The oxygen index test, also known as the limiting oxygen index (LOI) test, is one of the most widely used small-scale methods for evaluating the flammability of plastic materials by measuring the minimum oxygen concentration needed to sustain combustion. International standards such as ISO 4589, ASTM D2863, JIS K 7201 and GB/T 2406 define the apparatus, specimen requirements and procedures that make results comparable between laboratories. This article explains the technical basis of plastic oxygen index test standards, covering sample preparation, testing methods, typical LOI values and practical applications.
Technical Overview
The oxygen index (OI), more precisely the limiting oxygen index (LOI), is defined as the minimum concentration of oxygen, expressed as a volume percentage in a flowing mixture of oxygen and nitrogen, that will just support candle-like combustion of a vertically mounted plastic specimen under specified conditions. Since normal air contains approximately 20.9% oxygen, a material with an LOI of 21% or less will generally continue to burn in air once ignited, while higher values indicate progressively greater resistance to burning. The result is a quantitative, repeatable metric used worldwide for material screening, specification writing and quality control.
Although the underlying principle is common to all major standards bodies, the test is published under several technically aligned standards:
- ISO 4589-1: general guidance on oxygen index testing of plastics
- ISO 4589-2: the ambient-temperature oxygen index test, adopted in Europe as EN ISO 4589-2
- ISO 4589-3: oxygen index testing at elevated temperatures and determination of the temperature index
- ASTM D2863: the North American method, technically consistent with ISO 4589-2
- GB/T 2406.1, GB/T 2406.2 and GB/T 2406.3: the Chinese national standards adopting ISO 4589 Parts 1 to 3
- JIS K 7201-2: the Japanese equivalent for ambient-temperature testing
- NES 714: a legacy naval engineering standard still referenced in some defence specifications
In every case the principle is identical: a conditioned specimen is clamped vertically in a transparent chimney through which a metered oxygen–nitrogen mixture flows upward at a controlled velocity. The top of the specimen is ignited with a small gas flame, and the oxygen concentration is adjusted step by step until the limiting concentration that just supports combustion is determined. Because the LOI is highly sensitive to specimen geometry and test conditions, strict adherence to the published parameters is essential for valid comparisons.
Testing Samples
Specimen geometry has a strong influence on the measured oxygen index, so the standards define precise dimensions for each product form. At least 15 specimens are normally required for one determination, and spare specimens are strongly recommended because borderline responses consume additional samples.
Rigid Molding and Extrusion Materials
For compression-molded, injection-molded or machined thermoplastics and thermosets, ISO 4589-2 and GB/T 2406.2 specify bar specimens 80 mm to 150 mm long, 10 mm ± 0.5 mm wide and 4 mm ± 0.25 mm thick. ASTM D2863 uses bars 70 mm to 150 mm long, 6.5 mm ± 0.5 mm wide and 3 mm ± 0.25 mm thick. Specimens must be free of burrs, cracks and flash, and the edges should be smooth so that flame spread is not distorted.
Sheet, Laminate and Pipe Materials
Rigid sheets and laminates are typically tested at their as-supplied thickness of up to about 10.5 mm, with length and width as above. Pipe and tube materials are usually cut into longitudinal strips with the curvature retained or flattened only where the standard permits. Because LOI rises with thickness for most plastics, comparisons between materials are only valid when the specimen type and thickness are identical.
Film, Tape and Flexible Materials
Thin films, tapes and flexible substrates that would collapse or distort when clamped are supported during testing. ISO 4589-2 describes rolling a film strip onto a small rod, while ASTM D2863 provides alternative supported-mounting procedures. The use of any retainer or support must be clearly stated in the test report, since it can affect the burning behaviour and therefore the reported index.
Testing Items
Typical test items offered under the plastic oxygen index test standards include:
- Ambient-temperature limiting oxygen index (LOI) to ISO 4589-2, EN ISO 4589-2, ASTM D2863, JIS K 7201-2 or GB/T 2406.2
- Elevated-temperature oxygen index to ISO 4589-3 or GB/T 2406.3
- Temperature index determination: the temperature at which the oxygen index falls to 21%, indicating the point at which the material will burn in ordinary air
- Supplementary burning observations, including afterflame and afterglow time, flaming drips, char formation and specimen shrinkage
- Comparative LOI series across thicknesses, colors or flame-retardant formulations for R&D and specification development
The final LOI result is reported to one decimal place as a percentage of oxygen by volume, together with the specimen type, dimensions, ignition method and conditioning regime, all of which are part of the result.
Testing Methods
Apparatus and Test Conditions
The standard apparatus consists of a heat-resistant glass chimney, typically at least 450 mm tall with an internal diameter of 75 mm to 100 mm, a rod-and-clamp specimen holder, a gas mixing and metering system, and a small hand-held gas igniter producing a flame roughly 10 mm to 20 mm long. Oxygen and nitrogen of high purity, typically at least 98%, are blended so that the mixture travels upward through the chimney at a linear velocity of 40 mm/s ± 10 mm/s. The test is performed in a draught-free environment so that the upward gas flow fully governs the combustion behavior.
Specimen Conditioning
Before testing, specimens are conditioned at 23 °C ± 2 °C and 50% ± 5% relative humidity for at least 40 hours, or as specified by the relevant material standard. A reference mark is made 50 mm below the top edge of each specimen so the flame travel distance can be judged consistently. Hygroscopic materials may require additional drying or special conditioning agreed between the parties.
Ignition and Response Criteria
ISO 4589-2 defines two ignition modes. In top-surface ignition (Method A), the igniter flame is applied to the top of the specimen, with brief checks every few seconds and a maximum contact time of 30 seconds. In propagating ignition (Method B), the flame is applied to the upper portion of the specimen so as to promote downward flame spread, in short applications of about 5 seconds each. After the igniter is removed, the specimen is judged to have supported combustion at that oxygen concentration if it burns for 180 seconds or more, or if the flame front passes the 50 mm gauge mark; if neither occurs, the mixture is deemed too lean in oxygen to support continued burning.
Determination and Calculation
The oxygen concentration is adjusted in steps, beginning with coarse increments to bracket the limiting value and finishing with fine increments, commonly 0.2% oxygen, as required by ISO 4589-2. Once the responses alternate between burn and no-burn over a narrow range, the LOI is calculated from the final series of concentrations using the statistical formula in the standard, LOI = c + k·d, where d is the concentration step and k is a factor depending on the number of valid responses. Well-controlled laboratories typically achieve repeatability within about 0.5% oxygen, while reproducibility between different laboratories is commonly on the order of 1% to 1.5% oxygen.
Applications
Oxygen index data are used across the polymer supply chain, from compound formulation to end-product certification. Typical application areas include:
- Wire and cable compounds, where procurement specifications frequently call for LOI values of 28% or higher on insulation and sheathing materials, complementing the IEC 60332 series of flame tests
- Electrical and electronic components such as enclosures, connectors and switch parts, where LOI supports the results of UL 94 and IEC 60695-11-10/-11-20 burning tests
- Railway applications, where the European fire safety standard EN 45545-2 specifies oxygen index requirements for certain listed products
- Aerospace and defence programs that reference legacy methods such as NES 714 for interior and panel materials
- Building products, pipes, decorative laminates and automotive interior trim during material selection
- Research and quality control, including screening of halogenated, phosphorus-based, mineral and intumescent flame-retardant systems, batch release testing and incoming inspection by accredited third-party laboratories
Because the LOI test is fast, economical and reproducible, it is often the first flammability measurement performed on a new formulation, before more complex and expensive large-scale fire tests are undertaken.
FAQ
What does the oxygen index number actually mean?
The LOI tells you how oxygen-enriched the atmosphere must be for the plastic to keep burning like a candle. A value of 21% means the material will just barely burn in ordinary air; values above 21% mean the material will self-extinguish in air, and higher values indicate stronger flame retardancy. It is a comparative, small-scale measure of ignitability rather than a direct prediction of fire hazard in a real fire.
What LOI value is required for a plastic to be considered flame retardant?
Although acceptance limits are set by individual product specifications, common conventions are: below 21% easily combustible, 21% to 27% slowly burning or self-extinguishing in air, and 27% or above generally regarded as flame retardant. Indicative values for common plastics on standard-thickness specimens include:
- PTFE and other fluoropolymers: above 90%
- Rigid PVC: approximately 40% to 50%
- Phenolic resins and laminates: approximately 30% to 45%
- Polycarbonate: approximately 25% to 28%
- Polyamide (PA6, PA66): approximately 22% to 28%
- ABS: approximately 18% to 20%
- Polystyrene, polyethylene, polypropylene and PMMA: approximately 17% to 19%
Flame-retardant grades of these same polymers can reach 28% to 40% or higher depending on the additive system and loading.
Are ISO 4589-2, ASTM D2863, JIS K 7201-2 and GB/T 2406.2 mutually recognized?
The four standards are technically aligned: they use the same apparatus, gas flow, ignition principles and response criteria, and results for the same material generally agree within about 1% oxygen. However, the preferred specimen dimensions differ slightly between the ISO/GB/JIS family and ASTM D2863, so contracts and specifications should name the exact standard to be used. For international submissions, accredited third-party laboratories can usually test to several of these standards in parallel.
How many specimens are required and how long does a test take?
A single LOI determination normally consumes about 15 specimens, plus spares for borderline materials. The conditioning period of at least 40 hours dominates the schedule, while the bracketing test itself typically takes one to two hours once conditioning is complete. Elevated-temperature testing to ISO 4589-3 takes longer because each temperature point requires a separate determination and the chimney must stabilize thermally between readings.
Why must specimen thickness be reported with the LOI?
The oxygen index is a geometry-dependent property. For most plastics the LOI increases with specimen thickness, because thicker sections act as a heat sink and may form a more stable char, while very thin films show lower values. Two materials can therefore only be compared meaningfully when tested in the same specimen form, size and thickness, which is why the standard requires this information in every test report.
Is the oxygen index a pass/fail fire safety rating?
No. The LOI is a small-scale, comparative measurement, and specification writers set their own acceptance thresholds for each application. In practice it is used alongside other flammability tests such as UL 94 vertical and horizontal burning, IEC 60695-11-10 and IEC 60695-11-20, the IEC 60332 cable flame tests and the EN 45545-2 requirement sets. A complete fire hazard assessment always requires additional data on heat release, smoke production and flame spread, not the oxygen index alone.