The oxygen index (Limiting Oxygen Index, LOI) test is the most widely used small-scale method for evaluating the flammability of polymer foam materials such as polyurethane, polystyrene, polyolefin and phenolic foams. This article explains the principles of foam oxygen index analysis, the governing standards (ISO 4589-2, ASTM D2863 and GB/T 2406.2), specimen preparation requirements, step-by-step test procedures, typical result ranges and practical applications in quality control and flame-retardant material development.
Technical Overview
The oxygen index (OI), commonly called the limiting oxygen index (LOI), is defined as the minimum concentration of oxygen, expressed as a volume percentage in a mixture of oxygen and nitrogen, that will just support candle-like combustion of a vertically mounted specimen under specified laboratory conditions. Since normal air contains approximately 20.95% oxygen by volume, the LOI provides an intuitive benchmark of flammability: foam materials with an LOI below about 21% will continue to burn in ordinary air once ignited, while materials with an LOI of 26% or higher are generally regarded as self-extinguishing or flame retardant.
Polymeric foams are particularly susceptible to ignition and rapid flame spread because of their low density, extremely high specific surface area and, in many cases, open-cell structures that allow air to penetrate the material interior. Oxygen index analysis is therefore one of the most fundamental and cost-effective screening tools for characterizing the combustion behavior of cellular plastics. The method is standardized internationally in ISO 4589-2 and ASTM D2863, and in the Chinese national standard GB/T 2406.2, which is technically aligned with ISO 4589-2. ISO 4589-3 extends the same principle to elevated-temperature testing.
- Unmodified flexible polyurethane foam: approximately 17–18%
- Flame-retardant flexible polyurethane foam: typically 24–28%
- Rigid polyurethane and polyisocyanurate (PIR) foam: approximately 25–32%
- Expanded polystyrene (EPS), standard grade: approximately 18–19%; flame-retardant grade EPS: 30% or higher
- Extruded polystyrene (XPS): approximately 24–30%
- Phenolic foam: approximately 35–50%
- PVC and melamine-based foams: often well above 30%
Testing Samples
Specimen Geometry and Preparation
Under ISO 4589-2 and GB/T 2406.2, foam specimens are cut as rectangular strips 80 to 150 mm in length and 10 ± 0.5 mm in width. For cellular plastics with a density of 100 kg/m³ or greater, the standard specimen thickness is 10 ± 0.5 mm; for foams with a density below 100 kg/m³, a thickness of 20 ± 1 mm is used to ensure representative burning behavior. ASTM D2863 follows a closely comparable approach for cellular materials. A reference mark is drawn 50 mm from the top of each specimen so the operator can judge whether the flame front reaches the defined burning criterion.
Because many foams are anisotropic, specimens should be cut along both the rise (vertical) direction and the transverse direction whenever the material structure is directionally oriented, as in extruded boards or slabstock production. Specimens must be free of skin, densified layers, voids and machining damage unless those features are intentionally being evaluated. Before testing, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 88 hours in accordance with ISO 291. A minimum of 15 specimens, and often up to 20 for low-density foams, should be prepared to allow the bracketing sequence to be completed.
Typical Foam Materials Tested
- Rigid and flexible polyurethane foams, including PIR foam insulation boards
- EPS bead boards and molded parts, and extruded polystyrene (XPS) boards
- Polyolefin foams such as polyethylene (PE), polypropylene (PP) and EVA foams
- Phenolic and melamine foams used for thermal and acoustic insulation
- PVC and nitrile-butadiene rubber (NBR) foams for pipe and HVAC insulation
- Natural latex foam and specialty sponge materials for furniture and packaging
Testing Items
Accredited third-party laboratories typically report the following results for foam oxygen index analysis:
- The limiting oxygen index (LOI) value in volume percent oxygen, reported to one decimal place
- The ignition method used (top-surface ignition, Method A, or propagating ignition, Method B)
- Observed burning behavior, including afterflame and afterglow duration, flaming drips, shrinkage, curling and char formation
- Whether specimens reached the 50 mm reference mark within the criterion time
- A pass or fail judgment against the client specification, product standard or classification requirement
Testing Methods
Apparatus
The test is performed on an oxygen index apparatus consisting of a heat-resistant glass chimney (typically 70–100 mm in diameter and 450–600 mm high), a vertical specimen holder with a clamp, a mixing and metering system for oxygen and nitrogen, calibrated flow controllers or an oxygen analyzer, and a small hydrocarbon gas igniter producing a defined flame. The mixed gas passes through a bed of glass beads at the chimney base to ensure uniform, laminar upward flow around the specimen. The superficial gas velocity through the chimney is maintained at 40 ± 2 mm/s throughout the test, and the gases used should have a purity of at least 99.5%.
Test Procedure
- Condition and measure the specimens, then mount one vertically in the holder with at least 50 mm of the specimen exposed above the top of the holder.
- Set the initial oxygen concentration based on experience or published data for similar foams, allow the gas mixture to purge the chimney for at least 30 seconds, and verify the total flow rate.
- Apply the igniter flame to the top of the specimen for up to 10 seconds (Method A, top-surface ignition) or until the top surface ignites and the flame begins to spread downward (Method B, propagating ignition).
- Remove the igniter and start timing; record whether the specimen burns for 30 seconds or longer, or whether the flame front reaches or passes the 50 mm reference mark.
- Adjust the oxygen concentration upward or downward, typically in steps of 0.5% to 1% initially and 0.2% as the limiting value is approached, repeating the test on fresh specimens.
- Bracket the transition between burning and self-extinguishing behavior until the limiting oxygen concentration is established to the required precision.
Determination and Expression of Results
The LOI is taken as the oxygen concentration at which the material just sustains combustion according to the pass criterion, determined through the bracketing sequence described in the standard. The final value is reported as a volume percentage to one decimal place, together with the standard used, the ignition method, the specimen dimensions and orientation, the conditioning history, and any relevant burning observations such as dripping or melt collapse. When the difference between results for the two orientations exceeds a defined margin, both directional values should be reported separately.
Factors Affecting Results
- Density and cell structure: closed-cell and low-density foams often show different LOI values than higher-density or open-cell grades of the same polymer
- Specimen orientation relative to the foam rise direction
- Conditioning state: moisture absorbed by hygroscopic or flexible foams can noticeably raise measured LOI values
- Gas purity, flow calibration and the cleanliness of the chimney and bead bed
- Operator judgment of the ignition event and the burning criterion, which is why inter-laboratory comparisons require strict adherence to the standard
Applications
Oxygen index testing of foam materials supports a wide range of industrial and regulatory objectives:
- Building insulation: verifying flame-retardant grades of PU, PIR, EPS and XPS foams and supporting classification under systems such as GB 8624 and EN 13501 when combined with other fire test methods
- Furniture and bedding: screening flexible foam grades used in mattresses, upholstered furniture and public-transport seating for compliance with flammability requirements
- Automotive, rail and marine: evaluating interior foam components alongside horizontal burn tests such as FMVSS 302 and ISO 3795
- Research and development: quantifying the effectiveness of flame retardants, including phosphorus-based, halogen-based, nitrogen-based and mineral filler systems, and supporting reformulation work
- Quality control: routine incoming and outgoing inspection of foam production batches against declared LOI specifications
- Material selection: comparing candidate foams for enclosures, thermal insulation, packaging and acoustic applications where ignition resistance matters
FAQ
What LOI value qualifies a foam as flame retardant?
Although acceptance thresholds depend on the product standard, a common convention is that LOI values of 26% or above indicate self-extinguishing behavior, while values of 28–32% or higher are frequently specified for building insulation and transportation foams. Values below 21% indicate the material will burn readily in normal air.
Which standard should be chosen, ISO 4589-2 or ASTM D2863?
The two methods share the same principle and produce broadly comparable results, but they differ in chimney dimensions, some specimen details and procedural wording. ISO 4589-2 and the equivalent GB/T 2406.2 are commonly requested in Asia and Europe, while ASTM D2863 is prevalent in North American supply chains. The choice should follow the specification invoked by the buyer or the target regulation.
How many specimens are required?
At least 15 specimens are normally needed to complete the bracketing sequence, and 20 or more are recommended for low-density foams whose burning behavior is more variable. Additional specimens should be available for retesting when results are borderline.
Does the LOI value predict performance in a real fire?
The oxygen index test is a small-scale, well-ventilated screening method. It is excellent for ranking materials and quality control, but it does not by itself predict behavior in large-scale fires, where heat flux, geometry and oxygen depletion differ greatly. For comprehensive assessment, LOI should be combined with tests such as UL 94, ISO 3795 or FMVSS 302, smoke density testing and cone calorimetry per ISO 5660.
How long does foam oxygen index testing take?
After specimen conditioning, the instrument test itself is usually completed within a few hours, and accredited third-party laboratories typically issue reports within three to seven working days, depending on the number of materials and specifications involved.
