Oxygen Index (LOI) Determination Test for Plastic Polymers: Principles, Methods and Applications

The oxygen index determination test, also known as the Limiting Oxygen Index (LOI) test, measures the minimum concentration of oxygen in an oxygen/nitrogen mixture required to sustain flaming combustion of a plastic or polymeric material. Performed to internationally recognized methods such as ISO 4589-2, ASTM D2863 and GB/T 2406.2, it is one of the most widely used screening tools for evaluating the flame retardancy of polymers in accredited third-party laboratories. This article explains the testing principle, sample requirements, operating procedure, data interpretation and typical applications of the oxygen index method.

Technical Overview

The limiting oxygen index (LOI) is defined as the minimum volumetric concentration of oxygen, in a mixture of oxygen and nitrogen flowing upward through a vertical glass chimney, that will just support candle-like combustion of a vertically mounted specimen for a specified time or over a specified length. Because ambient air contains approximately 20.9% oxygen by volume, the LOI immediately indicates whether a material will continue to burn in air once ignited. The test is standardized in ISO 4589-2, ASTM D2863 and GB/T 2406.2, with related methods covering elevated-temperature determination (ISO 4589-3) and rubber materials (IEC 61144).

In a typical test, the specimen is clamped vertically inside the chimney, a precisely controlled oxygen/nitrogen mixture is established, and the top of the specimen is ignited with a small gas flame. The operator systematically raises or lowers the oxygen concentration until the limiting condition is bracketed, and the result is expressed as a volume percentage of oxygen, usually reported to the nearest 0.1%.

General interpretation of LOI values is as follows:

• Below 21%: the material burns readily in ambient air after ignition.
• 21% to 25%: the material burns slowly and marginally sustains combustion in air.
• 25% to 30%: the material shows a self-extinguishing tendency and is often regarded as flame retardant in many specifications.
• Above 30%: the material exhibits high flame retardancy, typical of grades used in railway, aerospace, cable and electronic applications.

It should be emphasized that the LOI is a small-scale laboratory ranking parameter. It complements, but does not replace, other flammability assessments such as UL 94 burning ratings or the IEC 60332 cable test series, and it does not by itself describe full-scale fire hazard.

Testing Samples

The oxygen index method is applicable to a broad range of self-supporting and, with appropriate fixtures, non-self-supporting polymeric materials. Common sample categories are described below.

Thermoplastics and Thermosetting Resins

Typical materials include polyethylene, polypropylene, polystyrene, ABS, PVC, polyamide, polycarbonate, POM, PET, PBT, polyphenylene ether, epoxy resins, phenolics, unsaturated polyesters and polyurethanes. Specimens are usually machined or cut from injection-molded or compression-molded plaques prepared under controlled conditions.

Elastomers and Rubber Compounds

Rubber materials, including cable insulation and sheathing compounds, can be evaluated using ISO 4589-2 or the rubber-specific IEC 61144 procedure at ambient temperature.

Cellular Plastics and Foams

Rigid and flexible foams are tested with the specimen dimensions adapted for cellular materials as specified in the standard, since density and cell structure significantly influence the result.

Films, Sheets and Flexible Materials

Thin or flexible materials that cannot stand upright are tested in special support frames or U-shaped holders, with the appropriate specimen category recorded in the report.

Fiber-Reinforced Composites and Laminates

Glass-fiber or carbon-fiber reinforced laminates, prepregs and pultruded sections are prepared as cleanly machined strips with smooth edges, since delamination or fiber exposure can affect burning behavior.

Specimen geometry is typically 80–150 mm in length and 10 mm in width, with a thickness of 4 mm or 10 mm for rigid plastics per ISO 4589-2, or product-relevant dimensions as required by ASTM D2863 or the material specification. A complete determination normally consumes 10–15 specimens per condition, and spares are recommended. Specimens should be free of cracks, flash and surface contamination, and should be conditioned, commonly at 23 ± 2°C and 50 ± 5% relative humidity, before testing.

Testing Items

The core deliverable of the experiment is the limiting oxygen index value, but several related determinations can be performed on the same apparatus:

• Limiting oxygen index at ambient temperature (23°C) to ISO 4589-2, ASTM D2863 or GB/T 2406.2.
• Limiting oxygen index at elevated temperatures to ISO 4589-3, including derivation of temperature-index information.
• Comparative LOI studies, for example before and after heat aging, UV exposure or humidity conditioning.
• Thickness-series characterization to document the dependence of LOI on specimen geometry.
• Formulation screening studies examining filler loading, synergist ratio or flame-retardant content.
• Supporting observations such as ignition ease, afterflame time, dripping and char formation.

Testing Methods

Apparatus

A standard oxygen index tester consists of a vertical glass chimney, typically 75–100 mm in internal diameter and at least 450 mm high; a specimen holder and clamp; purified oxygen and nitrogen supplies with pressure regulation; precision flow meters or mass-flow controllers with a mixing chamber; a small gas ignition burner; a timer; and pre-marked criteria for the 50 mm burned-length judgment. An oxygen analyzer is recommended to verify the actual mixture composition. The system should be periodically validated using a reference material such as PMMA, whose LOI lies in the range of approximately 17–18% oxygen.

Specimen Preparation and Conditioning

Specimens are prepared to the dimensions of the applicable category, marked with the 50 mm criterion, measured for thickness, and conditioned in a standard atmosphere. The material identity, color, density where relevant, and any prior treatment or aging are recorded, as these factors influence the measured index.

Test Procedure

• Mount the specimen vertically in the holder so that its top is positioned correctly within the chimney.
• Establish the total gas flow corresponding to the nominal linear velocity specified in ISO 4589-2, approximately 40 mm/s, and allow the system to purge and stabilize.
• Select an initial oxygen concentration based on experience or prior data for similar materials.
• Apply the ignition flame to the top of the specimen, using top-surface ignition with short applications or the propagation ignition method as appropriate to the standard.
• Observe the burning behavior and record whether the specimen exceeds the pass/fail criterion, typically a burning time exceeding three minutes or a burned length of 50 mm.
• Increase the oxygen concentration if the specimen extinguishes before the criterion and decrease it if the criterion is exceeded, using progressively smaller steps down to 0.2% oxygen in the final stage.
• Bracket the limiting concentration with fresh replicate specimens and calculate the LOI.

Data Reduction and Reporting

The test report should include the LOI in volume percent oxygen to the nearest 0.1%, the specimen category and actual thickness, the ignition procedure used, conditioning details, the standard and its edition, the gas flow conditions, and observations of dripping, glowing combustion or char formation. Good within-laboratory repeatability is typically on the order of ±0.5% oxygen, while interlaboratory reproducibility is somewhat wider, which is why consistent specimen preparation and apparatus verification are essential.

Applications

• Development and optimization of flame-retardant formulations, including halogen-free systems based on phosphorus-nitrogen synergists, aluminum trihydrate, magnesium hydroxide and intumescent packages.
• Quality control and lot-acceptance testing of resins, compounds, masterbatches and finished polymeric components.
• Wire and cable material qualification, complementing the flame propagation tests of the IEC 60332 series.
• Screening of materials for railway, aerospace, marine and construction applications, supporting property sets such as those referenced in EN 45545-2.
• Material selection and datasheet publication for electronic and electrical equipment enclosures, in conjunction with UL 94 ratings.
• Academic research, aging studies, competitor benchmarking and failure analysis of combustion-related field returns.

FAQ

What is considered a good LOI value for a flame-retardant plastic?

Values above 25–27% generally indicate self-extinguishing behavior in air. Many industrial and railway specifications require LOI values of at least 28% or 30% for flame-retardant grades, while unmodified commodity plastics such as polyethylene and polypropylene typically measure in the range of 17–19%.

How does the LOI test differ from UL 94 testing?

The LOI test quantifies the minimum oxygen concentration needed to sustain burning under a controlled atmosphere, producing a numerical value. UL 94 evaluates burning behavior in normal air and assigns ratings such as V-0, V-2, 5VA and 5VB. The two methods are complementary and are frequently reported together on material datasheets.

How many specimens are required for one determination?

A complete determination typically consumes 10–15 specimens, because each trial of the bracketing sequence uses a fresh specimen. Additional specimens are needed for elevated-temperature work or for studying multiple thicknesses and conditions.

Why can LOI results vary between laboratories or batches?

Variations arise from specimen thickness and geometry, conditioning history, material aging, accuracy of gas flow control, and operator judgment of the burning length criterion. Within-laboratory repeatability is usually within about ±0.5% oxygen, and regular verification with a reference material such as PMMA helps maintain consistency.

Can the oxygen index be measured at elevated temperature?

Yes. ISO 4589-3 defines the determination of LOI at elevated temperatures, which is valuable for materials used in warm environments and for deriving temperature-index information that describes how flame retardancy degrades with heat.

Does a high LOI guarantee fire safety in real applications?

No. The LOI is a screening and ranking parameter measured on small specimens under idealized conditions. A comprehensive fire-hazard assessment should combine LOI data with other tests such as UL 94, flame spread, heat release and smoke generation measurements.

How long does a typical determination take?

A single ambient-temperature determination, including apparatus stabilization and the full bracketing sequence, commonly requires one to two hours. Additional time is needed for specimen conditioning and for elevated-temperature or multi-condition test programs.