Rubber Smoke Density and Toxicity Testing: Standards, Methods, and Applications

Rubber smoke density and toxicity testing evaluates how much smoke a rubber material generates in a fire and how hazardous its combustion gases are, using internationally recognized methods such as ISO 5659-2, ASTM E662, and the IMO FTP Code. This technical guide explains the testing principles, applicable standards, specimen requirements, measured parameters, and typical applications of smoke emission and fire gas toxicity testing for rubber and elastomeric materials. Manufacturers of rubber components for rail vehicles, ships, buildings, cables, and industrial equipment rely on these tests to demonstrate compliance with fire safety regulations.

Technical Overview

In most fire scenarios, the leading cause of casualties is not direct flame contact but the inhalation of smoke and toxic combustion gases. Statistical studies of fire incidents consistently attribute the majority of fire deaths to smoke asphyxiation and intoxication, combined with reduced visibility that hampers evacuation. Rubber and elastomeric materials, which are widely used in enclosed environments such as rail carriages, ships, tunnels, and buildings, are therefore subject to strict fire safety requirements governing both the optical density of the smoke they produce and the toxicity of the gases released during decomposition and combustion.

Smoke density testing quantifies the attenuation of a light beam passing through smoke accumulated in a closed environmental chamber, yielding parameters such as specific optical density (Ds) and maximum specific optical density (Ds max). Smoke toxicity testing identifies and quantifies hazardous gases such as carbon monoxide, hydrogen cyanide, hydrogen halides, and sulfur dioxide generated under controlled fire conditions. Depending on the end-use sector, results are assessed against requirements in standards such as ISO 5659-2, ASTM E662, GB/T 8323, EN 45545-2, and the IMO FTP Code. Accredited third-party laboratories routinely perform these tests for product certification, material qualification, and research and development purposes.

Testing Samples

Raw Rubber Compounds and Vulcanized Sheets

Compound developers and material suppliers submit uncured or, more commonly, vulcanized rubber sheets for evaluation. Typical polymer families include natural rubber (NR), styrene-butadiene rubber (SBR), ethylene-propylene-diene rubber (EPDM), nitrile rubber (NBR/HNBR), chloroprene rubber (CR), butyl rubber (IIR), silicone rubber (VMQ), and fluoroelastomers (FKM). Standard specimens measure 75 mm × 75 mm, with a preferred thickness of 2 mm to 3 mm, and are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity prior to testing in accordance with ISO 291.

Rubber Components and Finished Products

Finished articles such as door and window seals, gaskets, O-rings, vibration isolators, hoses, bellows, rubber flooring, matting, and conveyor belt samples can be tested by cutting representative specimens from the product. Where components are laminated, coated, or reinforced with metal or textile inserts, the complete construction is tested so that the smoke and toxicity behavior reflects real service conditions. Specimen orientation and surface finish are recorded because they can influence burning behavior.

Rubber Cable Materials and Sheathing

Elastomeric insulation and sheath compounds used in low-smoke zero-halogen (LSZH) and flame-retardant cables are tested as plaques using chamber methods, while complete cables may additionally be evaluated for smoke emission using the 3 m³ chamber method described in IEC 61034. This verifies that halogen-free cable designs achieve the high light transmittance levels demanded in tunnels, power plants, and public infrastructure.

Testing Items

Smoke density and toxicity test programs for rubber materials typically cover the following parameters:

• Specific optical density (Ds), including Ds at 4 minutes and Ds max, measured under flaming and non-flaming exposure conditions
• VOF4, the cumulative smoke value over the first four minutes, required by EN 45545-2 for rail applications
• Mass optical density (MOD), normalizing smoke production to specimen mass loss
• Smoke Density Rating (SDR) per ASTM D2843 where applicable
• Light transmittance (%) through the accumulated smoke, used for cable tests per IEC 61034
• Concentrations of toxic gases, including CO, CO₂, HCN, HCl, HBr, HF, SO₂, NOₓ, formaldehyde, and acrolein
• Conventional Index of Toxicity (CIT) calculated in accordance with EN 45545-2 and EN 17077
• Fractional effective dose (FED) and fractional effective concentration (FEC) assessments based on ISO 13571
• Toxicity index for naval applications determined under NES 713
• Mass loss and residual mass of the specimen after exposure

Testing Methods

Smoke Density Chamber Method (ISO 5659-2 / ASTM E662 / GB/T 8323)

The single-chamber smoke density test, originally developed at the National Bureau of Standards, is the principal method for rubber materials. A 75 mm × 75 mm specimen is mounted in a holder and exposed, without an imposed ignition source (non-flaming or smoldering mode) or with a pilot flame (flaming mode), to radiant heat fluxes of 25 kW/m² or 50 kW/m² inside a closed 0.51 m³ chamber. A photometric system, using either a white light or laser light source, continuously measures the transmittance of a horizontal light beam across the chamber as smoke accumulates. The measured transmittance is converted into specific optical density, which is plotted against time. Typically, at least three valid specimens are tested per condition, and the test runs for up to 20 minutes or until minimum transmittance is reached. GB/T 8323 is technically aligned with the ISO 5659 series and is widely used for Chinese market certification.

Smoke and Toxicity Testing under the IMO FTP Code

For shipbuilding applications, the International Maritime Organization FTP Code, Annex 1, Part 2, specifies smoke generation and toxicity requirements for surface materials required to not exhibit excessive smoke and toxic gas production, including bulkhead and ceiling linings, wall and ceiling linings, and floor coverings. Smoke density is determined using the ISO 5659-2 chamber at 25 kW/m² in both flaming and non-flaming modes, and the combustion atmosphere is simultaneously sampled and analyzed for CO, CO₂, HCN, HCl, HBr, HF, and SO₂. Results must remain below the concentration limits defined in the Code for the material to be acceptable for use aboard ships subject to SOLAS requirements.

FTIR-Coupled Toxic Gas Analysis (EN 45545-2 / EN 17077)

In the railway sector, EN 45545-2 combines smoke density testing per ISO 5659-2 with online Fourier-transform infrared (FTIR) spectroscopy of the evolved gases. The FTIR analyzer continuously quantifies a defined set of gases during the chamber test, and the results are used to calculate the Conventional Index of Toxicity (CIT). Different product categories and hazard levels (HL1, HL2, HL3) within EN 45545-2 set corresponding limiting values for VOF4, Ds max, and CIT, with stricter requirements for vehicles operating in tunnels and unmanned scenarios with long evacuation times. EN 17077 further specifies the toxicity test methodology for materials and components used on trains, harmonizing the FTIR sampling and calculation approach.

Supplementary Toxicity Assessment Methods

Additional methods may be applied depending on the specification. ISO 19700, the steady-state tube furnace, evaluates toxic gas yields under controlled, dynamic combustion conditions at different equivalence ratios, providing data suitable for fire hazard engineering analysis. NES 713 determines a toxicity index from the gases produced by small specimens decomposed in a closed chamber and is often referenced in naval specifications. IEC 61034 measures the transmittance of smoke produced by horizontally mounted cables burned in a 27 m³ enclosure, which is the reference method for smoke performance of elastomeric cable systems. Guidance on gas sampling and analytical techniques for fire effluents is given in ASTM E800.

Applications

Smoke density and toxicity testing of rubber is applied across many industries:

• Railway vehicles: qualification of rubber window seals, floor mats, gangway bellows, hoses, and cable sheaths under EN 45545-2 and regional standards such as DIN 5510-2, BS 6853, and NF F 16-101
• Marine and offshore: certification of linings, floor coverings, gaskets, and sealing profiles under the IMO FTP Code for passenger ships, cargo vessels, and offshore units
• Building and civil engineering: classification of rubber flooring, expansion joint profiles, and sealing strips within national reaction-to-fire frameworks such as GB 8624 and EN 13501
• Mining and tunneling: verification of conveyor belts, ventilation ducting, and cable compounds where smoke control and self-rescue time are critical
• Wire and cable: development and certification of low-smoke, halogen-free elastomeric compounds using ISO 5659-2, IEC 61034, and related product standards
• Research and development: screening of flame-retardant formulations, comparison of filler systems such as aluminum hydroxide and magnesium hydroxide, and quality control during compound production

Test reports from accredited third-party laboratories are commonly required for type approval, supplier qualification, and regulatory submission in all of these sectors.

FAQ

What is the difference between smoke density and smoke toxicity testing?

Smoke density testing measures how strongly the smoke attenuates light, which relates to visibility during evacuation. Smoke toxicity testing measures the chemical composition and concentration of hazardous gases produced by the decomposing or burning material, which relates to incapacitation and lethal risk. Many specifications, such as EN 45545-2 and the IMO FTP Code, require both assessments, and modern test setups can obtain them simultaneously by coupling the smoke chamber with an FTIR gas analyzer.

Why are both flaming and non-flaming modes tested?

Real fires progress through smoldering, well-ventilated flaming, and under-ventilated stages, each producing different smoke yields and gas profiles. The non-flaming (smoldering) mode at 25 kW/m² represents the early pre-flashover stage, while the flaming mode with a pilot ignition source represents developed flaming combustion. Rubber compounds such as EPDM often perform very differently in the two modes, so regulators require data from both conditions to characterize the full hazard.

Which toxic gases are of greatest concern for rubber materials?

All rubbers produce carbon monoxide and carbon dioxide. Nitrile rubbers release hydrogen cyanide due to their nitrogen content; chloroprene rubber releases hydrogen chloride; fluoroelastomers can release hydrogen fluoride and other fluorinated species; and sulfur-vulcanized compounds release sulfur oxides. Brominated flame retardants, where present, contribute hydrogen bromide, while partial oxidation of hydrocarbons generates formaldehyde and acrolein.

How many specimens are required and how long does testing take?

Most chamber standards require a minimum of three specimens per exposure condition, and additional specimens are tested if results are scattered or if both 25 kW/m² and 50 kW/m² fluxes are specified. A single exposure lasts up to 20 minutes, but a complete program including conditioning, multiple modes, FTIR calibration, and reporting typically takes one to two weeks at an accredited laboratory.

How can the smoke density and toxicity of rubber be reduced?

Effective strategies include selecting halogen-free polymer bases such as EPDM or silicone, incorporating high loadings of mineral fillers such as aluminum hydroxide or magnesium hydroxide that release water on decomposition, optimizing the crosslink system to reduce sulfur dioxide formation, and using intumescent or nanocomposite additives that lower soot yield. Each formulation change should be verified by retesting, since smoke and toxicity performance is highly sensitive to compound composition.