Silicone Coating Horizontal Burning Test: Standards, Methods, and Evaluation Criteria

The silicone coating horizontal burning test is a standardized small-scale flammability procedure used to determine the ignition behavior, flame spread, and linear burning rate of cured silicone (polysiloxane) coatings in a horizontal orientation. Performed in accordance with recognized standards such as UL 94 (HB method), ASTM D635, ISO 1210, IEC 60695-11-10, and GB/T 2408, this test is essential for material qualification, quality control, and compliance verification of flame-retardant silicone coating systems. This article explains the sample requirements, test items, methodology, classification criteria, and typical applications of the horizontal burning test for silicone coatings.

Technical Overview

The horizontal burning test is one of the most widely used bench-scale fire test methods for evaluating the flammability characteristics of polymeric materials, including coatings, films, laminates, and coated textiles. In this method, a bar-shaped or strip-shaped specimen is mounted in a horizontal position and exposed to a small pilot flame from a laboratory burner for a defined application time. The test measures whether the specimen ignites, how far the flame front travels, and how quickly it propagates between reference marks on the specimen surface.

Silicone coatings are of particular interest in flammability evaluation because their inorganic silicon–oxygen backbone provides inherently favorable fire performance compared with most organic polymer coatings. During combustion, silicone materials tend to form a stable silica-like ash or ceramic residue that acts as a thermal barrier and insulates the underlying layers, suppressing further flame spread. Quantifying this behavior through a standardized horizontal burning test allows manufacturers to verify that a silicone coating formulation, application process, or coated article meets contractual, regulatory, and end-product flammability requirements.

The horizontal method is generally applied to materials expected to exhibit moderate to low flammability, and it produces objective, reproducible data including linear burning rate (in mm/min), extent of burned length, afterflame time, afterglow time, and dripping behavior. Testing is typically conducted by accredited third-party laboratories or in-house fire test facilities, and results are reported against the classification criteria of the applicable standard.

Testing Samples

Free-Standing Silicone Coating Films

When the flammability of the coating itself must be characterized independently of the substrate, a cured free film is prepared by casting or pressing the silicone coating at a controlled, uniform thickness. The film is then cut into standard strip specimens, typically 125 mm in length and approximately 13 mm in width, with the thickness recorded at multiple points. Free-film testing is commonly used for formulation development, batch comparison, and supplier material qualification.

Silicone-Coated Flexible Substrates

For silicone-coated fabrics, such as silicone-coated glass fiber cloth, aramid textiles, or composite flexible laminates used in welding blankets, fire curtains, and thermal insulation jackets, specimens are cut from the coated article with the coating layer fully intact. The specimen thickness is taken as the total laminate thickness, and the test evaluates the flammability of the coating-to-substrate system as it performs in service.

Silicone-Coated Rigid Substrates and Cable Materials

Silicone coatings applied to rigid panels, composite boards, and wire or cable constructions can also be evaluated in strip form where the geometry and thickness comply with the requirements of the selected standard. For elongated products such as coated wires, insulating sleeves, and cable jacket strips, the horizontal burning method provides a practical measure of surface flame propagation behavior.

Specimen Preparation and Conditioning

Specimens must be cut cleanly with smooth edges, since fraying, delamination, or irregular edges can distort ignition and flame spread behavior. Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 291 or ASTM D618, typically for a minimum of 48 hours. The cure schedule of the silicone coating should be fully documented, because incomplete curing can significantly alter ignition time, burning rate, and char formation. A minimum of three specimens is required by most standards, although five or more specimens are recommended to improve statistical confidence.

Testing Items

The horizontal burning test on silicone coatings generates the following core data points:

• Ignition behavior, including whether the specimen ignites during the 30-second flame application period.
• Time for the flame front to travel between the 25 mm and 100 mm reference marks, recorded to the nearest second.
• Linear burning rate in mm/min, calculated from the flame travel time between reference marks.
• Extent of burning and damaged length, including whether the flame front reaches or passes the 100 mm mark.
• Afterflame time (persistence of flaming after removal of the pilot flame) and afterglow time (persistence of glowing combustion without flame).
• Dripping behavior, distinguishing flaming drips from non-flaming drips, and whether drips ignite an indicator medium where the method requires it.
• Surface combustion phenomena characteristic of silicones, including silica ash or ceramic residue formation, char integrity, and afterglow behavior.
• Classification result against the applicable standard, such as UL 94 HB, IEC 60695-11-10 HB / HB 40 / HB 75, or the FH classifications of ISO 1210 and GB/T 2408.

Testing Methods

Apparatus and Test Conditions

The standard test setup consists of a draft-free enclosure or fire test chamber, a laboratory burner (typically a Bunsen or Tirrill burner fitted with a wing tip) adjusted to produce a blue flame of approximately 20 mm in height, a specimen clamp with a wire ring support that holds the specimen horizontally, reference marks at 25 mm and 100 mm from the ignition end, and a timing device accurate to at least one second. A dry, untreated cotton indicator pad is positioned beneath the specimen where the method requires evaluation of flaming drips. Ambient conditions in the chamber are maintained within the range specified by the standard, and air currents that could disturb the flame are excluded.

Test Procedure

The general procedure, common to UL 94 HB, ASTM D635, ISO 1210 method A, and GB/T 2408 horizontal method, is as follows:

• Condition the specimens, then mark reference lines at 25 mm and 100 mm from the end to be ignited.
• Mount the specimen horizontally on the support with the marked end free for flame application.
• Ignite the burner and adjust the flame to the specified height with a stable blue cone.
• Incline the burner to approximately 45 degrees and apply the flame to the free end of the specimen for 30 ± 1 seconds, then withdraw the burner.
• If the specimen does not ignite during the application period, record the result as no ignition.
• If the specimen ignites and the flame front reaches the 25 mm mark, start the timer and record the time required for the flame front to reach the 100 mm mark, or the point of self-extinguishment if the flame stops earlier.
• Record afterflame time, afterglow time, and any dripping behavior, noting whether drips are flaming or non-flaming.
• Repeat the test on the remaining specimens and report individual and averaged results together with the classification.

Calculation of Burning Rate

When the flame front passes both reference marks, the linear burning rate is calculated as the distance traveled (75 mm between the 25 mm and 100 mm marks) divided by the elapsed time, converted to mm/min. For example, a flame front requiring 150 seconds to travel from the 25 mm mark to the 100 mm mark corresponds to a linear burning rate of 30 mm/min. If the specimen self-extinguishes before the 100 mm mark, the burning rate is reported as not applicable and the result is based on the extinguishment behavior and damaged length.

Classification Criteria

Under the UL 94 HB and IEC 60695-11-10 horizontal criteria, a material is considered to meet the horizontal burning requirement when the specimens either extinguish before the flame front reaches the 100 mm mark, or exhibit a linear burning rate not exceeding 40 mm/min for specimens 3.0 mm to 13 mm in thickness, and not exceeding 75 mm/min for specimens less than 3.0 mm in thickness. The more restrictive HB 40 and HB 75 designations under IEC 60695-11-10 correspond to these thickness-dependent rate limits. Under ISO 1210 and the corresponding GB/T 2408 horizontal method, results are expressed as FH classifications that distinguish specimens on which the flame front does not pass the first reference mark, specimens that pass the 25 mm mark but not the 100 mm mark, and specimens that pass the 100 mm mark at rates below or above the specified limits. Because silicone coatings frequently self-extinguish or form an insulating ash layer that arrests flame propagation, well-formulated systems commonly achieve results at the most favorable end of these classifications.

Applications

Horizontal burning test data for silicone coatings support a broad range of engineering and compliance purposes:

• Wire and cable systems: qualification of silicone rubber insulation, jackets, and coated sleeving for high-temperature, halogen-free, and low flame spread constructions used in power, transit, and industrial installations.
• Fire protection textiles: verification of coated glass fiber and aramid fabrics for welding blankets, smoke curtains, fire barriers, expansion joints, and removable thermal insulation jackets.
• Electrical and electronic assemblies: evaluation of silicone conformal coatings, protective sleeves, and coated components where surface flammability control is required.
• Automotive and transportation: assessment of coated interior and engine-bay materials, including components evaluated against vehicle interior flammability requirements such as EN ISO 3795 where applicable.
• Aerospace and industrial thermal protection: screening and qualification of silicone-coated fabrics and composites for heat shields, thermal blankets, and protective covers.
• Building and construction: performance confirmation of silicone-coated weatherproofing and protective layers incorporated into fire-resistant assemblies.
• Quality control and R&D: batch-to-batch conformity checks, comparison of coating formulations and fillers, and support of certification files submitted to accredited third-party laboratories.

FAQ

Which standards govern the horizontal burning test for silicone coatings?

The most commonly applied standards are UL 94 (horizontal burning, HB method), ASTM D635, ISO 1210, IEC 60695-11-10, and the Chinese national standard GB/T 2408. These standards are technically harmonized in specimen geometry, burner flame application, and rate measurement, but differ in certain details, so the applicable standard should be confirmed with the specification owner or the end customer before testing begins.

Can a silicone coating be tested separately from its substrate?

Yes, provided a coherent cured free film of sufficient thickness and integrity can be prepared. Free-film testing characterizes the intrinsic flammability of the coating composition, while testing the coated article characterizes the system performance in service. Many certification programs require evaluation of the material in its end-use configuration, so both approaches may be relevant.

What do HB, HB 40, and HB 75 classifications mean?

These are horizontal burning classifications. HB indicates the material either self-extincedes before the 100 mm reference mark or burns at a rate below the applicable limit. HB 40 corresponds to the limit of 40 mm/min applied to specimens 3.0 mm to 13 mm thick, and HB 75 corresponds to the 75 mm/min limit applied to specimens thinner than 3.0 mm, as defined in IEC 60695-11-10.

How is the linear burning rate calculated?

The rate is calculated by dividing the 75 mm distance between the 25 mm and 100 mm reference marks by the measured flame travel time in seconds, then multiplying by 60 to express the result in mm/min. Specimens that extinguish before the 100 mm mark are reported on the basis of extinguishment rather than rate.

Do ash formation and dripping affect the result?

Silica-rich ash formation is a beneficial and characteristic behavior of silicone coatings, and a coherent residue that suppresses further burning often contributes to self-extinguishment. Dripping, particularly flaming drips, is recorded as part of the observations and may influence acceptance under certain specifications, so it should always be documented in the test report.

When should the horizontal method be used instead of the vertical method?

The horizontal method is appropriate for general material screening, quality control, and materials of moderate flammability. The vertical burning method, which applies the flame to the lower end of a vertically mounted specimen, is more severe and is typically required for engineering plastics and components in electrical equipment where higher flame retardancy ratings such as V-0 are specified.

How can reproducible results be ensured?

Reproducibility depends on strict control of specimen preparation, complete and documented curing of the silicone coating, accurate thickness measurement, standardized conditioning, draft-free test conditions, and consistent burner flame adjustment. Engaging accredited third-party laboratories with calibrated apparatus and experienced operators further improves the reliability and acceptance of test data.