Oxygen index combustion analysis determines the limiting oxygen index (LOI) of foam specimens — the minimum oxygen concentration, expressed as a volume percentage, that will just sustain flaming combustion of a vertically mounted sample. As one of the most widely used small-scale screening methods for cellular polymers such as polyurethane, polystyrene, polyolefin, and phenolic foams, LOI testing is performed to ISO 4589-2, ASTM D2863, and GB/T 2406.2 in accredited third-party laboratories. This article explains the test principle, specimen requirements, procedural steps, acceptance criteria, typical reference values, and industrial applications of oxygen index combustion analysis for foam materials.
Technical Overview
The limiting oxygen index (LOI), also known as the oxygen concentration index, is defined as the minimum concentration of oxygen in a flowing oxygen and nitrogen mixture that will just support candle-like flaming combustion of a specimen initially at ambient temperature. Because normal ambient air contains approximately 20.95% oxygen by volume, a foam material with an LOI above approximately 21% will not continue to burn easily once an ignition source is removed, while a material with an LOI below this threshold will propagate flame in ordinary air.
Oxygen index combustion analysis is a repeatable, quantitative, and relatively inexpensive bench-scale method. For foam specimens — which are often used in thermally sensitive or fire-critical applications such as building insulation, upholstered furniture, and vehicle interiors — the LOI value serves as a key indicator of flame-retardant performance for material development, quality control, and specification compliance.
- ISO 4589-2 — Plastics: Determination of burning behaviour by oxygen index, Part 2: Ambient-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, technically equivalent to ISO 4589-2
- ISO 4589-3 — Supplementary method for determination at elevated temperatures
It should be emphasized that oxygen index testing is a small-scale screening and characterization method. LOI results correlate with, but do not replace, large-scale reaction-to-fire tests such as those used in the GB 8624 and EN 13501 classification schemes, and they complement other bench tests such as UL 94 vertical and horizontal burning tests.
Testing Samples
Rigid Foam Specimens
Rigid cellular materials commonly submitted for LOI analysis include rigid polyurethane (PU) insulation foam, polyisocyanurate (PIR) foam, phenolic foam, extruded polystyrene (XPS), and expanded polystyrene (EPS). These materials are typically used as thermal insulation in walls, roofs, and pipe systems, where flame-retardant grades are frequently specified. Rigid foams are generally self-supporting and are straightforward to mount vertically in the test chimney.
Flexible Foam Specimens
Flexible foams include slabstock and molded flexible polyurethane foam for mattresses, seating, and packaging; cross-linked and non-cross-linked polyethylene (EPE) foam; ethylene-vinyl acetate (EVA) foam; and flexible elastomer foams. Because soft, highly compressible foams may deform under their own weight or buckle during testing, laboratories take additional care in specimen mounting, and very flexible specimens may require alternative specimen configurations described in the applicable standards.
Specialty and Elastomeric Foams
Silicone foam, melamine foam, polyvinyl chloride (PVC) foam, nitrile-butadiene rubber (NBR) insulation foam, and intumescent or fire-barrier foams are also analyzed by the oxygen index method. Many of these materials inherently exhibit high LOI values and are selected precisely for their resistance to ignition in air.
Specimen Dimensions and Preparation
Foam specimens are cut from production blocks, sheets, or finished parts using sharp blades or saws to produce clean, square edges without crushed or melted cell structure. Facings, skins, and adhesively bonded layers are removed unless the composite construction itself is to be evaluated.
- ISO 4589-2 preferred specimen: 80 mm to 150 mm long, 10 ± 0.5 mm wide, 10 ± 0.5 mm thick (typical configuration for self-supporting foams)
- ASTM D2863 preferred specimen: 70 mm to 150 mm long, 6.5 ± 0.5 mm wide, 3 ± 0.5 mm thick
- Alternative dimensions may be agreed between parties provided they are documented and applied consistently
- Specimens cut parallel to and perpendicular to the foam rise direction should be distinguished, as cell orientation can influence the result
- A reference mark is applied 50 mm below the top edge of each specimen to define the burning-length criterion
Conditioning Prior to Testing
Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours in accordance with ISO 291 prior to testing. Because open-cell foams can absorb moisture that affects combustion behavior, consistent conditioning is essential for reproducible and comparable results.
Testing Items
The primary measurand of oxygen index combustion analysis is the limiting oxygen index of the foam specimen, expressed as a volume percentage of oxygen in the oxygen and nitrogen mixture. In addition to the numerical LOI value, a complete test report documents supporting observations that characterize the burning behavior.
- Limiting oxygen index (LOI) in % v/v, reported to the nearest 0.1%
- Specimen description: foam type, nominal density, cell structure, thickness, and orientation
- Afterflame time and afterglow time following removal of the ignition source
- Occurrence of flaming drips, molten collapse, shrinkage, or intumescence
- Whether combustion was sustained by flaming, smoldering, or glowing
- Number of specimens tested and the conditioning regime applied
- Optional: oxygen index at elevated temperature per ISO 4589-3 for thermally demanding applications
Testing Methods
Apparatus
The oxygen index apparatus consists of a vertical heat-resistant glass chimney (typically at least 75 mm in internal diameter and 450 mm to 500 mm high), a specimen holder that supports the sample vertically at the center of the chimney, precision flow meters or mass flow controllers for oxygen and nitrogen, and a small propane or butane pilot-flame igniter. The mixed gas flows upward through the chimney at a controlled linear velocity, purging the column and surrounding the specimen with a uniform atmosphere of defined oxygen concentration.
Test Procedure
- Measure and condition the specimens, then mount the first specimen vertically so that the top of the specimen is well below the top of the chimney.
- Set the initial oxygen concentration based on prior knowledge or an estimated value, and sweep the chimney with the gas mixture for at least 30 seconds at a total flow corresponding to a linear velocity of 40 ± 2 mm/s (approximately 10.6 L/min in a standard 75 mm chimney).
- Apply the pilot flame to the top edge of the specimen in 5-second contact cycles for a maximum of 30 seconds until the specimen ignites.
- Observe the combustion: the trial is recorded as "burns" if flaming continues for more than 180 seconds after withdrawal of the igniter or if the burning front passes the 50 mm reference mark; otherwise the trial is recorded as "does not burn" at that concentration.
- Increase the oxygen concentration if the specimen extinguished early, or decrease it if the specimen met the burning criterion, and test a fresh specimen at each new concentration.
- Near the limiting concentration, bracket the result using concentration intervals of 0.2% (or 0.25%) oxygen until the burning behavior is balanced.
- Test a minimum of 15 specimens to complete a valid determination, verifying the final value with confirmation trials as required by the standard.
Calculation of the Oxygen Index
The oxygen concentration at each trial is calculated from the volumetric flow rates of the two gases, and the limiting oxygen index is determined from the bracketing series. The working relationship for each trial atmosphere is expressed as: LOI atmosphere (%) = 100 × V(O2) / [V(O2) + V(N2)], where V(O2) and V(N2) are the measured volume flow rates of oxygen and nitrogen, respectively. The final LOI is derived from the critical concentration in the balancing series and reported to one decimal place, for example 27.5%.
Interpretation of Results
LOI values provide an immediately meaningful ranking of the ease of ignition and flame-sustaining tendency of foam materials in air.
- Below 21% — readily combustible; the foam sustains flame in normal ambient air
- 21% to 25% — slow-burning tendency in air; improved ignition resistance
- 26% to 34% — generally self-extinguishing behavior in many practical environments
- Above 35% — highly flame-resistant; ignition is difficult to sustain even in enriched atmospheres
Within a single laboratory, repeatability is typically on the order of ±0.5% oxygen, while inter-laboratory reproducibility is generally within approximately ±1% to ±1.5%; differences smaller than these bands should not be treated as significant.
Typical LOI Ranges for Common Foams
The following indicative ranges are frequently encountered in practice and are useful for benchmarking, although formulation, density, and cell structure can shift values substantially.
- Untreated flexible polyurethane foam: 16% to 18%
- Flame-retardant flexible polyurethane foam: 24% to 30%
- Rigid polyurethane and PIR insulation foam: 24% to 30%
- Expanded polystyrene (EPS): 18% to 20%; flame-retardant grades: 24% to 35%
- Polyethylene and EVA foams: 17% to 22% untreated; flame-retardant grades: 25% to 32%
- Phenolic foam: 35% to 50%
- PVC and melamine foams: 35% and above
- Silicone foam: 28% to 35%
Applications
Oxygen index combustion analysis of foam specimens supports decision-making across the full product lifecycle, from research and formulation screening to production quality control and regulatory compliance verification. The method is particularly valuable because it requires only small quantities of material and delivers a numeric result that can be written directly into material specifications and purchase agreements.
- Flame-retardant development: screening halogen-free phosphorus, nitrogen, mineral filler, and intumescent additive systems in polyurethane and polyolefin foams
- Building products: supporting the evaluation of insulation foams used in walls, roofs, and cavity applications under classification frameworks such as GB 8624 and EN 13501
- Furniture and bedding: verifying the ignition resistance of flexible slabstock and molded seating and mattress foams
- Transportation: material qualification for automotive, railway (including EN 45545-2 referenced product groups), marine, and aerospace interior foams
- Wire and cable: assessing foamed insulation and jacketing compounds
- Quality control: monitoring batch-to-batch consistency of flame-retardant loading and foam density in continuous production
- Aging and durability studies: tracking LOI retention after heat aging, humidity exposure, or migration of flame retardants
- Academic and industrial research: benchmarking novel bio-based, flame-retarded, and nanocomposite foam systems
FAQ
What does the LOI value of a foam actually mean?
The LOI represents the minimum oxygen concentration, in volume percent, needed for a vertically mounted foam specimen to sustain candle-like flaming combustion for at least 3 minutes or over a burning length of 50 mm. Since air contains about 21% oxygen, foams with LOI values above 21% tend to self-extinguish in normal air once the ignition source is removed.
Which standards govern oxygen index testing of foam specimens?
The principal standards are ISO 4589-2 and its Chinese counterpart GB/T 2406.2 for ambient-temperature testing, and ASTM D2863 for equivalent testing under American practice. ISO 4589-3 extends the method to elevated temperatures. All are routinely performed by accredited third-party laboratories.
How many foam specimens are required for one LOI determination?
A valid determination normally requires a minimum of 15 usable specimens because the method relies on successive trials at different oxygen concentrations to bracket the limiting value. Additional specimens should be prepared to allow for retests in cases of anomalous behavior such as severe dripping or shrinkage.
Does specimen orientation affect the LOI result for foams?
Yes. Foams are anisotropic, and specimens cut parallel to the rise direction can burn differently from those cut perpendicular to it. The tested orientation must be recorded in the report, and comparisons between materials should always use consistently oriented specimens.
Does a high LOI guarantee compliance with fire safety regulations?
No. A high LOI demonstrates strong ignition resistance in a small-scale test, but regulatory approval for building, transport, and furniture applications typically depends on additional reaction-to-fire, flame spread, smoke, and full-scale performance tests. LOI data are best used for screening, specification, and quality control alongside the required application-specific tests.
What factors most commonly influence LOI results for foam materials?
Density, cell structure and orientation, specimen thickness, moisture conditioning history, flame-retardant loading and distribution, and the presence of skins or facings all influence the measured value. Consistent specimen preparation and conditioning are therefore essential for meaningful comparisons between laboratories and between production batches.
Can oxygen index testing be performed at temperatures above ambient?
Yes. ISO 4589-3 defines the procedure for determining the oxygen index at elevated temperatures, which is useful for foams used near heat sources. In general, LOI decreases as temperature rises, so ambient-temperature results should not be extrapolated to hot-service conditions without testing.