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 flammability characteristics of plastic materials. By measuring the minimum oxygen concentration required to sustain candle-like combustion of a vertically mounted specimen, the test provides a quantitative, highly reproducible flammability rating that supports flame-retardant material development, quality control, and regulatory compliance. This article explains the principle, standards, sample requirements, test procedure, and typical applications of oxygen index determination for plastics.
Technical Overview
The oxygen index (OI), also 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 combustion of a vertically mounted test specimen under specified test conditions. Because normal ambient air contains approximately 20.95% oxygen, an LOI value above approximately 21% indicates that the material will not continue to burn on its own under the test conditions, while lower values indicate that the material can sustain flaming combustion in ordinary air.
Oxygen index determination is governed by several internationally recognized standards, including ISO 4589-2 (ambient-temperature test), ISO 4589-3 (elevated-temperature test), ASTM D2863, EN ISO 4589-2, and the Chinese national standard GB/T 2406.2, which is technically aligned with ISO 4589-2. These standards specify the apparatus, specimen geometry, conditioning, ignition procedure, and calculation method required to obtain accurate and comparable results.
In industrial practice, materials are commonly classified according to their LOI values as follows:
- LOI below 21%: readily flammable materials that continue burning in normal air.
- LOI between 21% and 27%: slowly burning or self-extinguishing materials.
- LOI above 27%: generally regarded as flame-retardant materials suitable for demanding applications.
The LOI test is appreciated for its excellent repeatability, small specimen requirements, and quantitative numerical output, making it an effective screening tool for comparing resin formulations, flame-retardant additives, and compounding processes. It should be noted, however, that the oxygen index is a small-scale laboratory measurement and is typically used alongside other fire performance tests, such as UL 94 flame ratings, rather than as a standalone assessment of real-scale fire hazard.
Testing Samples
Oxygen index testing can be performed on a broad range of polymeric materials. Specimen geometry and preparation depend on the material category, and the standard test generally requires a minimum of 15 specimens per determination to complete the bracketing procedure.
Rigid Self-Supporting Plastics
Molded, extruded, or machined bars of thermoplastics and thermosets, such as polyethylene, polypropylene, polystyrene, ABS, polyamide, polycarbonate, PVC, epoxy, and phenolic materials, are tested as rectangular strips. Typical dimensions per ISO 4589-2 are 80 mm to 150 mm in length, 10 mm in width, and 4 mm to 10 mm in thickness, while ASTM D2863 defines a similar bar geometry for rigid specimens. Specimens must be free of voids, burrs, and surface defects that could influence burning behavior.
Flexible Plastics, Films, and Sheets
Thin or limp materials that cannot support themselves vertically are prepared as rolled specimens. For example, flexible film is typically wound onto a mandrel to form a rolled specimen approximately 100 mm in length with an outer diameter matching the standard requirement, secured at one end so that the free end can be ignited in the same manner as a rigid bar.
Cellular Plastics and Foam Materials
Thermoplastic and thermoset foams, such as polyurethane, polystyrene foam, and polyolefin foams, are tested using larger cross-sectional dimensions defined by the standard to account for their low density and high combustion rates. Special attention is paid to specimen orientation, since foam burning behavior can differ significantly between vertical and horizontal cell structures.
Fiber-Reinforced and Composite Materials
Regardless of material type, all specimens should be conditioned prior to testing, typically at 23°C ± 2°C and 50% ± 5% relative humidity for a defined period, unless a different conditioning regime is specified by the relevant material standard or the client.
Testing Items
Typical test items and deliverables within the scope of oxygen index determination include:
- Ambient-temperature limiting oxygen index (LOI) of plastics in accordance with ISO 4589-2, EN ISO 4589-2, ASTM D2863, and GB/T 2406.2.
- Elevated-temperature oxygen index determination in accordance with ISO 4589-3, evaluating how flammability changes as the test atmosphere and specimen are heated to temperatures up to approximately 300°C.
- Temperature index determination, identifying the temperature at which the oxygen index of a material falls to a specified critical value, often 21%.
- Flammability classification and comparison of different resin grades, flame-retardant formulations, filler loadings, or processing conditions.
- Verification testing for product specifications, incoming material inspection, and supplier qualification.
- Supporting data for compliance submissions involving flame-retardancy requirements in electrical, transportation, and construction sectors.
Testing Methods
The test is performed using an oxygen index apparatus consisting of a heat-resistant glass chimney, a specimen holder that supports the specimen vertically at the center of the chimney, a gas mixing and distribution system, precision flow meters or mass flow controllers for oxygen and nitrogen, and optionally an oxygen analyzer to verify the mixed gas concentration. A small propane-fueled diffusion flame igniter is used to ignite the top of the specimen.
The general test procedure is as follows:
- Specimens are conditioned and measured, and the apparatus is verified for gas-tightness and flow calibration.
- The specimen is clamped vertically in the holder, with the exposed length of the top of the specimen positioned at least a specified distance below the top rim of the chimney.
- An initial oxygen concentration is set, and the gas mixture is allowed to flow through the chimney for a sufficient purge period to establish a stable atmosphere.
- The top surface of the specimen is ignited using the igniter, with the flame applied for a defined duration until the specimen is well lit, after which the igniter is removed.
- The specimen is observed to determine whether combustion persists beyond the specified failure criteria, typically a burning time of 3 minutes or a burning extent of 50 mm, marked on the specimen.
- The oxygen concentration is adjusted stepwise, using either the bracketing procedure or the interval method defined in the standard, until the critical oxygen concentration is identified at which combustion is just sustained.
- The LOI is calculated and reported as the volume percentage of oxygen in the oxygen/nitrogen mixture.
The oxygen index is calculated as: LOI = 100 × VO2 / (VO2 + VN2), where VO2 and VN2 are the volumetric flow rates of oxygen and nitrogen at the critical concentration. Results are reported together with the test standard used, specimen dimensions, conditioning history, ignition observations, and any unusual burning behavior such as dripping, glowing combustion, or chatter. Repeatability is an important quality indicator, and replicate determinations on well-prepared specimens typically agree within a fraction of a percentage point of oxygen concentration.
Key operational parameters that influence the result include gas flow rate and mixing uniformity, chimney integrity, ignition technique, specimen thickness, and ambient conditions. For this reason, testing is best performed by trained operators in accredited third-party laboratories or in quality laboratories with calibrated equipment and documented procedures.
Applications
Oxygen index data are used across many industries in which polymer flammability is a safety, regulatory, or design consideration:
- Wire and cable insulation and sheathing materials, where LOI values support the selection of flame-retardant compounds for building wiring, power cables, and data cables.
- Electrical and electronic products, including connectors, switch housings, appliance enclosures, and circuit board laminates, often alongside UL 94 flame ratings.
- Rail transit and mass transportation interior materials, seating, and panels, where strict flame retardancy requirements apply to rolling stock components.
- Aerospace and aviation components, for which many material specifications define minimum oxygen index acceptance values.
- Automotive interior parts and under-hood plastic components requiring validated flame-retardant performance.
- Construction products, including pipes, fittings, wallcoverings, and thermal insulation materials subject to fire performance regulations.
- Flame-retardant research and development, where LOI testing enables rapid screening of halogenated, phosphorus-based, inorganic, and intumescent flame-retardant systems.
- Quality control and incoming inspection in plastics processing, compounding, and masterbatch manufacturing.
FAQ
What is a good oxygen index value for a flame-retardant plastic?
A material with an LOI above 27% is generally considered flame retardant, while values between 21% and 27% indicate slow-burning or self-extinguishing behavior. Values below 21% mean the material will continue to burn in normal atmospheric air under the test conditions. Many engineering specifications for rail, aerospace, and cable applications require LOI values well above 30%.
Which standards are used for oxygen index testing?
The most common standards are ISO 4589-2 and EN ISO 4589-2 for ambient-temperature testing, ISO 4589-3 for elevated-temperature testing, ASTM D2863, and GB/T 2406.2. These standards are broadly harmonized in principle, but they differ in certain specimen dimensions and procedural details, so the applicable standard should be confirmed before testing.
How many specimens are required for a test?
A full LOI determination normally requires a minimum of 15 specimens, since the bracketing procedure consumes several specimens at different oxygen concentrations before the critical value is established. Additional specimens may be needed for verification runs or when testing variable materials such as foams.
How does oxygen index testing differ from the UL 94 test?
The oxygen index test measures the minimum oxygen concentration needed to sustain combustion in a flowing gas mixture, producing a continuous numerical value. UL 94 methods, such as the UL 94 V-0 to V-2 vertical burn classifications, evaluate flame behavior in normal air with a defined ignition source and grade results into categories. The two methods are complementary, and many material datasheets report both LOI and UL 94 ratings.
What factors can affect the test result?
Specimen thickness, density, and geometry, moisture conditioning, aging or heat exposure history, filler and flame-retardant content, and the precision of gas flow control can all influence LOI results. Elevated temperature testing per ISO 4589-3 typically yields lower oxygen index values than ambient testing, reflecting the increased flammability of plastics at higher temperatures.
Can the oxygen index test be used for quality control?
Yes. Because the method is quantitative, repeatable, and requires only small specimens, it is widely used for batch-to-batch quality control, supplier material verification, and formulation comparison. Laboratories performing such testing should maintain calibrated flow instrumentation and validated procedures to ensure consistent results over time.