Smoke production testing for foam plastics measures how much light-obscuring smoke a cellular plastic material releases when exposed to radiant heat or flame, using closed-chamber photometric methods such as ISO 5659-2, ASTM E662 and ASTM D2843. The resulting parameters—specific optical density (Ds), maximum smoke density and VOF4—are decisive for compliance in building, railway, marine and aerospace applications. This article explains the test principle, specimen requirements, measured parameters, applicable standards and typical applications of foam plastic smoke generation testing.
Technical Overview
Foam plastics (cellular plastics) such as polyurethane, polystyrene, polyethylene and PVC foams combine low density with a very high surface-to-mass ratio. In a fire, this structure promotes rapid heating, melting, dripping and pool burning, which typically produces large volumes of dense, soot-laden smoke. Because smoke obscures escape routes and incapacitates occupants long before flame contact, smoke generation is one of the most critical fire-performance characteristics of polymeric materials and is strictly regulated in construction, rail, marine and aircraft applications.
Smoke production testing does not measure smoke "quantity" by mass. Instead, it quantifies the optical obscuration power of the smoke accumulated in a closed, calibrated chamber. A conditioned specimen is exposed to a radiant heat flux, with or without a pilot ignition flame, while a vertical light beam and photometer continuously record the transmittance of the smoky atmosphere. Applying the Beer-Lambert relationship, the measured light attenuation is converted into specific optical density (Ds), which normalizes the result to the exposed specimen area and the optical path length. For standard chamber geometry this simplifies to Ds = 132 × log10(100/T), where T is the percentage light transmittance.
For foam plastics specifically, the test method must accommodate behaviors such as specimen shrinkage, melt collapse and flaming drips. Retaining grids, foil wrapping of the specimen edges and post-test mass-loss recording are commonly applied to ensure repeatable and comparable results. Testing is normally performed by accredited third-party laboratories under ISO/IEC 17025 quality systems.
Testing Samples
Rigid Thermoset Foams
Rigid polyurethane (PU) and polyisocyanurate (PIR) foam boards used as building and appliance insulation, phenolic foam panels, and melamine acoustic foams. These materials are frequently tested to demonstrate compliance for wall, roof and HVAC applications.
Thermoplastic Foams
Expanded polystyrene (EPS) and extruded polystyrene (XPS) insulation boards, polyethylene (PE and XLPE) foams, EVA foams and rigid or semi-rigid PVC foams used in packaging, cushioning, flotation and construction.
Flexible Foams
Flexible polyurethane slabstock and molded foams for seating, mattresses and public-transport interiors, where smoke density limits are often more stringent than for construction products.
Specimen Preparation
- Specimen size for ISO 5659-2 / GB/T 8323.2: 75 mm × 75 mm, with product thickness up to a maximum of 25 mm.
- ASTM E662 uses the same 75 mm × 75 mm format in the NBS-type chamber.
- Specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity (ISO 291) for a minimum of 24 hours before testing.
- The lower face and sides of each specimen are wrapped in aluminum foil to direct decomposition products toward the exposed surface; initial and residual mass are recorded.
- A minimum of three specimens per exposure mode (flaming and non-flaming) is customary, with additional specimens retained for repeat testing.
Testing Items
Depending on the specified standard and end-use regulation, the following parameters are reported:
- Specific optical density (Ds) as a function of time, from continuous transmittance measurement.
- Maximum specific optical density (Dm or Ds max) within the test duration.
- VOF4 — the cumulative specific optical density over the first four minutes, calculated as the sum of Ds at 1, 2, 3 and 4 minutes; a key acceptance parameter in EN 45545-2 railway applications.
- Ds at fixed times, such as Ds(1.5) and Ds(4.0) in ASTM E662 or Ds(4) in aerospace smoke tests.
- Smoke Density Rating (SDR) derived from maximum optical density values in ASTM D2843.
- Time to reach Ds = 16, an established visibility threshold related to tenability.
- Specimen mass loss and visual observations (ignition time, flaming, dripping, collapse).
- Optional gas analysis of the chamber atmosphere (CO, CO2, HCN, HCl, HF, SO2, NOx, etc.) by FTIR for toxicity assessment under ISO 19021, EN 45545-2 or IMO FTP Code Part 2.
Testing Methods
ISO 5659-2 / GB/T 8323.2 — Single-Chamber Smoke Density Test
The most widely applied method for foam plastics. A 75 mm × 75 mm specimen is placed in a holder in the base of an approximately 0.5 m³ closed chamber and exposed to a radiant furnace at 25 kW/m² (50 kW/m² for certain specifications), in either flaming mode (with a small pilot flame) or non-flaming (smoldering) mode. A vertical 914 mm optical path with a light source and photometer measures transmittance at short intervals for the standard 10-minute duration, which may be extended to 20 minutes for certain rail and marine programs. The method underpins EN 45545-2, the IMO FTP Code and many national building regulations.
ASTM E662 — Specific Optical Density (NBS Smoke Chamber)
The North American counterpart using the same 0.5 m³ NBS-type chamber geometry. Specimens are tested at 25 kW/m² in flaming and non-flaming modes; the standard reports Ds at 1.5 and 4.0 minutes and the maximum specific optical density Dm. It is referenced in US regulations for electrical, rail transit and specialty applications.
ASTM D2843 — Smoke Density Chamber (XP-2 Type)
Specimens are burned directly in a propane flame in a smaller chamber, producing a transmittance curve from which the Smoke Density Rating (SDR) is derived. The method has historically been cited for foam plastics and plastic building components in certain North American codes.
Sector-Specific Programs
- EN 45545-2: railway fire protection, combining ISO 5659-2 smoke data (Ds max, VOF4) with FTIR gas toxicity across hazard levels HL1–HL3.
- IMO FTP Code (2010), Annex 1, Part 2: smoke generation and toxicity for surface materials, including foams, on ships and high-speed craft.
- FAR 25.853 Appendix F, Part V and equivalent CS-25 requirements: 4-minute smoke chamber exposure for aircraft cabin materials.
- GB 8624 classification in China, where smoke production parameters from GB/T 8323 and SMOGRA-type data from GB/T 20284 contribute to the smoke sub-classification of building products.
Irrespective of the standard, rigorous calibration is mandatory: the radiant heat flux is verified with a water-cooled heat flux meter, the photometric system is zeroed and spanned before every test, and chamber tightness is confirmed so that smoke dilution cannot distort the optical density curve.
Applications
Smoke generation data for foam plastics directly support the following activities:
- Compliance demonstration for building insulation products under GB 8624, EN 13501-1 and regional codes, where smoke sub-classes (e.g., s1–s3 or t0–t2) govern permissible applications.
- Material qualification for railway rolling stock under EN 45545-2 for interior foams, seating and thermal insulation.
- Marine type approval of accommodation and structural foams under the IMO FTP Code.
- Aerospace interior qualification of foams for seats, insulation and panels under FAR/CS 25.853.
- Research and development of low-smoke and halogen-free foam formulations, where Ds max and VOF4 serve as quantitative optimization targets.
- Factory production control, supplier verification and failure analysis support provided by accredited third-party laboratories.
FAQ
Q: What is the difference between flaming and non-flaming test modes?
A: In flaming mode, a small pilot flame contacts the specimen surface to force flaming decomposition; in non-flaming mode, the specimen is exposed only to radiant heat. Some foams produce more smoke in the smoldering, non-flaming condition, so most specifications require both modes.
Q: Which result is most important for approval?
A: It depends on the regulation. Railway applications focus on Ds max and VOF4; aerospace focuses on Ds at 4 minutes; ASTM D2843 programs focus on SDR. Accredited laboratories report all parameters required by the applicable specification.
Q: What are typical acceptance limits?
A: Limits vary by hazard level and product category. For example, EN 45545-2 applies progressively stricter Ds max and VOF4 limits from HL1 to HL3, while aircraft specifications typically cap Ds(4). The governing limit must always be taken from the specific end-use regulation.
Q: How many specimens are required?
A: A minimum of three valid specimens per exposure mode is standard, plus spares. Foam materials showing melt collapse or dripping may require repeats until consistent results are obtained.
Q: How long does testing take?
A: Each exposure runs 10–20 minutes, but a complete project—including conditioning, calibration, both modes, data reduction and reporting—typically requires one to two weeks, longer when FTIR toxicity screening is added.
Q: Can smoke density results be compared between ISO 5659-2 and ASTM E662?
A: The chambers and photometry are similar, but differences in calibration procedures, reporting points and flame application mean results are indicative rather than strictly interchangeable. Always test to the standard named in the purchase specification or regulation.