Oxygen index (Limiting Oxygen Index, LOI) combustion testing is the principal laboratory method for quantifying how readily foam cushioning materials sustain burning, making it essential for furniture, automotive, and public-transport seating compliance. This article explains the LOI testing principle, applicable standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, specimen requirements, test procedures, and typical acceptance thresholds used by accredited third-party laboratories and manufacturers worldwide.
Technical Overview
The oxygen index test, also known as the limiting oxygen index (LOI) test, is a small-scale laboratory method that measures the minimum concentration of oxygen, expressed as a volume percentage in a flowing oxygen and nitrogen mixture, required to just sustain candle-like combustion of a vertically mounted specimen. Because ordinary air contains approximately 21% oxygen, the LOI value immediately indicates whether a foam will continue to burn in normal air once an ignition source is removed.
The result is calculated as follows: LOI (%) = 100 × oxygen flow ÷ (oxygen flow + nitrogen flow). A higher value means more oxygen is required to keep the specimen burning, and the material is therefore considered more flame-resistant.
- Below 21% – the foam continues to burn readily in normal ambient air once ignited.
- Around 21–26% – marginal performance; self-extinguishing behavior depends on geometry, density, and heat feedback.
- Above 26% – generally accepted as flame-retardant; the most common procurement threshold for seating foams.
- Above 28–32% – frequently required for rail, marine, and high-occupancy public seating specifications.
Foam cushions are particularly fire-sensitive because their cellular structure provides a very large surface area at low density, open cells allow rapid oxygen access to the combustion zone, and thermoplastic or thermoset polymers decompose into flammable gases. For this reason, flame-retardant grades must be verified by oxygen index testing before use in regulated applications.
Testing Samples
Flexible Polyurethane Foam (PU)
Flexible PU foam is the dominant cushioning material for sofas, office chairs, mattresses, and vehicle seats, in both slabstock and molded high-resilience forms. Unmodified grades typically show LOI values of roughly 17–18%, while flame-retardant grades reach 24–30% or higher. Specimens are normally cut from the bare core foam of the finished cushion.
Viscoelastic (Memory) Foam
Slow-recovery viscoelastic foam is widely layered with conventional PU in premium seating. Its LOI depends strongly on the flame-retardant package used, so testing verifies both the base formulation and any post-treatment.
Polyolefin Foams (EPE, XPE, EPP)
Polyethylene and polypropylene foams are used in packaging, flotation cushions, and child restraint padding. These materials melt and drip while burning, and flame-retardant modifications must be confirmed by oxygen index measurement.
Rubber Foams (NBR-PVC, EPDM, Latex)
Natural and synthetic latex cushions and NBR-PVC or EPDM comfort foams are also tested. Halogen-containing NBR-PVC compounds naturally achieve higher oxygen index values than hydrocarbon-based foams.
Specimen Preparation and Conditioning
Typical specimens measure 100–150 mm in length with a 10 mm × 10 mm cross-section, or the actual product thickness where specified. Specimens must be cut cleanly without crushed cells, skins, or compressed edges. Fabric covers, adhesives, and coatings are removed so that the bare foam is evaluated. Conditioning is performed at 23 ± 2 °C and 50 ± 5% relative humidity for at least 40 hours before testing, in line with ISO 291 or ASTM D618.
Testing Items
- Limiting oxygen index at ambient temperature (23 °C) – the core measurement for material classification and release.
- Specification conformity verification – pass or fail assessment against contractual thresholds such as LOI ≥ 26% or ≥ 28%.
- Flame-retardant lot control – batch-to-batch comparison to confirm correct dosing of additive masterbatches or treatments.
- Temperature index determination – the temperature at which the oxygen index falls to approximately 21%, evaluated under ISO 4589-3 for materials exposed to elevated service temperatures.
- Post-aging oxygen index – repeat testing after heat or humidity aging to simulate long-term service performance.
- Comparative screening for research and development – ranking of candidate formulations, fillers, and flame-retardant systems.
Testing Methods
Reference Standards
- ISO 4589-2 / EN ISO 4589-2 – Determination of burning behaviour by oxygen index, ambient-temperature test.
- ASTM D2863 – Standard test method for measuring the minimum oxygen concentration to support candle-like combustion of plastics.
- GB/T 2406.2 – Chinese national standard, technically aligned with ISO 4589-2.
- ISO 4589-3 – Elevated-temperature oxygen index procedure used to derive the temperature index.
Apparatus
The test uses an oxygen index chamber consisting of a vertical heat-resistant glass column, typically about 450 mm high, a specimen holder that supports the sample vertically inside the column, a gas mixing and flow control system for oxygen and nitrogen, and a small premixed gas ignition burner. Calibrated flow meters or an oxygen analyzer are used to set the mixture accurately, with the total gas flow adjusted to give a linear velocity of approximately 40 mm/s through the column.
Test Procedure
- Step 1: Mount the conditioned specimen vertically in the holder and position the assembly inside the chimney.
- Step 2: Purge the column with the oxygen and nitrogen mixture for at least 30 seconds to establish a stable atmosphere.
- Step 3: Apply the ignition flame to the top of the specimen for the defined period, using either top-surface ignition or propagation ignition as required by the standard method.
- Step 4: Observe whether combustion is sustained for 3 minutes or spreads over 50 mm of specimen length; either criterion constitutes sustained burning.
- Step 5: Adjust the oxygen concentration using the bracketing, or staircase, approach – increasing oxygen after the specimen fails to burn and decreasing it after sustained burning occurs, with progressively smaller steps.
- Step 6: Calculate the limiting oxygen index from the final series of results and confirm the value by verification tests at the rounded concentration.
- Step 7: Report the LOI to one decimal place, together with specimen dimensions, orientation, conditioning, ignition method, and the standard used.
Precision and Influencing Factors
Within-laboratory repeatability is typically better than 0.5 percentage points of oxygen, while interlaboratory reproducibility is generally on the order of one percentage point. Results for foams depend on density, cell structure, specimen thickness, uniformity of flame-retardant distribution, and aging history, so meaningful comparisons require like-for-like specimen geometry and documented conditioning.
Applications
Oxygen index testing of foam cushions supports product qualification, regulatory compliance, and quality control across multiple industries.
- Public-assembly furniture – cinemas, theaters, auditoriums, and stadium seating where fire codes mandate flame-retardant filling materials.
- Rail and bus interiors – seat foam suppliers demonstrate oxygen index thresholds required by procurement specifications for mass transit.
- Automotive seating – tier suppliers qualify flame-retardant foam grades before production release.
- Office and institutional furniture – purchasers use LOI limits, commonly 26% or above, as objective acceptance criteria.
- Marine and offshore seating – flame-retardant verification for confined-environment applications.
- Manufacturing quality control – incoming resin inspection, additive dosing checks, and batch release of finished foam blocks.
- Research and development – screening of halogen-free flame retardants such as melamine, phosphorus compounds, and expandable graphite.
FAQ
What oxygen index value should a compliant foam cushion achieve?
Most specifications treat 26% as the minimum for flame-retardant classification. Demanding rail and public-transport applications often require 28–32%, while unmodified flexible PU foam typically measures only 17–18% and is not suitable for regulated seating.
Does a high oxygen index mean the cushion cannot burn?
No. The LOI is a relative ranking of how easily combustion is sustained in a controlled atmosphere. Real fires involve higher heat fluxes and complex interactions, so a full fire-risk assessment should combine oxygen index data with cone calorimeter testing to ISO 5660, flame spread, smoke density, and full-scale component tests.
Can a finished cushion with its fabric cover be tested directly?
No. The oxygen index method applies to the foam material itself. Covers, stitching, and adhesives are evaluated with separate ignition and flame spread tests, so specimens should be cut from the bare core foam.
How many specimens are required?
A complete determination typically consumes 15 to 20 specimens, each approximately 100–150 mm long with a 10 mm × 10 mm cross-section. A small offcut block of about 200 × 100 × 100 mm is generally sufficient material.
Are ASTM D2863, ISO 4589-2, and GB/T 2406.2 results interchangeable?
The methods are technically aligned and usually produce very similar values, but minor differences exist in apparatus verification and procedural details. Confirm which standard the applicable specification mandates before testing.
How long does the test take?
Individual specimen evaluations take only a few minutes, but including conditioning, calibration checks, and the staircase procedure, a full determination is normally completed within one to two working days at accredited third-party laboratories.
Is oxygen index testing sufficient for regulatory compliance?
It is usually one element of a broader compliance package. Upholstered furniture flammability regulations and transport codes additionally require smoldering cigarette tests, open-flame ignition tests, and sometimes full-scale mock-up burning, so an integrated test program is recommended.