The vertical burning test for silicone coatings is a standardized bench-scale flammability evaluation in which coated specimens are exposed to a controlled 20 mm methane flame in a vertical orientation and classified as V-0, V-1, or V-2 based on afterflame time, afterglow behavior, and dripping characteristics. Performed in accordance with recognized schemes such as UL 94, IEC 60695-11-10, ISO 1210, ASTM D3801, and GB/T 2408, the method is widely used by manufacturers, quality engineers, and accredited third-party laboratories to verify the flame-retardant performance of silicone-coated fabrics, sleeving, cable materials, and composite substrates. This article explains the sample requirements, measured parameters, step-by-step test procedure, classification criteria, and typical applications of the method.
Technical Overview
Silicone coatings are applied extensively over glass-fiber fabrics, braided sleeving, wires and cables, and various flexible or rigid substrates in order to combine high thermal stability, weathering resistance, and electrical insulation with inherent flame-retardant behavior. Because end products containing these coated materials are frequently deployed in regulated fire-safety environments, their combustion characteristics must be quantified in a reproducible, laboratory-controlled manner. The vertical burning method is one of the most widely accepted small-scale techniques for this purpose, and it forms the basis of the well-known V-0 / V-1 / V-2 classification system.
In a vertical burning test, a strip-shaped specimen is clamped with its long axis vertical, and a standardized premixed blue flame approximately 20 mm in height is applied to the lower end of the specimen for a defined duration. The behavior of the specimen after the flame is withdrawn — including afterflame time, afterglow time, flaming drips, and whether burning reaches the clamp — is recorded and compared against acceptance limits to assign a flammability classification.
The test is most commonly performed under the following frameworks, all of which are technically harmonized in their core 50 W vertical burning procedure:
- UL 94, Tests for Flammability of Plastic Materials for Parts in Devices and Appliances (vertical burning, V-0/V-1/V-2 method)
- ASTM D3801, covering the comparative burning characteristics of solid plastics in a vertical position
- IEC 60695-11-10, fire hazard testing of electrotechnical products using the 50 W vertical flame application
- ISO 1210, determination of burning characteristics by horizontal and vertical burning
- GB/T 2408 and GB/T 5169.16, Chinese national standards technically aligned with the corresponding IEC methodology
Accredited third-party laboratories routinely conduct this test for material qualification, supplier approvals, and regulatory compliance submissions.
Testing Samples
Silicone-coated materials reach the laboratory in many product forms. The laboratory prepares test specimens of the specified dimensions, verifies coating continuity, and records substrate type, coating construction, and thickness, since all of these factors influence burning behavior.
Silicone-Coated Glass Fiber Fabrics
Coated glass cloth used for welding blankets, thermal insulation jackets, expansion joints, and fire barriers is cut into strips of nominally 125 mm in length and 13 mm in width. Specimens are cut in both machine and transverse directions when fabric orientation may affect burning behavior. Damaged length and char formation may additionally be evaluated using complementary vertical flame methods for textiles such as GB/T 5455.
Silicone-Coated Sleeving, Tapes, and Cable Materials
Braided fiberglass sleeving with a silicone coating, self-fusing or adhered tapes, and insulation or jacket compounds intended for wire and cable are tested as strips of the standard dimensions, or, where product-specific schemes apply, on finished constructions mounted in a representative vertical orientation.
Silicone-Coated Rigid and Flexible Substrates
Free-standing coating plaques, coated polymer films, coated composites, and metal-backed specimens can also be evaluated. When the coating alone is of interest, specimens of the cured coating are prepared at the nominal applied thickness so that the classification reflects the coating system itself rather than the substrate.
Specimen Preparation and Conditioning
A standard evaluation uses five specimens per set. Specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 48 hours prior to testing, and additional oven preconditioning may be applied where the qualification program requires it. Thickness is measured and reported, and both the thinnest and thickest commercially available constructions should be considered for a complete material qualification.
Testing Items
The vertical burning method quantifies the following parameters for each specimen and for the complete set of five:
- Afterflame time after the first flame application (t1), in seconds
- Afterflame time after the second flame application (t2), in seconds
- Afterglow time after the second flame application (t3), in seconds
- Total afterflame time summed over all ten flame applications for the set
- Whether flaming drips or fallen particles ignite the cotton indicator placed beneath the specimen
- Whether any specimen exhibits afterflame or afterglow extending up to the clamp
- Qualitative observations such as charring, intumescence, coating delamination, and substrate exposure
- Final classification of the material as V-0, V-1, V-2, or no classification (failure)
Together these items describe both the ignition resistance of the silicone-coated material and its tendency to propagate fire through flaming debris.
Testing Methods
Testing is performed in an enclosed, ventilated burning chamber equipped with a specimen clamp that grips the top of the strip, a laboratory burner supplied with methane of at least 98 % purity, a timing device, and a pad of dried absorbent cotton positioned 300 mm below the lower edge of the specimen.
The general procedure is as follows:
- Mount the specimen vertically with its lower edge 10 mm above the top of the burner tube.
- Adjust the burner to produce a blue flame of 20 mm height with a well-defined inner cone.
- Apply the flame to the lower edge of the specimen for 10 seconds, then withdraw it and record the afterflame time t1.
- After flaming ceases, reapply the flame for a further 10 seconds, withdraw it, and record the afterflame time t2 and the subsequent afterglow time t3.
- Observe whether flaming drips ignite the cotton indicator and whether burning reaches the clamp.
- Repeat the sequence for all five specimens, sum the afterflame times, and assign the classification.
Classification Criteria
The acceptance limits for each class are summarized below:
- V-0: afterflame time of each individual specimen does not exceed 10 seconds; total afterflame time for the set does not exceed 50 seconds; afterflame plus afterglow of each specimen after the second application does not exceed 30 seconds; no flaming drips ignite the cotton.
- V-1: afterflame time of each individual specimen does not exceed 30 seconds; total afterflame time for the set does not exceed 250 seconds; afterflame plus afterglow after the second application does not exceed 60 seconds; no flaming drips ignite the cotton.
- V-2: identical timing limits to V-1, except that flaming drips that ignite the cotton indicator are permitted.
- Failure or lower classification: the material receives no vertical rating if any specimen burns up to the clamp, if glowing or flaming persists beyond the limits, or if the timing criteria are exceeded.
Where individual specimens produce marginal results, the standards prescribe supplementary testing of additional specimen sets before the final classification is confirmed.
Test Report
The report should identify the material and coating system, specimen dimensions and thickness, conditioning history, the standard and edition applied, individual and total afterflame and afterglow times, cotton ignition results, all observations, and the assigned classification.
Applications
Vertical burning data for silicone-coated materials support design, procurement, and compliance decisions across many industries:
- Wire and cable: qualification of silicone-based insulation, jackets, and coated braiding used in appliance cordage, harness protection, and high-temperature wiring.
- Electric vehicles and energy storage: fire-barrier fabrics, cell-wrapping materials, and thermal runaway mitigation layers evaluated as part of battery safety programs.
- Rail transit, marine, and aerospace: interior and insulation materials where flame spread and dripping behavior are tightly regulated.
- Industrial thermal insulation: silicone-coated glass cloth for removable insulation blankets, welding curtains, and expansion joints.
- Electronics and electrical equipment: conformal silicone coatings and coated components assessed under UL 94 and IEC 60695-11-10 as part of enclosure and part-level flammability qualification.
- Building and construction: fire curtains, smoke seals, and coated membrane materials requiring documented flame-retardant performance.
FAQ
What does a V-0 rating mean for a silicone coating?
V-0 is the highest rating in the vertical burning scheme. It means the coated specimen self-extinguishes within 10 seconds after each of the two flame applications, the cumulative afterflame time for five specimens remains below 50 seconds, and no flaming drips ignite the cotton indicator.
Can a silicone coating by itself guarantee a V-0 rating?
No. The result depends on the coating formulation (for example platinum-cured systems and flame-retardant filler loading), applied thickness, substrate type, and specimen geometry. Verification by testing at the minimum and maximum production thicknesses is recommended.
How does the vertical burning test differ from the horizontal burning test?
The vertical orientation is more severe because the specimen is heated by its own flame from below and along its length, producing stronger piloted ignition and upward flame spread. Materials that pass the horizontal test may still fail the vertical test, so the vertical method is generally specified for more demanding applications.
Which standards are mutually recognized for this test?
UL 94, IEC 60695-11-10, ISO 1210, ASTM D3801, and GB/T 2408 describe essentially the same 50 W vertical flame procedure, with minor editorial differences. Purchasers should confirm which specific standard and edition is referenced by the applicable product regulation.
How long does testing take?
The burning sequence itself is short, but standard conditioning of at least 48 hours, specimen preparation, and reporting mean a typical turnaround of several working days at an accredited third-party laboratory, and longer when oven preconditioning or additional specimen sets are required.