The oxygen index test, also known as the limiting oxygen index (LOI) test, is one of the most widely used laboratory methods for evaluating the ignition and combustion behavior of plastic foams. This article explains the standard test methods for foam oxygen index testing, covering the applicable international and national standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, along with specimen requirements, test procedures, calculation of results, and typical industrial applications.
Technical Overview
The limiting oxygen index (LOI) is defined as the minimum concentration of oxygen, expressed as a percentage by volume, in a flowing mixture of oxygen and nitrogen that will just support candle-like flaming combustion of a vertically mounted specimen under specified test conditions. Because ambient air contains approximately 20.9% oxygen, foam materials with an LOI below 21% will generally continue to burn in air once ignited, while materials with higher LOI values are progressively harder to ignite and more likely to self-extinguish.
In the test, a bar-shaped foam specimen is clamped vertically inside a transparent glass chimney. A metered oxygen/nitrogen mixture flows upward through the chimney at a controlled velocity, the top of the specimen is ignited with a small burner flame, and the operator records whether burning is sustained beyond the specified criteria. The oxygen concentration is adjusted stepwise until the critical concentration is bracketed, and this value is reported as the oxygen index.
The principal standards governing foam oxygen index testing include:
- ISO 4589-2 – Plastics – Determination of burning behaviour by oxygen index – Part 2: Ambient-temperature test
- ISO 4589-3 – Part 3: Elevated-temperature test
- ASTM D2863 – Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index)
- GB/T 2406.2 – Chinese national standard for the ambient-temperature oxygen index test, technically aligned with ISO 4589-2
- JIS K 7201-2 – Japanese industrial standard equivalent to ISO 4589-2
It should be noted that the oxygen index test is a small-scale screening and quality-control method. LOI values are valuable for comparing formulations and supporting material qualification, but they do not by themselves define fire hazard in real-scale fire scenarios.
Testing Samples
Almost all families of polymer foam can be evaluated by the oxygen index method, provided that suitable bar specimens can be cut and that the material does not melt, drip, or collapse so severely that a valid vertical burn observation is impossible. Typical sample categories are described below.
Flexible Polyurethane Foam
Used in seating, mattresses, and acoustic padding, flexible polyurethane foam is one of the most frequently tested materials. Because of its low density and open-cell structure, specimens are usually cut with a larger cross-section, commonly 10 mm × 10 mm bars of 80–150 mm length in accordance with the cellular-material specimen type of ISO 4589-2 and GB/T 2406.2.
Rigid Polyurethane and PIR Foam
Rigid PUR/PIR insulation foams are tested to support building-product qualification and flame-retardant formulation development. Specimens are cut from boards or bun stock with cut surfaces flat and parallel, and edges free of skins where the standard requires the core material to be evaluated.
Polyolefin Foams (PE, EVA, PP)
Closed-cell polyethylene, ethylene-vinyl acetate, and polypropylene foams used in footwear midsoles, packaging, and flotation products are commonly submitted for LOI testing, often alongside horizontal burn and UL 94 foam classifications.
Elastomeric PVC/NBR Foam
Flexible elastomeric insulation foam for HVAC piping and ductwork is tested to verify flame-retardant grades. The dense, closed-cell structure generally yields stable vertical burning behavior and reproducible LOI values.
Phenolic, Melamine, and Specialty Foams
High char-forming foams such as phenolic and melamine foam typically achieve high oxygen index values and are tested to demonstrate compliance with demanding fire-performance specifications in mass transport and construction.
Specimen Preparation and Conditioning
- Standard bar dimensions: typically 80–150 mm long × 10 mm wide × 10 mm thick for cellular plastics per ISO 4589-2/GB/T 2406.2; ASTM D2863 uses 70–150 mm × 6.5 mm × 3 mm bars for solid plastics, with larger cross-sections permitted for cellular materials.
- Quantity: a minimum of 15 specimens is recommended for the full bracketing procedure, with additional specimens retained for repeat testing.
- Conditioning: 23 ± 2 °C and 50 ± 5% relative humidity for at least 40 hours (ISO 291 or ASTM D618), unless otherwise specified by the relevant product standard.
- Specimens must be free of skin, dense surface layers, or defects unless the skin itself is the object of the test, and dimensions must be verified before mounting.
Testing Items
The primary and auxiliary test items for a foam oxygen index evaluation typically include:
- Limiting oxygen index (LOI) at ambient temperature, reported to 0.1%
- Oxygen index at elevated temperature in accordance with ISO 4589-3, for example at 70 °C, used in certain transport and cable specifications
- Ignitability observations: ease of ignition, afterflame time, and whether flaming or glowing combustion is sustained
- Quantified burning response at fixed oxygen concentrations: burning time and damaged or charred length versus the standard pass criteria (typically 3 minutes or 50 mm)
- Behavioral observations such as dripping, shrinking, collapse of the cellular structure, and soot or smoke release during the test
- Comparative LOI before and after thermal ageing or humidity exposure, where required by the specification
Testing Methods
Apparatus and Test Conditions
- A vertical borosilicate glass chimney, typically about 450–500 mm high with an internal diameter of 70–100 mm, mounted on a base with gas inlets and a mixing chamber.
- Supplies of high-purity oxygen and nitrogen with pressure regulation and calibrated flow controllers; gas temperature equilibrated to 23 ± 2 °C before entering the column.
- A gas velocity through the chimney of approximately 40 mm/s, maintained constant throughout the concentration sweep.
- A specimen holder that clamps the lower end of the specimen so that the top edge lies well below the top of the chimney.
- A small propane or butane igniter used for either Procedure A (top-surface ignition) or Procedure B (top and face ignition to promote flame propagation).
Test Procedure
- Condition the specimens, verify dimensions, and select the ignition procedure appropriate to the material.
- Set an initial oxygen concentration based on experience or expectation, purge the chimney, mount the specimen, and allow the gas flow to stabilize before ignition.
- Apply the igniter flame to the top of the specimen; record whether flaming combustion exceeds the failure criteria (burning longer than 3 minutes or damaged length greater than 50 mm for standard bar specimens).
- Increase the oxygen concentration if the specimen fails to sustain burning, or decrease it if the criteria are exceeded, using progressively smaller steps (down to about 0.2% oxygen) as the critical concentration is approached.
- Confirm the bracketing result with repeat specimens until the response sequence satisfies the standard, then calculate the oxygen index.
Calculation of the Oxygen Index
The oxygen index is the volume percentage of oxygen in the inlet gas mixture at the critical condition:
LOI = 100 × V(O2) / [V(O2) + V(N2)]
ISO 4589-2 and GB/T 2406.2 additionally define a calculation method of the form LOI = cF + k·d, where cF is the final test concentration, d is the concentration interval used, and k is a coefficient derived from the recorded sequence of burning and non-burning responses. The reported result is rounded to 0.1%.
Interpretation of Results
- LOI below 21%: the foam will typically continue to burn in normal air after ignition.
- LOI between 21% and about 26%: marginal burning behavior; generally not considered self-extinguishing.
- LOI of 26% to 28% and above: commonly accepted as indicating flame-retardant, self-extinguishing character for many foam specifications.
- LOI of 32% and above: frequently required for high-grade flame-retardant foams in demanding applications such as insulation boards and mass-transport components.
The test report should identify the standard and specimen type used, the ignition procedure, specimen dimensions and density, conditioning history, the oxygen index value, and all observations such as dripping, shrinkage, or glowing combustion.
Applications
Oxygen index data for foams are used across many industries for material development, quality control, and regulatory support:
- Automotive interior foams: development and verification of flame-retardant seating, headrest, and trim foams supporting regulations such as FMVSS 302 programmes.
- Furniture and bedding: evaluation of flexible slabstock and molded polyurethane foams against flammability specifications and voluntary safety schemes.
- Building insulation: qualification of rigid PUR/PIR and phenolic foam boards within classification frameworks such as GB 8624 and related product standards.
- Footwear and packaging: screening of EVA and PE foams for ignition resistance and for compliance with buyer flammability requirements.
- HVAC and industrial insulation: verification of flame-retardant PVC/NBR elastomeric foam grades.
- Rail and aerospace interiors: comparative evaluation of specialty foams used in seat cushions, panels, and thermal barriers.
- Research and development: quantifying the effectiveness of phosphorus, nitrogen, halogen, mineral-filler, and intumescent flame-retardant systems, and studying the influence of density, cell structure, and ageing on burn performance.
Accredited third-party laboratories routinely perform these tests to ISO/IEC 17025 quality requirements, providing results that are accepted by regulators, buyers, and certification bodies worldwide.
FAQ
What is the oxygen index of a foam material?
It is the minimum oxygen concentration, in an oxygen/nitrogen mixture, at which a vertically mounted foam specimen will just sustain flaming combustion under standardized conditions. It is expressed as a volume percentage; higher values indicate greater resistance to burning.
What LOI value is considered flame retardant for foams?
Although acceptance limits depend on the product specification, foams with an LOI of 26% to 28% or above are generally regarded as flame retardant and self-extinguishing, and some high-specification applications require values of 32% or greater.
Which standards apply to foam oxygen index testing?
The most common are ISO 4589-2 and its equivalents ASTM D2863, GB/T 2406.2, and JIS K 7201-2 for ambient-temperature testing, and ISO 4589-3 for elevated-temperature oxygen index determination.
How many specimens are needed for a test?
A minimum of 15 conditioned bar specimens is recommended for a full determination, because the bracketing procedure requires testing at several oxygen concentrations plus confirmation runs at the critical concentration.
Can all flexible foams be tested by this method?
Most can, but very thin, highly flexible, or heavily dripping foams may not exhibit valid candle-like vertical burning. In such cases, alternative methods such as the UL 94 foam classifications (HBF, HF-1, HF-2) or horizontal burn tests are more appropriate.
How does LOI relate to UL 94 ratings?
They are independent tests: LOI quantifies the oxygen concentration needed to sustain burning, while UL 94 evaluates burning behavior in air after removal of a small flame. A high LOI generally correlates with better UL 94 performance, but one cannot be substituted for the other in specifications.
What factors influence the accuracy of LOI results?
Specimen dimensions and density, cell structure and skin condition, conditioning history, gas purity and flow calibration, chimney temperature, ignition procedure, and operator technique all affect repeatability. Interlaboratory reproducibility is typically on the order of one to two percentage points of oxygen.
Is the oxygen index test sufficient for fire safety certification?
No. It is a small-scale screening and quality-control test. Regulatory fire classification of foam products also relies on ignition, spread-of-flame, heat release, smoke, and toxicity tests performed at bench or real scale according to the relevant application standards.