Determination of Specific Optical Density of Cellular Plastics (Foam Plastics): Principles, Methods

The specific optical density test is the internationally recognized method for quantifying the smoke generation potential of cellular plastics (foam plastics) during combustion or thermal decomposition. Performed in a closed smoke chamber in accordance with ASTM E662, ISO 5659-2 and GB/T 8323.2, this test determines the specific optical density (Ds) by measuring light attenuation caused by accumulated smoke. This article explains the test principle, specimen requirements, measurement procedure, reported parameters and typical regulatory applications of smoke density testing for foam materials.

Technical Overview

Cellular plastics, commonly known as foam plastics, are low-density polymer materials with a cellular structure that provides excellent thermal insulation, cushioning and lightweight properties. Because of their high specific surface area and organic composition, foam plastics can release large volumes of dense smoke when exposed to heat or flame. In many fire scenarios, smoke obscuration and reduced visibility constitute a greater hazard to occupants than heat or flame itself. The determination of specific optical density (Ds) was therefore developed to quantitatively characterize the smoke-generating tendency of these materials.

The specific optical density test is typically conducted in a closed 0.51 m³ (18 ft³) smoke chamber, historically referred to as the NBS smoke chamber. A square specimen is exposed to a controlled radiant heat flux of 25 kW/m² under either flaming conditions (with a pilot ignition burner) or non-flaming conditions (smoldering decomposition, no pilot flame). As smoke accumulates inside the sealed chamber, a vertical collimated light beam passing through the chamber is progressively attenuated. The decrease in light transmittance over time is recorded by a photometric system and converted into the specific optical density Ds.

The specific optical density normalizes the measured optical density with respect to the exposed specimen area and the optical path length, making results independent of chamber geometry and therefore comparable between laboratories. It is calculated as:

• Ds = G × log10(I0/I), where G = V/(A × L)
• V = chamber volume (approximately 0.51 m³)
• A = exposed specimen area (65 mm × 65 mm, approximately 0.0042 m²)
• L = optical path length (approximately 914 mm)
• I0 = initial light intensity (100% transmittance); I = instantaneous light intensity
• For a standard chamber configuration, the geometric factor G is approximately 132

The principal standards governing this test include ASTM E662 (Standard Test Method for Specific Optical Density of Smoke Generated by Solid Materials), ISO 5659-2 / EN ISO 5659-2 (Plastics – Smoke generation – Determination of optical density by a single-chamber test), GB/T 8323.2 (the Chinese national standard technically aligned with ISO 5659-2), IMO FTP Code Part 2 for marine applications, and the smoke density provisions of FAR 25.853 Appendix F, Part V for aircraft cabin materials.

Testing Samples

Thermosetting Foams

Rigid polyurethane (PUR), polyisocyanurate (PIR) and phenolic foams are widely used as building and industrial insulation materials. These materials are tested to demonstrate compliance with fire performance classifications such as GB 8624, EN 13501 and EN 45545-2. Their charring behavior under radiant heat strongly influences the smoke density curve, and both faced (with aluminum foil or facings) and unfaced products can be evaluated.

Thermoplastic Foams

Expanded polystyrene (EPS), extruded polystyrene (XPS), polyethylene (PE) and polypropylene (PP) foams tend to shrink, melt and drip under radiant exposure. The smoke chamber records the combined optical effect of soot, tarry aerosols and decomposition products. Test laboratories must take care that melting or dripping does not disturb the heat flux calibration of the furnace during exposure.

Elastomeric and Flexible Foams

Flexible polyurethane slabstock and molded foams, PVC/NBR elastomeric insulation foams and rubber foams used in furniture, seating, cushioning and pipe insulation represent a third category. These low-density, open-cell materials typically ignite readily in the flaming mode and may reach maximum smoke density within the first few minutes of the test.

Specimen Preparation and Conditioning

Standard specimens measure 75 mm × 75 mm, with a thickness not exceeding 25 mm; products thicker than 25 mm are generally cut down, while thin materials may be tested at full thickness or stacked according to the product specification. All faces except the exposed surface are wrapped in aluminum foil and the specimen is mounted in a holder that exposes a 65 mm × 65 mm area. Prior to testing, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity until moisture equilibrium is reached. A typical test program requires a minimum of three specimens for the flaming mode and three specimens for the non-flaming mode.

Testing Items

The smoke density test yields a time-dependent curve of specific optical density, from which the following characteristic parameters are reported:

• Ds(1.5) – specific optical density at 1.5 minutes of exposure
• Ds(4) – specific optical density at 4 minutes of exposure, the most widely cited index and the basis of many regulatory limits
• Ds(max) – the maximum specific optical density reached during the test
• VOF4 – cumulative smoke parameter defined as the sum of Ds values over the first four minutes (Ds1 + Ds2 + Ds3 + Ds4), used primarily in EN 45545-2 railway requirements
• t16 – the time required to reach Ds = 16, an indicator of the rate of smoke development and its implication for escape visibility
• Mass loss and mass loss rate – recorded when the chamber is equipped with an integrated weighing system
• Full light transmittance versus time curve under both flaming and non-flaming conditions

Testing Methods

Testing is performed in an accredited third-party laboratory using a single-chamber smoke density apparatus consisting of a sealed test chamber, a radiant electric furnace, a specimen holder with pilot burner, a vertical photometric system (light source, collimating optics and photodetector), an exhaust and purge system, and computerized data acquisition. The general procedure is as follows:

• System verification: the radiant flux at the specimen position is calibrated to 25 kW/m² (ISO 5659-2 additionally permits 50 kW/m² for higher-severity assessment), and the optical system is set and checked at 0% and 100% transmittance.
• Specimen loading: the foil-wrapped, conditioned specimen is placed in the holder and positioned facing the radiant furnace, with the chamber door closed to create a sealed environment.
• Exposure: in the flaming mode, the pilot burner flame contacts the specimen surface at the start of the test; in the non-flaming mode, the pilot burner is extinguished and the specimen undergoes smoldering pyrolysis under radiant heat alone.
• Measurement: light transmittance through the chamber is recorded continuously (typically for 10 minutes, or 20 minutes where specified), while the specimen surface behavior (ignition, flaming, glowing, dripping, collapse) is observed and documented.
• Calculation: transmittance data are converted to Ds using the geometric factor G, and the characteristic indices Ds(4), Ds(max), VOF4 and t16 are derived.
• Reporting: results for each mode are reported as the mean of valid replicates, together with the complete Ds–time curve, test conditions, specimen description and any atypical behavior such as shrinkage or melt dripping.

Because smoke density results are sensitive to specimen density, cell structure, thickness, facing layers and conditioning history, strict adherence to the standard procedure and detailed documentation of the material configuration are essential for reproducible and comparable data.

Applications

Specific optical density data for foam plastics are used across multiple industries and regulatory frameworks:

• Building products: smoke performance inputs for fire classification of insulation materials under GB 8624 and EN 13501, and for product certifications in construction.
• Railway vehicles: EN 45545-2 requires ISO 5659-2 smoke density results (Ds4, VOF4, t16) for interior and structural materials in all R-sets, including foams used in thermal insulation and seating.
• Aviation: aircraft cabin and cargo compartment materials must satisfy smoke density limits (commonly Ds(4) ≤ 200) verified per the FAR/CS 25.853 Appendix F Part V method.
• Marine: the IMO FTP Code Part 2 adopts ISO 5659-2 for smoke generation assessment of materials used in ship construction and outfitting.
• Research and development: foam formulators use Ds curves to evaluate the effectiveness of flame retardants, smoke suppressants, char promoters and reactive modifications, and to benchmark competitive materials.
• Quality control: routine smoke density verification supports batch conformity of foam production, supplier qualification and incoming material inspection.

FAQ

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

In the flaming mode, a small pilot flame impinges on the specimen surface, forcing ignition and flaming combustion, which typically produces soot-laden smoke from gas-phase combustion. In the non-flaming mode, the specimen is heated by radiant flux alone and decomposes by smoldering pyrolysis without piloted ignition. Both modes are required because a material may perform well in one condition yet generate severe smoke in the other.

Why is specific optical density used rather than raw light transmittance?

Raw transmittance depends on chamber size, specimen area and optical path length. Specific optical density normalizes the measurement by these geometric factors, so that results from different laboratories and chamber designs are directly comparable on a common scale.

Which standard should be selected: ASTM E662, ISO 5659-2 or GB/T 8323.2?

The choice depends on the target market and regulatory scheme. GB/T 8323.2 is technically aligned with ISO 5659-2 and is used for Chinese and IMO-related assessments; EN ISO 5659-2 is required by EN 45545-2 for railway applications; ASTM E662 is the reference method for North American and aviation smoke density requirements. The chambers are similar, but irradiance levels, specimen details and reported indices differ, so the applicable specification should always be confirmed before testing.

How many specimens are required for a complete test?

A full evaluation normally requires at least three valid specimens in the flaming mode and three in the non-flaming mode, plus spare specimens to account for invalid runs caused by melting, dripping or non-representative behavior.

Do smoke chamber results predict full-scale fire behavior?

Specific optical density is a small-scale, single-parameter screening measurement. It quantifies smoke obscuration under standardized radiant exposure but does not by itself replicate complex full-scale fire dynamics. It is therefore used in combination with other fire test data—such as flame spread, heat release and toxicity assessments—within overall fire safety engineering and classification frameworks.