Combustion Performance Testing of Spray Polyurea Coatings: Standards, Methods, and Classifications

Spray polyurea coatings are high-performance elastomers widely used for waterproofing, corrosion protection, and impact resistance, yet as organic polymer materials they must be evaluated for fire safety in construction, transportation, and industrial applications. Combustion performance testing of spray polyurea measures ignitability, flame spread, heat release, smoke production, and fire effluent toxicity in accordance with standards such as GB 8624, EN 13501-1, ISO 4589-2, ASTM E1354, and UL 94. This article provides a technical overview of the specimen requirements, test items, methods, and typical classification outcomes for spray polyurea flammability evaluation.

Technical Overview

Spray polyurea is a two-component elastomer formed by the rapid reaction of an isocyanate prepolymer with an amine-terminated polyether blend, applied through plural-component spray equipment to produce a seamless, fast-curing membrane. Because polyurea is a crosslinked organic polymer, it is combustible under sustained flame or high heat flux, even though many formulations exhibit self-extinguishing behavior, rapid char formation, and no melt dripping due to their high crosslink density.

Combustion performance testing is therefore essential whenever spray polyurea is installed in fire-regulated environments such as tunnels, metro systems, railway bridges, building roofs, pipelines, and chemical storage facilities. The testing program evaluates the principal fire hazards of the material: ease of ignition, lateral and vertical flame propagation, rate of heat release, total heat released, smoke density, burning droplet formation, and the toxicity of combustion gases. Results are used to assign a fire classification under national or international schemes, most commonly GB 8624 in China, EN 13501-1 in Europe, and ASTM E84 or UL 723 tunnel classifications in North America.

Accredited third-party laboratories typically conduct these tests on specimens prepared under controlled spray conditions, with the coating either tested as a free film or applied to its intended end-use substrate, since the substrate strongly influences the observed burning behavior.

Testing Samples

Free-Film Specimens

Free films are produced by spraying the polyurea system onto a release surface (polyethylene sheet or PTFE-lined board), allowing full cure, and then cutting panels to the dimensions required by each test method. Typical film thicknesses are 1.5 mm to 3.0 mm, matching common field application ranges. Free-film testing characterizes the intrinsic combustion properties of the elastomer itself, such as limiting oxygen index and UL 94 burning behavior, and is frequently used for quality control and formulation comparison of flame-retarded polyurea systems.

Coated Substrate Panels

For classification-oriented testing, the polyurea is spray-applied to the substrate used in service, such as concrete slabs, steel plates, plywood, or rigid polyurethane foam insulation. End-use configuration testing is mandated by most building product classification standards because flame spread, smoke production, and heat release depend significantly on the thermal properties and combustibility of the substrate. Coating thickness, spray parameters, primer use, and substrate preparation must be documented in the test report.

Specimen Conditioning and Quantity

Specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours (or to constant mass) in accordance with ISO 291 / GB/T 2918 prior to testing. Most methods require replicate sets, for example five to six specimens for oxygen index and UL 94 testing, three specimens for cone calorimetry, and three to four panels for single burning item testing. Free films and coated panels are sealed on the rear face and edges where the standard requires it, so that only the exposed surface participates in combustion.

Testing Items

Depending on the target market and application, a complete combustion performance evaluation of spray polyurea may include the following test items:

• Limiting oxygen index (LOI) to ISO 4589-2 / ASTM D2863 / GB/T 2406.2
• Single-flame source ignitability to GB/T 8626 / EN ISO 11925-2
• Single burning item (SBI) test to GB/T 20284 / EN 13823 for FIGRA, THR600s, LFS, and SMOGRA values
• Building materials burning behavior classification to GB 8624 or EN 13501-1
• Surface burning characteristics (flame spread and smoke developed indices) to ASTM E84 / UL 723 Steiner tunnel
• Heat release rate and total heat release by cone calorimetry to ISO 5660-1 / ASTM E1354 / GB/T 16172
• Smoke density rating to GB/T 8627 or specific optical density to ASTM E662
• Vertical and horizontal burning (50 W and 500 W flame) to UL 94 / IEC 60695-11-10 / GB/T 2408 / GB/T 2407
• Toxicity classification of fire effluents to GB/T 20285
• Afterflame time, afterglow time, and burning droplet or debris observation

Testing Methods

Limiting Oxygen Index (LOI)

In the oxygen index method (ISO 4589-2, ASTM D2863, GB/T 2406.2), a bar-shaped specimen is clamped vertically and ignited at its top edge in a glass chimney through which a controlled mixture of oxygen and nitrogen flows upward. The minimum oxygen concentration, in volume percent, that just supports candle-like flaming combustion for 50 mm of burning or 180 seconds is determined. Conventional polyurea typically shows an LOI of about 21% to 24%, while flame-retarded formulations can reach 27% to 32% or higher. A higher LOI indicates better ignition resistance in air (ambient oxygen content is approximately 21%).

Single-Flame Source Test (GB/T 8626 / EN ISO 11925-2)

A small propane flame (typically 15 s or 30 s exposure) is applied to the bottom edge or surface of a vertically mounted specimen. Pass criteria require that flaming does not reach the 150 mm gauge line within a prescribed time and that no flaming droplets ignite filter paper below the specimen. This screening test is a prerequisite for Class E and higher classifications under both GB 8624 and EN 13501-1, and well-formulated polyurea coatings generally pass because they char rapidly and do not produce flaming drips.

Single Burning Item (SBI) Test (GB/T 20284 / EN 13823)

The SBI test exposes a corner assembly of two specimens (495 mm × 1500 mm long wing and 495 mm × 800 mm short wing) to a 30 kW sandbox burner placed at the base of the vertical corner. Instrumentation measures heat release rate by oxygen consumption and smoke production by laser optical attenuation. The key outputs, FIGRA (fire growth rate), THR600s (total heat release within 600 s), LFS (lateral flame spread), and SMOGRA (smoke growth rate), determine whether the product achieves B1, B2, C, or D classification levels with associated smoke classes s1 to s3 and droplet classes d0 to d2.

Steiner Tunnel Test (ASTM E84 / UL 723)

For North American projects, the specimen is mounted as the ceiling of a 7.6 m tunnel and exposed to a controlled gas burner with a 240 km/h airflow for 10 minutes. Flame spread index (FSI) and smoke developed index (SDI) are calculated against red oak and inorganic board references. Sprayed elastomeric coatings applied to noncombustible substrates are commonly evaluated for Class A (FSI ≤ 25, SDI ≤ 450) acceptance in wall, ceiling, and tunnel lining applications.

Cone Calorimetry (ISO 5660-1 / ASTM E1354 / GB/T 16172)

A 100 mm × 100 mm horizontal specimen is exposed to a radiant cone heater at irradiance levels of 25 to 75 kW/m², with ignition by a spark igniter. Oxygen consumption calorimetry yields time to ignition, peak and average heat release rate, total heat release, effective heat of combustion, mass loss rate, and smoke production rate. Cone calorimeter data are valuable for research, fire modeling, and comparing flame-retardant polyurea formulations, though they do not by themselves provide a regulatory classification.

Smoke Density Testing (GB/T 8627 / ASTM E662)

Smoke production is assessed in a closed chamber where the specimen is exposed to a radiant source (with or without piloted ignition) and the attenuation of a light beam across the chamber is recorded. The smoke density rating or specific optical density (Ds) values feed into the smoke classification (s1, s2, s3 or S-class) required by GB 8624 and by tunnel and metro project specifications, where low smoke generation is often a mandatory purchase requirement.

Vertical and Horizontal Burning (UL 94 / IEC 60695-11-10 / GB/T 2408)

Small-specimen burning tests apply a 50 W (or 500 W) premixed flame to molded or cut bars. Afterflame time, afterglow time, cotton ignition by flaming drips, and burn-up behavior determine ratings from V-0, V-1, V-2, and 5VA down to HB. These ratings are most relevant for polyurea used in electrical enclosures, transportation equipment, and component-level applications.

Fire Effluent Toxicity (GB/T 20285)

Specimens are decomposed under defined smoldering and flaming conditions, and the concentrations of asphyxiant and irritant gases, including carbon monoxide, carbon dioxide, hydrogen cyanide, hydrogen chloride, hydrogen fluoride, sulfur dioxide, nitrogen oxides, and formaldehyde, are quantified. Hazard quotients are used to assign toxicity grades (AQ1, AQ2, AQ3), which are frequently specified alongside GB 8624 fire classes for enclosed spaces such as road tunnels and rail vehicles.

Applications

Combustion performance data for spray polyurea support fire safety compliance and material selection across many demanding sectors:

• Rail transit and road tunnels, where B1 classification, low SMOGRA, and AQ-level toxicity grades are typically mandated for protective and waterproofing linings
• Building roofing and underground waterproofing systems requiring GB 8624 or EN 13501-1 classification for the installed assembly
• Oil and gas pipelines, tank exteriors, and secondary containment, where coatings must resist ignition during hot-work or fire exposure scenarios
• Marine and offshore structures subject to SOLAS-related surface flammability and low flame spread requirements
• Military, blast, and impact protection installations combining energy absorption with verified flame retardance
• Industrial equipment linings, chutes, and wear surfaces near ignition sources or high-temperature operations
• Theme park scenery and architectural elements needing Class A or B surface burning characteristics over various substrates

FAQ

Is spray polyurea flammable? Yes, as an organic elastomer, polyurea will ignite and burn under sustained high heat flux. However, its rapid gel, high crosslink density, and tendency to form a protective char layer mean most commercial grades self-extinguish after removal of the flame and produce no flaming melt drips, allowing flame-retarded formulations to reach B1 (GB 8624) or Class B/C (EN 13501-1) levels.

Which classification is usually required for tunnel and metro projects? Specifications commonly call for B1 (GB 8624) with a low smoke grade (s1) and smoke toxicity grade AQ1 or AQ2 per GB/T 20285, supported by SBI results (FIGRA, THR600s, SMOGRA) and single-flame source testing. Projects in North America instead reference ASTM E84 Class A.

Does the substrate affect the test results? Significantly. The same polyurea film tested free-standing, over concrete, or over combustible polyurethane foam will show different heat release, flame spread, and smoke results. Classification testing must therefore use the end-use substrate and thickness declared for the application.

What specimen thickness should be tested? The thickness used in actual installation, typically 1.5 mm to 3.0 mm, should be tested and clearly stated in the report, since LOI, heat release, and afterflame behavior can vary with thickness.

How long does a combustion test program take? Including specimen preparation, spray application, curing, conditioning, replicate testing, and reporting, a standard program covering LOI, single-flame source, SBI, smoke density, and toxicity generally takes two to four weeks at an accredited third-party laboratory, with multi-substrate programs requiring additional time.