Determination of Smoke Generation from Plastics: Test Methods, Standards and Applications

Smoke generation testing quantifies the amount of visible smoke that plastic materials produce under controlled fire conditions, using internationally recognized methods such as ISO 5659-2, ASTM E662 and ASTM D2843. This technical guide explains the measurement principles, sample requirements, key reported parameters and typical regulatory applications of smoke generation determination, helping manufacturers and material developers select the appropriate test for rail, marine, construction and cable products.

Technical Overview

Smoke generated by burning or thermally decomposing plastics is one of the principal hazards in real fires. Dense smoke reduces visibility along escape routes, slows evacuation, impedes firefighting operations, and carries toxic combustion products. Because smoke production depends strongly on polymer chemistry and formulation, regulators and industry specifications limit the smoke released by plastic components used in trains, ships, aircraft, buildings and cable systems. Determination of smoke generation is therefore a core reaction-to-fire test for polymeric materials.

Smoke generation measurement is predominantly photometric: a test specimen is exposed to a defined radiant heat flux, with or without a pilot ignition source, inside a closed chamber of known volume. The smoke accumulates and attenuates a horizontal light beam, and the reduction in light transmission is converted, through the Beer-Lambert relationship, into optical density values. The main instrument platforms are the single-chamber smoke density apparatus (NBS-type chamber) specified in ISO 5659-2, ASTM E662 and GB/T 8323.2, the XP2 chamber specified in ASTM D2843, and the cone calorimeter specified in ISO 5660-1, ASTM E1354 and GB/T 16172. These methods yield parameters such as specific optical density, cumulative smoke indices and smoke production rates that feed directly into product certification schemes such as EN 45545-2 for railway applications and the IMO FTP Code for ships.

Testing Samples

Thermoplastic and Thermoset Sheet Materials

Rigid sheets, plates and molded plaques of PVC, PE, PP, PC, PMMA, ABS, polyamide, epoxy and phenolic resins are the most common sample forms. For the single-chamber method, specimens are typically cut to 75 mm by 75 mm with the end-use thickness, generally up to 25 mm, and are tested in both the as-received surface orientation and, where required, in transverse orientation.

Foam and Cellular Materials

Flexible and rigid polyurethane foams, polystyrene and polyolefin foams used in seating, insulation and panels can be tested, with special holders used to prevent specimen distortion while maintaining the defined exposed area.

Composite and Reinforced Plastics

Glass-fiber and carbon-fiber reinforced laminates, sandwich panels and GMT or SMC moldings used in structural and interior applications are tested at their nominal production thickness to reflect real fire exposure.

Cable Compounds and Cable Assemblies

Insulation and sheathing compounds, as well as complete low-smoke cables, are evaluated either as 75 mm square plaques in the smoke chamber or as full cable runs in the 3 m cube test of IEC 61034.

Coatings, Adhesives and Surface Finishes

Paints, laminates and decorative films are applied at specified mass per unit area onto standard substrates so that the measured smoke reflects the finished product rather than the substrate alone.

Prior to testing, specimens are conditioned to equilibrium at 23 degrees C and 50 percent relative humidity, and mass and dimensions are recorded to allow calculation of mass-loss-based indices.

Testing Items

Depending on the selected standard, the following quantities are determined and reported:

• Light transmission versus time and minimum transmission value
• Specific optical density Ds as a function of time, including Ds at 1.5 minutes and Ds at 4 minutes
• Maximum specific optical density Dm and the time to reach Dm
• Clearing correction factor and corrected Dm value
• VOF4, the cumulative specific optical density over the first four minutes, required by EN 45545-2
• Smoke Density Rating, SDR, determined in the ASTM D2843 XP2 chamber
• Mass loss and, where required, mass optical density in flaming and non-flaming modes
• Smoke production rate, rate of smoke release and total smoke production from cone calorimeter testing
• Smoke extinction area per unit mass of material consumed
• Optional gas concentration data, such as CO, CO2, HCN, HCl, HF, HBr, NOx and SO2, obtained by FTIR analysis, and the derived conventional toxicity index CIT

Testing Methods

Single-Chamber Method: ISO 5659-2 and GB/T 8323.2

A 75 mm by 75 mm specimen is mounted vertically in a 0.51 cubic meter sealed chamber and exposed to a radiant furnace. Standard exposure levels are 25 kW per square meter with a pilot flame (flaming mode), 25 kW per square meter without a pilot flame (smoldering or non-flaming mode) and, where specified, 50 kW per square meter. A horizontal photometric system with a light path of approximately 0.91 m continuously records transmission, from which specific optical density is calculated. This method is the basis of smoke assessment in EN 45545-2 and the IMO FTP Code.

NBS Smoke Chamber Method: ASTM E662

ASTM E662 uses an equivalent chamber geometry and photometric principle. A 76 mm by 76 mm specimen is exposed to a radiant flux of 2.5 W per square centimeter in flaming and non-flaming modes. The standard requires reporting of Ds at 1.5 and 4 minutes, Dm, and VOF4 for applications such as rail interior materials in North America. Related procedures include the older naval engineering standard NES 711.

XP2 Chamber Method: ASTM D2843

In this method, small specimens are burned with a propane flame inside a 0.07 cubic meter chamber, and light transmission is measured across a short path. The integrated result is expressed as a Smoke Density Rating on an indexed scale, widely used for engineering plastics and building products in specifications referencing ASTM D2843 and equivalent ISO 1133-adjacent national requirements.

Cone Calorimeter Method: ISO 5660-1, ASTM E1354 and GB/T 16172

A 100 mm by 100 mm specimen is exposed to a selectable irradiance of 10 to 100 kW per square meter under the cone heater. Smoke released into the exhaust duct is measured by laser photometry, allowing calculation of the instantaneous smoke production rate and total smoke production, together with heat release rate and mass loss rate. This method provides the richest dataset for material development and fire modeling.

Cable Smoke Test: IEC 61034

Complete cables are burned horizontally over a defined alcohol or burner source inside a 3 m cubic enclosure, and light transmission is measured over a 3 m horizontal path. Low-smoke zero-halogen cables are typically required to maintain a minimum light transmission of 60 percent during the test.

Combined Smoke and Toxicity Testing

Under EN 45545-2 and the IMO FTP Code Annex 1 Part 2, smoke density testing in the ISO 5659-2 chamber is combined with FTIR gas sampling to determine gas concentrations and the CIT index in both flaming and non-flaming conditions, giving a complete picture of smoke obscuration and smoke toxicity.

Applications

Smoke generation data are used across many regulated sectors:

• Railway rolling stock: classification of interior and exterior materials under EN 45545-2 against hazard levels HL1, HL2 and HL3, using Ds, VOF4 and CIT requirements
• Marine and offshore: compliance of bulkhead linings, ceilings, floor coverings and pipes with the IMO FTP Code and SOLAS Chapter II-2 fire safety requirements
• Building products: support for European classification under EN 13501-1, where smoke classes s1, s2 and s3 are assigned, and for North American specifications referencing chamber methods
• Wire and cable: qualification of low-smoke and low-smoke zero-halogen constructions to IEC 61034 and related EN requirements
• Aviation interiors: demonstration of specific optical density limits for cabin materials under FAA requirements such as FAR 25.853
• Research and development: comparison of flame-retardant formulations, halogen-free compounds, bio-based polymers and nanocomposites, and generation of input data for fire and evacuation modeling

FAQ

What is the difference between flaming and non-flaming test modes?

In the flaming mode, a small pilot flame contacts the specimen to force sustained combustion. In the non-flaming mode, the specimen is exposed to radiant heat alone, so it decomposes and smolders without flaming. Both modes are important because different polymers produce their worst smoke output under different conditions, and standards such as EN 45545-2 require both sets of results.

Why is VOF4 so important?

VOF4 sums the specific optical density over the first four minutes of exposure, which corresponds to the critical escape window in a real fire. High early smoke production is more dangerous to evacuating passengers than smoke released later, so railway and marine specifications place strict limits on VOF4.

Which specimen thickness should be tested?

Materials are normally tested at their end-use thickness, because heat penetration, melt dripping and char formation all depend on thickness. Where several grades exist, the worst-case or maximum thickness representative of the application is chosen, and this must be documented in the test report.

Are smoke density and smoke toxicity tested together?

They can be. In the single-chamber method, gas sampling lines allow simultaneous FTIR analysis of toxic gases, enabling calculation of the CIT index alongside optical density. This combined approach is mandatory for many railway and marine product categories.

Which method should be selected for a new material?

The choice follows the target regulation: ISO 5659-2 or GB/T 8323.2 with VOF4 and CIT for railway and marine certification, ASTM E662 where North American specifications apply, ASTM D2843 for Smoke Density Rating listings, and cone calorimetry when detailed material characterization or fire modeling data are needed. Accredited third-party laboratories can advise on the correct combination of tests and on witness testing of product families.