Oxygen index testing determines the minimum oxygen concentration required to sustain candle-like combustion of low smoke halogen-free (LSZH/LS0H) foam materials, making it one of the most widely used bench-scale flammability tests for flame-retardant polymers. Performed in accordance with ISO 4589-2, ASTM D2863 and GB/T 2406.2, the limiting oxygen index (LOI) provides a quantitative, highly reproducible measure of flammability for halogen-free foams used in rail transit, wire and cable, and thermal-management applications. This article explains the test principle, sample requirements, methodology, acceptance criteria and frequently asked questions.
Technical Overview
The oxygen index test, also known as the limiting oxygen index (LOI) test, measures the minimum concentration of oxygen in a flowing mixture of oxygen and nitrogen that will just support candle-like combustion of a vertically mounted specimen. Since ambient air contains approximately 20.9% oxygen by volume, a material with an LOI above 21% cannot sustain flaming combustion under normal atmospheric conditions. This makes LOI a convenient, numerical indicator of flame retardancy that is particularly well suited to quality control and material qualification.
Low smoke halogen-free (LSZH, also written LS0H or LSFH) foams are engineered to replace halogenated flame-retardant materials in applications where combustion toxicity, corrosive gases and smoke obscuration are critical concerns. Because these formulations achieve flame retardancy through metal hydroxide fillers, phosphorus-based additives, intumescent systems or inherently resistant polymer chemistries rather than halogenated compounds, oxygen index testing is the primary method used to verify that their flame-retardant performance meets specification. Typical requirements for LSZH foam materials range from LOI ≥ 28% for general industrial use to LOI ≥ 32–35% for demanding sectors such as rail transit and marine engineering.
The dominant test standards are ISO 4589-2, ASTM D2863 and GB/T 2406.2, which are technically harmonized and mutually recognized by most certification schemes. ISO 4589-3 extends the method to elevated temperatures and defines the temperature index (TI). Older naval specifications based on NES 714 have largely been superseded by the ISO method.
Testing Samples
Halogen-Free Polyolefin Foams (PE, XLPE, EVA and Blends)
Cross-linked polyethylene, ethylene-vinyl acetate and polyolefin blend foams are the most common LSZH foam families. They are widely used in cable bedding, cushioning and sealing components. Their LOI values typically range from 22% for unfilled grades to above 32% for heavily filled flame-retardant compounds, and testing is routinely used to verify filler dispersion and batch consistency.
Silicone Foams
Silicone elastomer foams are inherently halogen-free and exhibit high LOI values, frequently exceeding 30–35% due to the formation of protective silica residue during combustion. They are commonly tested for EV battery thermal-barrier and fire-protection applications where sustained self-extinguishing behavior is required.
EPDM, TPE and TPU Foams
Terpolymer elastomer and thermoplastic polyurethane foams are tested when used as gaskets, expansion joints and acoustic insulation in rolling stock and infrastructure projects. Compounders rely on LOI data to optimize mineral filler loading against mechanical and density targets.
Specimen Preparation and Conditioning
For rigid and semi-rigid foams, ISO 4589-2 recommends specimens of 80–150 mm in length, 10 ± 0.5 mm in width and 10 ± 0.5 mm in thickness; when the product form is thinner than 10.5 mm, the actual thickness is retained and recorded. Specimens must be cut cleanly without crushing or sealing the cell structure, since deformed cells alter burning behavior. Unless otherwise specified, specimens are conditioned at 23 ± 2°C and 50 ± 5% relative humidity for a minimum of 88 hours in accordance with ISO 291, and the foam density of each specimen is recorded because LOI correlates strongly with density and cell geometry.
Testing Items
Accredited third-party laboratories typically report the following results for low smoke halogen-free foam submissions:
- Limiting oxygen index (LOI) at ambient temperature (23°C), expressed as a volume percentage to one decimal place.
- Pass/fail verification against a specification limit, such as LOI ≥ 28% or LOI ≥ 32%.
- Temperature index (TI) determined at elevated temperatures per ISO 4589-3, where required by rail or marine specifications.
- Observation data including afterflame time, melting, dripping, curling, intumescence and char formation, which assist in failure analysis.
- Optional complementary flammability and smoke items frequently requested alongside LOI: UL 94 or IEC 60695-11-10 vertical/horizontal burning, ISO 5659-2 or ASTM E662 smoke density, and EN 14582 or IEC 60754 halogen and acidity determination.
Testing Methods
Test Apparatus
The standard apparatus consists of a heat-resistant glass chimney (typically 70–100 mm in diameter and 450–600 mm high), a vertical specimen holder with a support ring, a precision oxygen and nitrogen mixing and flow-control system with calibrated analyzers or flowmeters, and a small propane or butane diffusion igniter. The total gas flow through the column is maintained at 40 ± 2 mm/s superficial velocity, and oxygen concentration at the column exit is verified with an oxygen analyzer accurate to ± 0.1 volume percent.
Test Procedure
The specimen is clamped vertically in the holder, which is then positioned in the center of the chimney. The column is purged with the oxygen-nitrogen mixture for at least 30 seconds before ignition. The operator applies the igniter flame to the top of the specimen for up to 30 seconds, removing it briefly every 5 seconds to observe whether the specimen ignites. For each oxygen concentration, the specimen behavior is judged against the failure criteria: burning for more than 180 seconds or flame front reaching the 50 mm downward measurement mark.
Testing begins with a preliminary concentration estimated from experience or prior data. The concentration is then adjusted in steps to bracket the transition between burning and extinguishing, followed by fine increments of 0.2% oxygen to locate the critical point precisely. In the final determination phase, a fresh specimen is tested at the candidate concentration, and three confirmation specimens are tested; the LOI is validated when the behavior is correctly predicted for at least two of the three verification specimens, as prescribed by ISO 4589-2.
Calculation and Expression of Results
The oxygen index is calculated as the volume fraction of oxygen in the total gas mixture at the critical concentration: LOI = 100 × V(O₂) / [V(O₂) + V(N₂)]. The result is reported to one decimal place together with the standard used, specimen dimensions, density, conditioning history and a description of burning behavior. Because foam morphology introduces anisotropy, results are reported separately for machine and transverse directions when specimen orientation differs.
Acceptance Criteria and Interpretation
Materials are broadly classified as follows: LOI below 21% burns readily in air, 21–27% indicates self-ext extinguishing or slow-burning behavior in air depending on geometry, and above 27% is generally regarded as flame retardant. Most LSZH foam specifications in rail, cable and marine applications impose limits between 28% and 35%. It should be noted that LOI is a relative, geometry-dependent ranking tool rather than a direct fire-hazard assessment; large-scale and reaction-to-fire performance must be confirmed with the appropriate system-level tests.
Applications
Oxygen index testing of low smoke halogen-free foam supports material qualification and production control across multiple industries:
- Rail transit: qualification of seat cushions, thermal insulation, sealing foams and LSZH cable compounds against material requirements referenced in EN 45545-2 fire-safety schemes, where suppliers are frequently required to demonstrate LOI ≥ 28–32%.
- Wire and cable: incoming inspection and formulation development of halogen-free insulation, bedding and sheathing compounds specified in IEC and EN low-smoke cable families.
- Electric vehicles: verification of silicone and polyolefin foam thermal barriers and compression pads used in battery packs, where high LOI values are a core design input.
- Building and infrastructure: fire-rated gaskets, expansion seals and acoustic foams installed in escape routes and tunnels.
- Aerospace and marine: lightweight halogen-free foam components qualified against flame-retardance acceptance limits using ISO 4589-2 or ASTM D2863 data.
- Consumer electronics: padding and sealing foams in enclosures where halogen-free content and self-extinguishing behavior are both mandated.
FAQ
What oxygen index value is required for low smoke halogen-free foam? Most industrial specifications require a minimum LOI of 28%, while rail transit, marine and battery-protection applications commonly demand 32% or higher. Values above 35% are achievable with highly filled or inherently resistant compounds such as silicone foam.
Why do foams usually show a lower LOI than solid materials of identical composition? The cellular structure of foam provides a large surface area, efficient heat feedback and ready access of oxygen to the polymer matrix, all of which accelerate combustion. This is why specimen density, cell size and orientation are always reported with the LOI result.
Is an LOI result equivalent to a UL 94 rating? No. LOI ranks ease of extinction in a controlled oxygen atmosphere, while UL 94 and IEC 60695-11-10 evaluate burning behavior in normal air against defined afterflame and ignition criteria. The two tests are complementary, and many qualification programs require both.
Are ISO 4589-2, ASTM D2863 and GB/T 2406.2 results interchangeable? The methods are technically aligned and use the same apparatus and criteria; however, minor differences in verification procedures and reporting conventions exist, so the applied standard should always be stated. Most certification schemes accept data from any of the three when performed by accredited third-party laboratories.
Does oxygen index testing measure smoke production or halogen content? No. LOI assesses only flammability. Smoke optical density is measured by ISO 5659-2 or ASTM E662, while halogen content and corrosive gas evolution are determined by EN 14582 and IEC 60754 series methods, which are typically ordered together to fully characterize an LSZH foam.
How many specimens are required? A full determination generally consumes 15 to 20 specimens, including preliminary screening, fine bracketing and the three confirmation specimens. Additional specimens should be supplied when retests, orientation studies or density variation checks are anticipated.
