Synthetic Leather Combustion Smoke Toxicity Testing: Standards, Methods and Evaluation

Synthetic leather combustion smoke toxicity testing quantifies the hazardous gases released when polyurethane (PU), PVC, and microfiber leather materials burn or thermally decompose. This technical article explains the key toxic combustion products, internationally recognized test methods such as ISO 5659-2 and NF X 70-100, the evaluation criteria used by EN 45545-2, the IMO FTP Code and GB/T 20285, and how accredited third-party laboratories support fire safety compliance for transport and interior applications.

Technical Overview

Synthetic leather is a polymer-based material produced by coating or impregnating a textile substrate with polyurethane (PU), polyvinyl chloride (PVC), or other resin systems, together with plasticizers, stabilizers, pigments, flame retardants and finishing agents. When such materials are exposed to fire, they first pyrolyze and then combust, generating a complex mixture of hot smoke and toxic gases. Statistical studies of fire fatalities consistently show that the majority of deaths in enclosure fires are caused by smoke inhalation and asphyxiation rather than by burns, which makes smoke toxicity a decisive fire safety parameter for materials used in confined spaces.

Combustion smoke toxicity testing reproduces controlled fire scenarios (smoldering, non-flaming thermal decomposition, and well-ventilated or under-ventilated flaming) in standardized apparatus. The evolved gases are collected and quantitatively analyzed, and the measured concentrations are converted into toxicity indices or hazard classifications. These data allow manufacturers, designers and regulators to compare materials, optimize low-smoke low-toxicity formulations, and demonstrate compliance with fire safety requirements for railway, marine, automotive, aviation and building applications.

Depending on the polymer system, synthetic leather can release the following classes of hazardous combustion products:

• Asphyxiant gases: carbon monoxide (CO), carbon dioxide (CO2) and hydrogen cyanide (HCN), the latter being characteristic of nitrogen-containing polyurethane and polyamide microfiber systems.
• Inorganic irritant and corrosive gases: hydrogen chloride (HCl) from PVC, hydrogen fluoride (HF) and hydrogen bromide (HBr) from halogenated flame retardants, sulfur dioxide (SO2) from sulfur-containing additives, and nitrogen oxides (NOx).
• Organic irritants and hazardous volatile compounds: formaldehyde, acrolein, phenol, benzene, toluene, styrene and residual isocyanate-derived species.
• Particulate smoke, characterized separately through optical density measurements.

Testing Samples

Polyurethane (PU) Synthetic Leather

PU synthetic leather consists of one or more polyurethane coating layers on a woven or knitted textile backing, produced by solvent, waterborne or solvent-free processes. Thermal decomposition of the urethane and isocyanate segments at approximately 250–450 °C generates significant quantities of carbon monoxide and hydrogen cyanide, together with amines, nitriles and aromatic hydrocarbons. PU systems are therefore typically screened for both asphyxiant and irritant gas species.

PVC Synthetic Leather

PVC leather contains a substantial chlorine fraction in the polymer backbone plus phthalate or alternative plasticizers. Dehydrochlorination begins at relatively low temperatures (approximately 200–300 °C), producing high yields of hydrogen chloride, which is both toxic and corrosive. Additional products include carbon monoxide, benzene and acrolein from plasticizer decomposition, so PVC grades are evaluated with particular attention to HCl emission.

Microfiber Synthetic Leather

Microfiber (ultrafine fiber) synthetic leather combines a polyamide or polyester microfiber nonwoven with a polyurethane impregnation matrix. The polyamide component contributes hydrogen cyanide, ammonia and nitrogen oxides, while the polyurethane matrix contributes CO and HCN. These materials generally require full-spectrum gas analysis.

Specialty and Functional Grades

Flame-retardant, halogen-free, low-smoke, semi-PU, thermoplastic polyurethane (TPU) and bio-based synthetic leather variants may be tested to verify that the incorporation of flame retardants does not unduly increase toxic gas yields. Sample preparation typically involves conditioning at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 24 hours in accordance with ISO 291, with specimen geometry and number determined by the selected test method.

Testing Items

Typical test programs for synthetic leather combustion smoke toxicity include the following measurable parameters:

• Quantitative concentrations of CO, CO2, HCN, HCl, HF, HBr, SO2, NOx (NO and NO2) and NH3 in the combustion effluent.
• Organic gas species, including formaldehyde, acrolein, benzene, toluene, styrene, phenol and other volatile organic compounds.
• Smoke production characteristics: specific optical density (Ds), maximum specific optical density (Ds max) and cumulative smoke (VOF4) where required.
• Mass loss and combustion duration under the applied heat flux.
• Calculated toxicity metrics: fractional effective dose (FED), fractional effective concentration (FEC), conventional toxicity index (CIT per EN 45545-2), conventional toxicity index per NF X 70-100, or the smoke toxicity hazard classification per GB/T 20285.
• Comparison of gas yields and indices against regulatory limit values for the target application.

Testing Methods

Smoke Chamber Method (ISO 5659-2 / ASTM E662)

The single-chamber smoke density test exposes 75 mm × 75 mm specimens to a radiant heat flux of 25 kW/m² with or without piloted ignition, or 50 kW/m² without pilot flame, while optical density of the accumulating smoke is continuously recorded. When combined with gas sampling and analysis, typically by Fourier transform infrared spectroscopy (FTIR) in accordance with ISO 19701 or the annexes of EN 45545-2, the method yields time-dependent concentrations of the eight regulated gases (CO2, CO, HF, HCl, HBr, HCN, NO2, SO2) used to calculate the conventional toxicity index CIT for railway applications. GB/T 8323.2 is the corresponding Chinese adoption for smoke density determination.

Tubular Furnace Methods (NF X 70-100, ISO/TS 19700)

Small specimens (approximately 1 g) are pyrolyzed or combusted in a quartz tube furnace at controlled temperatures (typically 400, 600 and 800 °C) under a defined air flow. The effluent is analyzed for a broad set of gases, and results are expressed as a conventional toxicity index that sums the ratios of each gas concentration to its reference incapacitation concentration. The steady-state tube furnace of ISO/TS 19700 additionally allows equivalence-ratio control to simulate under-ventilated flaming conditions. These methods are widely referenced in railway and defense specifications.

Marine Fire Test Procedure (IMO FTP Code)

For shipbuilding materials, the FTP Code requires smoke generation testing in the chamber configuration with concurrent measurement of the principal toxic gases, each of which must remain below specified maximum concentrations (for example, limits on CO, HCl, HCN and HF). Synthetic leather used in marine furniture, wall coverings and upholstered seating is evaluated against these acceptance criteria.

Smoke Toxicity Hazard Grading (GB/T 20285)

The Chinese dynamic method decomposes specimens in a flowing-air furnace across staged temperature conditions, and the evolved gas concentrations are used to assign a smoke toxicity hazard classification used in conjunction with the GB 8624 reaction-to-fire classification for building products and interior finishing materials.

Supporting Analytical Techniques

• FTIR spectroscopy for simultaneous multi-gas quantification.
• Ion chromatography for halogen acids (HCl, HBr, HF) and SO2 trapped in absorbing solutions.
• Gas detector tubes for on-site or screening measurements in line with ASTM E800 guidance.
• NDIR or paramagnetic analyzers for CO and CO2; chemiluminescence for NOx; ion-selective electrodes for HCN and NH3.
• Gas chromatography and GC-MS for volatile organic species.
• Toxicity index calculation and fire hazard interpretation in accordance with ISO 13344 and ISO 13571.

Applications

Combustion smoke toxicity data for synthetic leather are directly used in the following sectors:

• Railway rolling stock: compliance demonstration against the hazard level requirements of EN 45545-2 (R sets) for seats, wall and ceiling coverings, and interior trim.
• Marine and offshore: qualification of upholstery, decorative finishes and accommodation linings under the IMO FTP Code.
• Automotive interiors: development and acceptance testing of seat and door trim materials, supplementing horizontal flammability requirements such as GB 8410 and FMVSS 302.
• Aviation: material qualification programs for cabin components referencing fire, smoke and toxicity (FST) requirements.
• Buildings and public spaces: smoke toxicity grading of upholstered furniture, wall coverings and decorative laminates under GB/T 20285 and related building regulations.
• Furniture, footwear and consumer goods: formulation benchmarking and low-toxicity product development, including validation of halogen-free and flame-retardant grades.
• Quality control and R&D: comparison of resin systems, flame retardant packages and process changes on toxic gas yields.

FAQ

Q: Which toxic gases are most critical for synthetic leather?

A: For PU and microfiber materials, carbon monoxide and hydrogen cyanide dominate the asphyxiant hazard; for PVC materials, hydrogen chloride is the controlling species. Irritant organics such as formaldehyde and acrolein are monitored for all polymer types.

Q: How do I choose the right test method?

A: Method selection follows the end-use regulation: EN 45545-2 with ISO 5659-2 based gas analysis for railway applications, the FTP Code for marine applications, GB/T 20285 with GB 8624 for building products in China, and NF X 70-100 or ISO/TS 19700 where a broad-spectrum toxicity index is specified.

Q: What sample sizes and quantities are required?

A: Typical requirements are 75 mm × 75 mm panels at the supplied thickness for smoke chamber testing (usually a minimum of six to nine specimens covering all irradiance conditions) and approximately 1 g portions for tubular furnace testing, with replicate determinations per temperature.

Q: How are results interpreted?

A: Gas concentrations are compared with reference incapacitation or permissible exposure levels and converted into indices such as CIT, FED or FEC. Values below the applicable limit for the required fire hazard level indicate acceptable smoke toxicity performance.

Q: Does a flame-retardant treatment guarantee lower smoke toxicity?

A: No. Halogenated flame retardants can suppress flame spread while increasing yields of HCl, HBr or CO in the smoke. Smoke toxicity testing is therefore conducted independently of, and in addition to, ignitability and flame spread tests.