6413, ISO 6941, UL 94 and IEC 60695-11-10, the method supports compliance verification for flame-retardant textiles, plastics, protective clothing and transportation interior materials. This article explains the testing principle, specimen requirements, procedures, burning rate calculation and typical industrial applications.
Technical Overview
Vertical burning rate determination is a laboratory flammability test in which a specimen is clamped in a vertical orientation, ignited at its lower edge or face with a calibrated gas flame, and its combustion behavior is recorded and quantified. Because hot combustion gases rise and preheat the unburned material above the flame front, the vertical configuration represents the most severe and most realistic orientation for evaluating upward flame spread, making it the preferred method for materials that must resist rapid fire growth.
During the test, the position of the flame front is monitored as a function of time, either visually or with marker-thread timing devices, and the vertical burning rate is calculated as the distance traveled by the flame front per unit of time, typically expressed in mm/s. Complementary observations, including afterflame time, afterglow time, damaged (char) length, surface flash, melt dripping and whether the specimen burns through to its top edge, complete the burning-behavior profile of the material.
The most widely applied national and international standards include:
- GB/T 5455 – Burning behaviour of textiles, vertical method (Chinese national standard)
- ISO 6940 / ISO 6941 – Ignitability and flame spread rate of vertically oriented textile specimens
- ISO 15025 – Limited flame spread test methods for protective clothing
- ASTM D6413/D6413M – Flame resistance of textiles (vertical test)
- UL 94 and IEC 60695-11-10 / GB/T 2408 – 20 mm vertical flame tests for plastics (V-0, V-1, V-2 classification)
- ASTM D5048 – 500 W vertical flame tests for plastics (5VA / 5VB classification)
- EN 1102 / EN 1103 – Burning behaviour of curtains and drapes in vertical orientation
- JIS L 1091 – Testing methods for flammability of textiles (Japanese national standard)
Testing Samples
Textile Fabrics and Flame-Retardant Treated Materials
Woven, knitted and nonwoven fabrics, including durably and non-durably flame-retardant finished textiles, workwear fabrics, curtains and drapery. Specimens are prepared in both lengthwise and widthwise directions, because flame spread behavior can differ significantly with fabric construction, yarn twist and finish distribution.
Plastic and Polymeric Materials
Rigid and flexible plastics, thermoplastics, thermosets, compounds, films and molded plaques tested as vertical bar or sheet specimens for classification under UL 94 and IEC 60695-11-10. Specimens are generally tested at the minimum and maximum thicknesses intended for the end product, since burning behavior is strongly thickness-dependent.
Protective Clothing and Technical Textile Components
Materials for flame-retardant protective garments, fire-fighting apparel, welding clothing and their linings, interlinings, sewing threads and accessories. These may be evaluated as individual materials or as assembled multi-layer systems reflecting the end-use configuration.
Coated Fabrics, Composites and Interior Trim Materials
Coated or laminated fabrics, textile-faced foams, seat covers, wall panels and decorative laminates used in furnishings and transportation interiors, which are frequently tested at their end-use construction, thickness and color variants.
Testing Items
Depending on the applicable standard and the end-use specification, the following parameters are determined or observed:
- Ignition time and ease of ignition of the specimen
- Afterflame time, the duration of continued flaming after removal of the ignition flame (s)
- Afterglow time, the duration of glowing combustion without flame (s)
- Damaged length or char length, the extent of destroyed or charred material along the specimen (mm)
- Vertical burning rate or flame spread rate, flame-front travel distance per unit time (mm/s)
- Occurrence of surface flash across the specimen face
- Melting and dripping behavior, and whether flaming drips ignite an indicator medium
- Burning to the top edge, complete consumption, or burning up to the holding clamp
- Material classification, such as flame-retardant grades or UL 94 V-0 / V-1 / V-2 ratings
Testing Methods
Specimen Preparation and Conditioning
Specimens are cut to the dimensions prescribed by the standard, for example 300 mm × 80 mm for the GB/T 5455 vertical textile method or approximately 305 mm × 76 mm for ASTM D6413, with the specimen length aligned with the direction of flame spread to be evaluated. A representative number of specimens, typically at least five per direction, is tested, and additional replicate sets may be required after laundering cycles when wash-durable flame-retardant finishes are being verified. Textile specimens are normally conditioned in a standard atmosphere, commonly 20 ± 2 °C and 65 ± 4 % relative humidity for at least 24 hours, while plastic bar specimens for UL 94 type tests are typically conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 48 hours.
Apparatus and Test Conditions
Testing is performed in a ventilated enclosure or cabinet that prevents ambient drafts while allowing extraction of combustion products. A calibrated gas burner supplied with propane, butane, natural gas or methane as specified delivers a standardized flame, commonly 40 ± 2 mm high for textile vertical methods and a nominal 20 mm blue flame for the UL 94 small-flame vertical test. Precision timers, a graduated scale for char length measurement, absorbent cotton indicators for drip detection in plastics tests, and regulated gas flow controls complete the setup. Ambient conditions, burner-to-specimen geometry and flame application time are strictly controlled, because small deviations can measurably affect results.
Vertical Flame Test Procedure for Textiles
The specimen is clamped in a frame so that it hangs vertically, and the standard flame is applied to the center of the lower edge or, where specified, to the fabric face. After a defined ignition period, typically 12 s under GB/T 5455 and ASTM D6413, the ignition flame is withdrawn and timing begins. The operator records afterflame time, afterglow time, surface flash, whether burning reaches the upper or side edges, and any melting or dripping. Char or damaged length is measured after the specimen has cooled, using a specified tearing or weighted-pin technique that follows the char line to its farthest point. For flame spread rate determination under ISO 6941, marker threads linked to timers are attached at defined heights, and the times at which the advancing flame front severs each thread are used to compute the spread rate.
Vertical Burning Test Procedure for Plastics
For the UL 94 and IEC 60695-11-10 vertical test, bar specimens are suspended vertically above a layer of dry absorbent cotton. The 20 mm blue flame is applied to the lower edge of the specimen for 10 ± 0.5 s, withdrawn, and then re-applied for a second 10 s period once flaming stops or is extinguished. Afterflame and afterglow times after each application, cumulative afterflame time across all specimens, whether the specimen burns up to the clamp, and whether flaming drips ignite the cotton are recorded and compared against the classification criteria to assign a V-0, V-1 or V-2 rating. More severe 500 W vertical tests, corresponding to UL 94 5VA and 5VB classifications under ASTM D5048, apply a larger flame repeatedly to both bar and plaque specimens for the most demanding material requirements.
Calculation of Burning Rate
The vertical burning rate is derived from the flame-front displacement and the corresponding elapsed time. For marker-thread methods, the rate for each interval is calculated as v = d / t, where v is the flame spread rate in mm/s, d is the distance between two successive marker threads in mm, and t is the time for the flame front to travel between them in seconds. For char-length based methods, the average damaged length combined with the total burning time can likewise be used to express an equivalent propagation rate. Reports must state the standard used, specimen direction and thickness, conditioning history, flame height, ignition time and gas type so that results are reproducible and comparable between laboratories.
Applications
- Flame-retardant workwear, fire-fighting clothing and welding protective garments, verifying self-extinguishing behavior and limited char length
- Children's sleepwear and nightwear fabrics intended for regulated consumer markets
- Curtains, draperies, bedding, upholstery and other home furnishing textiles
- Transportation interior materials for automotive, railway, marine and aircraft cabins
- Plastic enclosures, connectors, switchgear housings and other electrical and electronic components requiring V-class materials
- Building interior finishes, decorative laminates and coated banner materials
- Research and development of flame-retardant fibers, finishes and polymer formulations, incoming material inspection and production quality control
FAQ
What is vertical burning rate determination?
It is a flammability test method in which a vertically clamped specimen is ignited by a standardized gas flame and the upward propagation of the flame front is timed and measured, together with parameters such as afterflame time, afterglow time and damaged length, to quantify how quickly and how severely a material burns.
Why is the vertical orientation considered the most severe?
In the vertical position, hot gases and flame rise along the specimen and continuously preheat the unburned material above the flame front, so combustion is self-accelerating. A material that passes a vertical test will generally perform even better in horizontal or inclined configurations, which is why the most demanding applications specify the vertical method.
How is the vertical burning rate calculated?
The rate equals the flame-front travel distance divided by the corresponding time, usually in mm/s. In ISO 6941 type tests, marker threads at known spacings stop individual timers when burned through, giving precise interval times from which average and interval-by-interval spread rates are reported.
What is the difference between UL 94 V-0, V-1 and V-2?
All three classifications use the same 20 mm flame applied twice for 10 s to vertical bar specimens. V-0 requires the shortest afterflame times, no more than 10 s per application and 50 s in total for the specimen set, with no flaming drips igniting the cotton indicator. V-1 allows longer afterflame times, up to 30 s per application and 250 s in total, with no flaming drips igniting cotton. V-2 uses the same timing limits as V-1 but tolerates flaming drips. In terms of severity, V-0 is the most demanding rating, followed by V-1 and then V-2.
How does the vertical test differ from horizontal burning tests?
Horizontal methods, widely used for automotive interior trim materials, measure flame spread along a horizontally mounted specimen where upward preheating is limited, so they yield slower rates and are less severe. Vertical methods impose upward flame spread with strong self-preheating, producing shorter times and greater char propagation, and material selection should always follow the orientation specified by the applicable end-product standard or regulation.
Can vertical burning rate results be used for product certification?
Yes, when the tests are performed by accredited third-party laboratories or properly equipped in-house facilities in strict accordance with the referenced standard, and when specimen construction, thickness and conditioning represent the end-use configuration. Because flammability results are sensitive to thickness, aging and washing history, compliance programs often combine vertical burning rate determination with complementary assessments such as limiting oxygen index, glow-wire or smoldering cigarette tests.