Determination of Oxygen Index (LOI) of Plastics: Testing Standards, Methods, and Applications

The oxygen index (Limiting Oxygen Index, LOI) test is one of the most widely used methods for evaluating the flammability characteristics of plastic materials. This article provides a comprehensive technical overview of plastic oxygen index determination, covering the test principle, applicable standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, sample requirements, testing procedures, and typical industrial applications.

Technical Overview

The oxygen index, also known as 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 flaming combustion of a material under specified test conditions. Since ambient air contains approximately 20.9% oxygen, materials with an oxygen index below this value will generally continue to burn in air, while materials with a higher oxygen index will tend to extinguish themselves when the ignition source is removed.

In the oxygen index test, a vertically mounted specimen is ignited at its top surface inside a heat-resistant glass chimney. A precisely controlled gas mixture of oxygen and nitrogen flows upward through the chimney at a constant rate. The oxygen concentration is progressively adjusted until the critical concentration that just sustains candle-like burning for a defined period or over a defined distance is determined. This critical value is reported as the LOI of the material.

The oxygen index method offers excellent reproducibility and is a small-scale laboratory screening test. It is widely specified in product standards, material datasheets, and regulatory requirements as a quantitative indicator of the relative flammability of polymers. It should be noted, however, that LOI results describe behavior under the specific test conditions and are not intended to directly assess fire hazard in full-scale fire scenarios.

Relevant Standards

• ISO 4589-2 – Plastics – Determination of burning behaviour by oxygen index – Part 2: Ambient-temperature test
• ISO 4589-3 – Determination of burning behaviour by oxygen index – 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, technically aligned with ISO 4589-2
• IEC 60695-2 series and related IEC product standards that reference LOI limits for insulating materials

Testing Samples

Oxygen index determination is applicable to a broad range of plastic materials in solid form. Sample geometry significantly influences the test result, so specimens must be prepared strictly according to the requirements of the selected standard.

Rigid Plastics and Molded Materials

Typical specimens are 80 to 150 mm in length and 10 mm in width, with preferred thicknesses of 4 mm or 10 mm. Specimens should be free from defects such as bubbles, cracks, and flash that could affect burning behavior. Injection-molded, compression-molded, or machined specimens are all acceptable provided the surface is smooth and dimensions are uniform.

Flexible Films and Sheets

Thin films are usually tested with a special specimen support frame or rolled specimen holder that maintains the required vertical geometry during testing. Typical film specimens are 140 mm long and 52 mm wide, rolled into a cylindrical form as specified in the relevant standard.

Cellular Plastics (Foams)

Rigid and flexible foam materials are tested using larger cross-sections, typically 10 mm to 15 mm in thickness and up to 15 mm in width, because of their low density and rapid burning characteristics.

Fabrics, Coated Textiles, and Composites

Polymer-coated fabrics, fiber-reinforced composites, laminates, and elastomeric materials can also be evaluated, provided that appropriate specimen dimensions and support fixtures are used.

Testing Items

The primary deliverable of the test is the oxygen index value of the material, expressed as a percentage of oxygen in the O2/N2 mixture. Additional parameters and observations typically reported include:

• Limiting Oxygen Index (LOI) value, in volume percent (V/V)
• Burning behavior observations, such as afterflame time, dripping, and soot formation
• Specimen dimensions, orientation, and conditioning state
• Test standard applied (ISO 4589-2, ASTM D2863, or GB/T 2406.2) and ignition procedure used
• Elevated-temperature oxygen index, where ISO 4589-3 is specified
• Compliance judgment against specified LOI limits in product standards or material specifications

Testing Methods

Apparatus

The test apparatus consists of a heat-resistant glass chimney (typically 450 to 500 mm high), a specimen holder that clamps the specimen vertically at its base, a gas mixing and flow control system with calibrated oxygen and nitrogen metering, an igniter capable of producing a defined flame (approximately 10 to 20 mm long), and gas analyzers or flowmeters to verify the oxygen concentration of the delivered mixture.

Specimen Conditioning

Unless otherwise specified, specimens are conditioned to equilibrium in a standard atmosphere of 23 °C and 50% relative humidity prior to testing, in accordance with ISO 291 or ASTM D618. Some specifications require dry conditioning (desiccator storage) or specific pre-treatment depending on the end-use requirements.

Test Procedure

• Measure and record specimen dimensions; condition the specimens as required by the standard.
• Mount the specimen vertically in the holder so that the top of the specimen is at least 100 mm below the top of the chimney.
• Purge the chimney with the gas mixture at the specified total flow rate for at least 30 seconds to establish a stable atmosphere.
• Apply the ignition flame to the top of the specimen for the prescribed time (typically up to 30 seconds), withdrawing and reapplying as necessary.
• Observe whether burning continues for the specified criterion, such as 3 minutes of burning time or 50 mm of burned distance for many geometries.
• Adjust the oxygen concentration upward or downward in incremental steps and repeat the test on fresh specimens until the critical oxygen concentration is bracketed.
• Calculate the LOI from the oxygen concentrations of the last passing and first failing tests using the calculation or bracketing method defined in the standard.
• Verify the result by repeating confirmation tests on multiple specimens at the determined concentration.

Calculation and Expression of Results

The oxygen index is calculated as the volume fraction of oxygen in the gas mixture, often determined from the flow rates of oxygen and nitrogen using the formula LOI = 100 × VO2 / (VO2 + VN2). The final reported value is typically the average of replicate determinations, rounded to 0.1% or reported to the nearest 0.5% depending on the method used. A small-scale and large-scale verification series may be specified in some standards to confirm the bracketing result.

Interpretation of Results

• LOI below 21%: the material generally burns readily in normal air and is considered flammable.
• LOI between 21% and approximately 27%: the material is slow-burning or self-extinguishing under certain conditions.
• LOI above approximately 27%: the material is generally considered to exhibit self-extinguishing or flame-retardant behavior in air.

Applications

Oxygen index determination is specified across many industries as a material qualification, quality control, and research tool:

• Wire and cable insulation and sheathing compounds, where LOI limits are referenced in electrical product standards
• Construction products, including pipes, panels, thermal insulation foams, and decorative materials subject to fire performance classifications
• Electrical and electronic equipment housings and internal components evaluated against flammability requirements
• Railway, marine, and aerospace interior materials, where LOI is one element of a multi-tier fire safety assessment
• Flame-retardant polymer development, comparing the effectiveness of halogenated, phosphorus-based, inorganic, and intumescent flame retardant systems
• Incoming material inspection and batch release testing by manufacturers and accredited third-party laboratories
• Academic and industrial research into polymer combustion mechanisms, char formation, and smoke suppression

FAQ

What is the oxygen index of a plastic material?

The oxygen index is the minimum volume percentage of oxygen in an oxygen/nitrogen mixture that will just sustain flaming combustion of a vertically mounted specimen under standardized laboratory conditions. It is a quantitative, reproducible measure of relative flammability: the higher the oxygen index, the less flammable the material.

Which standards govern plastic oxygen index testing?

The most commonly applied standards are ISO 4589-2 (ambient temperature), ISO 4589-3 (elevated temperature), ASTM D2863, and GB/T 2406.2, which is technically aligned with ISO 4589-2. The standards are broadly equivalent, but procedural details such as specimen dimensions, ignition time, and the calculation method differ, so the specific standard must be agreed before testing.

What oxygen index value is considered flame retardant?

As a common convention, plastics with an LOI of 21% or lower will usually continue to burn in normal air, while materials with an LOI above approximately 27% are generally regarded as flame-retardant or self-extinguishing. However, acceptance limits are set by individual product specifications and regulations, which must always take precedence over general conventions.

What factors influence oxygen index test results?

Results can be affected by specimen thickness and geometry, material density, filler and flame-retardant content, conditioning history (moisture and temperature), ignition procedure, gas flow stability, and the accuracy of oxygen concentration control. Strict adherence to the specified specimen preparation and conditioning requirements is essential for valid, repeatable results.

How many specimens are required for a test?

A typical determination uses a series of specimens because the oxygen concentration must be adjusted stepwise to bracket the critical value, followed by confirmation tests. Depending on the material and standard, a complete test may require 10 to 20 specimens or more to establish a statistically valid LOI value.

Can the oxygen index test be performed at elevated temperatures?

Yes. ISO 4589-3 extends the method to elevated temperatures using a heated chimney, and certain specifications reference a temperature index based on this method. Elevated-temperature LOI data are important for applications where materials are exposed to heat in service, such as electrical equipment and cable systems.

Is oxygen index testing a full fire hazard assessment?

No. The LOI test is a small-scale laboratory screening test that measures relative ignitability and burning sustenance under controlled conditions. For a complete fire safety evaluation, it is normally combined with other tests such as vertical/horizontal burning tests, glow-wire tests, cone calorimetry, and large-scale fire performance testing as required by the applicable product standards.