Determination of Smoke Generation from Burning Plastics

Determination of smoke generation from burning plastics quantifies the optical density of smoke released when polymeric materials decompose or burn under controlled laboratory conditions. Testing in accordance with ISO 5659-2, ASTM E662, ASTM D2843, and ISO 5660-1 provides the specific optical density (Ds), VOF4, and smoke production data required for fire-safety compliance in rail, marine, building, and electrical applications. This article explains the principles, specimens, measured parameters, test methods, and typical applications of plastic smoke generation determination performed by accredited third-party laboratories.

Technical Overview

Smoke is one of the most critical fire hazard characteristics of plastic materials. In real fire scenarios, smoke reduces visibility along escape routes, impedes evacuation and rescue operations, and transports toxic and corrosive combustion products such as carbon monoxide, hydrogen chloride, and hydrogen cyanide. Fire statistics consistently show that the majority of fire fatalities result from smoke inhalation rather than direct flame contact, which makes quantitative evaluation of smoke generation a fundamental requirement in the fire-safety qualification of polymeric materials.

Because plastics are carbon-based organic polymers, their thermal decomposition and flaming combustion can generate substantial volumes of soot-laden smoke. Determination of smoke generation is therefore performed using photometric (optical) methods: a conditioned test specimen is exposed to a defined radiant heat flux or direct flame inside a closed smoke chamber or beneath an exhaust hood, and the attenuation of a light beam passing through the accumulated smoke is continuously recorded. The measured light transmittance is converted into optical density or specific optical density, which normalizes the result by the exposed specimen area and the optical path length of the chamber.

The principal standards framework includes ISO 5659-1 (guidance on optical-density testing), ISO 5659-2 (single-chamber smoke density test), ASTM E662 (specific optical density using the NBS-type chamber), ASTM D2843 (smoke density of plastics), ISO 5660-1 (cone calorimeter with exhaust-duct smoke measurement), and the Chinese national standard GB/T 8323.2, which technically corresponds to ISO 5659-2. Accredited third-party laboratories conduct these tests to support regulatory compliance, product certification, material selection, and fire-hazard engineering assessment.

Testing Samples

Molded Thermoplastics

Granules or powders of polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), acrylonitrile butadiene styrene (ABS), polystyrene (PS), polycarbonate (PC), polymethyl methacrylate (PMMA), polyamide (PA), and their flame-retarded compounds are compression-molded or injection-molded into flat test plaques with a smooth, uniform surface.

Thermosetting Polymers

Phenolic resins, epoxies, unsaturated polyesters, melamine formaldehyde, and polyurethane systems are cast or laminated into plaques representative of the cured commercial product.

Cellular Materials and Foams

Rigid and flexible polyurethane foams, expanded polystyrene (EPS), extruded polystyrene (XPS), and other insulation foams are tested at their nominal product density and thickness, since cellular structure strongly influences smoke yield.

Composites and Finished Components

Glass-fiber-reinforced panels, sandwich constructions, laminates, cable sheathing and insulation compounds, appliance housings, and rail or aircraft interior components are sampled directly from production parts, preserving the actual surface finish and construction.

Specimen Preparation Requirements

• Specimen dimensions of 75 mm × 75 mm at the actual product thickness (maximum 25 mm) for ISO 5659-2, GB/T 8323.2, and ASTM E662.
• Specimen dimensions of 100 mm × 100 mm for ISO 5660-1 cone calorimeter testing.
• A minimum of three specimens per exposure condition (flaming and non-flaming), with additional specimens when results show excessive scatter.
• Conditioning at 23 ± 2 °C and 50 ± 5 % relative humidity to constant mass prior to testing.
• Measurement and recording of specimen mass and thickness before and after each test for mass-loss correlation.

Testing Items

The following parameters are typically determined during smoke generation testing of plastics:

• Specific optical density (Ds) under flaming and non-flaming (smoldering) exposure modes.
• Maximum specific optical density, Ds(max), reached during the test period.
• Specific optical density at defined times, such as Ds at 1.5 minutes and Ds at 4 minutes.
• VOF4, the cumulative smoke parameter calculated by integrating Ds over the first four minutes, as required by EN 45545-2 for railway applications.
• Smoke Density Rating (SDR) determined according to ASTM D2843.
• Continuous light transmittance and optical density curves versus time.
• Smoke production rate (SPR), total smoke production (TSP), and specific extinction area (SEA) measured in the exhaust duct per ISO 5660-1.
• Mass loss and mass loss rate of the specimen during the test.

Testing Methods

ISO 5659-2 / GB/T 8323.2 Single-Chamber Smoke Density Test

A 75 mm × 75 mm specimen is mounted horizontally on a load cell in a closed 0.51 m³ chamber and exposed to a radiant heat flux of 25 kW/m² from a conical heater. The test is performed in two modes: without piloted ignition (non-flaming decomposition) and with a small propane pilot flame applied to the specimen surface (flaming combustion). A horizontal light beam, typically from a white-light or laser source, traverses the chamber, and the transmitted light intensity is recorded continuously for up to 10 minutes. The specific optical density is calculated as Ds = (V / (A × L)) × log10(100/T), where V is the chamber volume, A is the exposed specimen area, L is the optical path length, and T is the percent transmittance. The maximum value Ds(max) and the cumulative value VOF4 are reported, making this method the reference procedure for EN 45545-2 and IMO FTP Code compliance.

ASTM E662 Specific Optical Density Test

ASTM E662 employs an NBS-type smoke chamber in which a vertically or horizontally oriented specimen is exposed to a 25 kW/m² radiant furnace. Testing is conducted in both a flaming mode, using a multi-jet propane pilot burner, and a smoldering mode with the pilot flame extinguished. The specific optical density is computed from light transmittance data in the same manner as ISO 5659-2, and Ds(max) values for both modes are reported. This method is widely specified in North American material specifications for transportation, electrical, and military applications.

ASTM D2843 Smoke Density Test

In the ASTM D2843 smoke density chamber, specimens supported on a wire grid are exposed directly to the flame of a propane burner. The cumulative specific optical density area recorded over a four-minute exposure is used to calculate the Smoke Density Rating (SDR), a single-figure index from 0 to 100 that allows straightforward comparison of the smoke-producing tendencies of different plastics. The method also identifies whether materials exhibit flaming drips or unusual burning behavior during exposure.

ISO 5660-1 Cone Calorimeter Method

The cone calorimeter subjects a 100 mm × 100 mm horizontally or vertically oriented specimen to irradiance levels between 10 kW/m² and 100 kW/m² under a calibrated exhaust hood. A laser photometer mounted in the duct continuously measures smoke extinction, from which the smoke production rate, total smoke production, and specific extinction area are derived, simultaneously with heat release rate data. Because results are expressed per unit area and per unit mass loss, this method provides inputs suitable for fire modeling and performance-based fire safety engineering.

Applications

Smoke generation determination supports material qualification and regulatory compliance across multiple industries:

• Railway applications: EN 45545-2 defines smoke density requirements for interior materials using EN ISO 5659-2, with Ds(max) and VOF4 limit values assigned according to the hazard levels HL1, HL2, and HL3 of the train design category.
• Marine applications: the International Maritime Organization (IMO) FTP Code requires smoke generation testing of bulkhead, ceiling, and finish materials for shipbuilding approval.
• Building and construction: smoke density data contribute to the fire classification of linings, insulation products, pipes, and cable systems used in escape routes and protected areas.
• Electrical and electronics: low-smoke compounds for cable sheathing, connectors, and enclosures are verified for optical density to protect occupied spaces and sensitive equipment.
• Research and development: comparative smoke density data guide the formulation of low-smoke, halogen-free flame-retardant polymer systems.
• Quality control: batch-to-batch verification confirms that production materials consistently meet the smoke performance established during type approval.

FAQ

Why are both flaming and non-flaming test modes required?

The two modes represent different stages of a real fire. The non-flaming (smoldering) mode simulates a material exposed to radiant heat from an adjacent fire before ignition, while the flaming mode represents fully developed combustion. Some materials produce more smoke under smoldering conditions than when flaming, so regulations such as EN 45545-2 evaluate both modes to characterize worst-case behavior.

What is the difference between specific optical density and Smoke Density Rating?

Specific optical density (Ds) normalizes the measured optical density to the exposed specimen area and the optical path length of the chamber, allowing comparison between chambers and specimens. The Smoke Density Rating (SDR) is a dimensionless 0–100 index derived from the cumulative specific optical density area over four minutes in the ASTM D2843 flame-exposure test and is intended for simple ranking of plastics.

How should a standard be selected for a specific project?

Standard selection follows the applicable regulation or specification. Railway projects in Europe reference EN ISO 5659-2 through EN 45545-2; marine approvals reference the IMO FTP Code; North American material specifications frequently cite ASTM E662 or ASTM D2843; and performance-based fire engineering studies typically require ISO 5660-1 cone calorimeter data. Accredited third-party laboratories can confirm the correct method and acceptance criteria for each end use.

Does low smoke density guarantee that a material is safe?

No. Smoke density measurement quantifies only optical obscuration. Toxicity depends on the chemical composition of the gases evolved, which requires separate analysis of combustion gases using Fourier-transform infrared spectroscopy or ion chromatography in combination with the smoke chamber. Comprehensive fire-hazard assessment combines smoke density, gas toxicity, flammability, and heat release data.

How long does a typical smoke generation test take?

A single exposure in the ISO 5659-2 or ASTM E662 chamber lasts approximately 10 minutes, but a complete evaluation including specimen conditioning, both exposure modes, replicate specimens, chamber cleaning, and calibration typically requires several working days in an accredited laboratory.