Limiting Oxygen Index (LOI) Test Principle for Plastics: Determination, Methods and Applications

The limiting oxygen index (LOI) is one of the most widely used small-scale indicators of plastic flammability, defining the minimum oxygen concentration in an oxygen–nitrogen mixture required to just sustain candle-like combustion of a vertical specimen. Determined according to internationally recognized procedures such as ISO 4589-2, ASTM D2863 and GB/T 2406.2, the LOI value enables manufacturers and researchers to rank and compare the flame-retardant performance of polymeric materials. This article explains the underlying measurement principle, specimen requirements, test procedure, calculation method, and typical industrial applications.

Technical Overview

The limiting oxygen index (LOI), also called the oxygen index (OI) or critical oxygen index (COI), is defined as the minimum concentration of oxygen, expressed as volume percent, in a mixture of oxygen and nitrogen that will just support combustion of a material under specified test conditions. The test is standardized in ISO 4589-2, ASTM D2863, GB/T 2406.2 and JIS K 7201-2, all of which share the same fundamental principle.

The scientific basis of the method rests on the fact that combustion is a gas-phase oxidation reaction whose rate depends strongly on the partial pressure of oxygen. Pure nitrogen is chemically inert at flame temperatures and acts only as a diluent. By progressively diluting oxygen with nitrogen in a controlled, vertically rising gas stream, the test determines the precise oxygen fraction at which the flame front on a top-ignited vertical specimen is on the boundary between self-sustained propagation and extinction.

A specimen is clamped vertically in a heat-resistant glass chimney. A metered oxygen–nitrogen mixture of known composition flows upward through the column at a constant total flow rate. The top of the specimen is ignited with a small diffusion flame, and the gas composition is adjusted until the minimum oxygen concentration is found at which burning is just sustained. The limiting oxygen index is calculated as:

LOI = 100 × V(O2) / [V(O2) + V(N2)]

Because normal ambient air contains approximately 20.9% oxygen, a material with an LOI below 21% will generally continue to burn in air once ignited, whereas a material with an LOI well above 21% tends to self-extinguish after removal of the ignition source. Higher LOI values therefore indicate greater resistance to ignition and flame propagation under these laboratory conditions.

Testing Samples

Specimen geometry has a significant influence on the measured oxygen index, so standards prescribe dimensional requirements for different categories of plastics.

Rigid, Self-Supporting Plastics

Molded or sheet specimens of typical dimensions 80 mm to 120 mm in length, 10 mm in width and 4 mm in thickness are clamped vertically in the specimen holder. Thicker or thinner specimens may be tested, but the reported thickness must always accompany the LOI value because heat sink effects shift the result.

Flexible Films and Thin Sheets

Flexible materials that cannot stand upright are wrapped around a vertical mandrel or rod, or supported on a frame, so that a defined burning length is maintained during the test. A common specimen size for films is 140 mm × 52 mm rolled into a tube.

Cellular Plastics and Rubbers

Expanded materials and elastomers are tested using modified specimen holders specified in the relevant standards; for cellular plastics the same top-ignition principle applies with adjusted dimensions and support.

Specimen Conditioning

Prior to testing, specimens are normally conditioned at 23 °C ± 2 °C and 50% ± 5% relative humidity for a defined period (commonly 88 hours unless otherwise agreed), because absorbed moisture can suppress combustion and raise the apparent LOI.

Testing Items

Accredited third-party laboratories and in-house quality control departments typically offer the following oxygen index determinations:

• Limiting oxygen index at ambient temperature (23 °C) per ISO 4589-2, ASTM D2863 or GB/T 2406.2.
• Limiting oxygen index at elevated temperatures per ISO 4589-3, which measures LOI at temperatures up to approximately 200 °C using a heated chimney.
• Thickness-dependent LOI comparison for product design and material selection.
• LOI evaluation before and after heat aging, water immersion or UV exposure to assess retention of flame-retardant performance.
• Comparative LOI testing of neat polymers versus flame-retardant compounds for research and development.

Testing Methods

Apparatus

A standard oxygen index apparatus consists of a heat-resistant glass test chimney of specified diameter, a specimen holder and clamp, a gas mixing and metering system with calibrated flow controllers for oxygen and nitrogen, an ignition source (typically a small propane or butane burner), and an oxygen analyzer for verifying the delivered mixture. A timing device and graduated height marks are used to judge the burning criterion.

Test Procedure

• Mount the conditioned specimen vertically in the holder and position it centrally in the chimney so the top of the specimen is a specified distance below the chimney outlet.
• Set the initial oxygen concentration based on prior knowledge or a trial ignition, and establish the total gas flow rate specified by the standard.
• Purge the column with the gas mixture for at least 30 seconds to ensure a stable, homogeneous atmosphere.
• Apply the ignition flame to the top of the specimen for a defined time (for example, up to 30 seconds with occasional removal to observe whether the specimen ignites) until the full width of the top is burning.
• Observe whether the burning front reaches the criterion mark (typically 50 mm below the top of the specimen) within a specified time (typically 180 seconds), or whether the flame self-extinguishes.
• Using the bracketing or interval method, vary the oxygen concentration in defined increments until two adjacent concentrations differing by a small interval bracket the pass/fail boundary, then verify repeatability with multiple specimens.

Calculation and Reporting

The LOI is reported as the volume percentage of oxygen at the limiting condition, calculated from the flow rates of oxygen and nitrogen, rounded to 0.1%. The test report also states the standard used, specimen dimensions, conditioning history, whether ignition was top-ignition (Method A) or lengthwise propagation (Method B), and the number of specimens tested. ISO 4589-2 additionally defines a Method C for materials that exhibit erratic burning behavior, based on a statistical evaluation of multiple specimens at a single oxygen concentration.

Factors Influencing Results

• Specimen thickness and geometry: thicker specimens act as heat sinks and usually yield higher LOI values.
• Temperature: LOI decreases as ambient temperature rises, which is why elevated-temperature testing per ISO 4589-3 can reveal weaknesses not seen at room temperature.
• Moisture content and conditioning history.
• Additives, fillers, plasticizers and flame retardants, which alter both gas-phase and condensed-phase combustion chemistry.
• Gas flow stability and accuracy of oxygen concentration control.

Applications

The limiting oxygen index test is applied extensively across the polymer industry because it is quick, reproducible, inexpensive and requires only a small quantity of material.

• Flame retardant research and development: screening halogenated, phosphorus-based, nitrogen-based, inorganic and intumescent flame retardant systems in thermoplastics, thermosets, elastomers and composites.
• Quality control: verifying lot-to-lot consistency of flame-retarded compounds, masterbatches and finished compounds before processing.
• Material qualification and specification: LOI limits are written into many procurement specifications for wire and cable insulation, connectors, printed circuit board laminates, construction panels and railway interior components.
• Regulatory screening: although LOI itself is rarely a pass/fail requirement in end-product fire codes, it is widely used as a precursor screening test before larger-scale flame tests such as UL 94, IEC 60695-based tests or the glow-wire test.
• Academic and comparative studies: providing a single numeric index for ranking the relative flammability of polymers such as PE, PP, PS, ABS, PC, PA, PVC, epoxy and phenolic resins.

Typical indicative values include LOI around 17%–18% for polyethylene and polypropylene, roughly 19% for polystyrene, approximately 22% for polycarbonate, and above 35% for PVC and highly filled flame-retarded compounds. These figures illustrate the practical significance of the 21% threshold corresponding to normal air.

FAQ

What is considered a good LOI value for a plastic?

As a general guideline, materials with an LOI below 21% burn readily in normal air, values between 21% and 25% indicate slow burning or partial self-extinguishing behavior, values between 26% and 34% are commonly described as self-extinguishing, and values above 35% indicate strong flame resistance. Acceptance limits, however, must always be defined by the relevant product specification.

Does a higher LOI mean a product is fire safe?

Not by itself. The LOI is a small-scale laboratory ranking tool that measures ease of extinction under a gentle top-ignition condition. It does not replicate real fire scenarios involving high heat flux, flame spread, smoke production or dripping. Real-world fire compliance relies on end-product standards and larger-scale fire tests appropriate to the application.

What are the differences between ISO 4589-2 and ASTM D2863?

Both standards apply the same candle-like top-ignition principle and produce closely comparable results. Differences exist in details such as chimney dimensions, flow rates, ignition timing, and the way the final value is derived from the bracketing sequence, so the standard used must always accompany the reported value.

Why is nitrogen used as the diluent gas?

Nitrogen is chemically inert at flame temperatures, thermally similar to oxygen in heat capacity, inexpensive and safe. It dilutes the oxygen without participating in the combustion chemistry, allowing the oxygen partial pressure to be varied as the single controlled variable of the experiment.

Can the LOI test be performed at elevated temperatures?

Yes. ISO 4589-3 specifies a heated-column apparatus for determining the temperature index, which is the temperature at which the LOI drops to a specified value, typically 20.9%. This is valuable for materials used near heat sources, such as electrical enclosures and cable systems.