Plastic Oxygen Index (LOI) Flame Retardancy Testing: A Complete Technical Guide

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 flame-retardant performance of plastic materials. Conducted in accordance with standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, the test determines the minimum oxygen concentration in an oxygen/nitrogen mixture required to sustain candle-like combustion of a vertically mounted specimen. This guide explains the test principle, sample requirements, testing procedure, result interpretation, and typical applications for manufacturers, quality engineers, and flame-retardant material developers.

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 flowing mixture of oxygen and nitrogen that will just support candle-like combustion of a plastic specimen ignited at its top. Because ambient air contains approximately 20.9% oxygen, the LOI value provides an immediate indication of whether a material will continue to burn in normal atmospheric conditions: materials with an LOI below approximately 21% tend to burn readily in air, while materials with an LOI well above 21% tend to self-extinguish once the ignition source is removed.

The oxygen index method is a small-scale laboratory screening test. It is reproducible, quantitative, and relatively insensitive to specimen geometry when standard dimensions are used, which makes it particularly valuable for quality control, batch-to-batch comparison of flame-retardant compounds, and research into new formulations. The principal standards governing the test include:

• ISO 4589-2 — Plastics: Determination of burning behaviour by oxygen index, Part 2: Ambient-temperature test
• ISO 4589-3 — Elevated-temperature oxygen index test
• ASTM D2863 — Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics
• GB/T 2406.2 — Chinese national standard, technically aligned with ISO 4589-2
• JIS K 7201-2 — Japanese industrial standard for oxygen index determination

Although the LOI value does not by itself predict behavior in a full-scale fire, it remains a fundamental parameter in material datasheets, flame-retardant formulation development, and many industry procurement specifications.

Testing Samples

Rigid Thermoplastics and Thermosets

The standard specimen for molded plastics is a rectangular bar, typically 80 to 150 mm in length, 10 mm in width, and 4 mm in thickness, cut or injection-molded from the material under evaluation. Edges should be smooth and free from burrs, cracks, or stress concentrations that could influence ignition and flame spread.

Films, Sheets, and Flexible Materials

Thin films and flexible sheets require modified specimen preparation because they cannot stand freely in the specimen holder. Depending on the standard and material behavior, specimens may be tested in a rolled form supported on a mandrel or rod, using adjusted dimensions appropriate to the product form.

Cellular Materials and Foams

Rigid and flexible foams can also be evaluated using the oxygen index method, with specimen dimensions adapted to the cellular structure. Test reports for foams should clearly state density, cell structure, and specimen geometry, since these factors significantly affect the measured LOI.

Textiles and Fiber-Reinforced Composites

Fabrics are frequently characterized by oxygen index using dedicated textile procedures such as GB/T 5454. For fiber-reinforced composites, specimens are cut from laminated panels with the reinforcement orientation documented, as anisotropy can influence burning behavior.

Specimen Quantity and Conditioning

A minimum of 15 to 20 specimens is generally recommended, because the bracketing procedure requires multiple ignition trials at different oxygen concentrations. Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity in accordance with ISO 291; cellulosic and hygroscopic materials typically require at least 88 hours of conditioning before testing.

Testing Items

Accredited third-party laboratories offering oxygen index testing typically provide the following test items and reporting options:

• Ambient-temperature limiting oxygen index (LOI) of plastics in accordance with ISO 4589-2, ASTM D2863, or GB/T 2406.2
• Elevated-temperature oxygen index determination in accordance with ISO 4589-3
• Temperature index (TI) evaluation, identifying the temperature at which the oxygen index falls to a specified critical value such as 21%
• Comparative LOI testing before and after heat aging, humidity exposure, or UV weathering to assess flame-retardant durability
• LOI testing for quality control of incoming resins, masterbatches, and finished compounds
• Verification of whether a material meets a specified minimum LOI requirement in a customer or industry specification

Test reports normally state the LOI value to one decimal place, together with specimen dimensions, material thickness, ignition details, observed burning behavior such as dripping or afterflame characteristics, and the applicable standard.

Testing Methods

Test Apparatus

The oxygen index apparatus consists of a heat-resistant glass chimney, typically at least 450 mm in height with an internal diameter of 75 to 100 mm, in which a vertically mounted specimen is held on a supporting rod. Oxygen and nitrogen of high purity are supplied from compressed gas cylinders, metered through calibrated flow controllers or rotameters, mixed, and introduced at the base of the chimney so that the gas mixture rises with a nominal linear velocity of approximately 40 mm/s past the specimen. A small propane or butane diffusion flame igniter is used to ignite the top of the specimen, and timers are used to record burning time. Many modern instruments integrate an oxygen analyzer to verify the actual oxygen concentration of the mixture.

Test Procedure

The specimen is clamped vertically in the holder so that at least the top portion extends into the chimney, and the gas flow is allowed to purge the column for approximately 30 seconds to establish a stable atmosphere. The operator sets an initial oxygen concentration, ignites the top of the specimen with the igniter flame for a controlled period of up to 30 seconds, and then observes the combustion behavior against the pass criterion, which is typically that the specimen must not burn for more than 3 minutes after removal of the igniter and must not consume more than 50 mm of its length.

Using the bracketing method, the oxygen concentration is adjusted progressively: large concentration steps are used initially to approach the critical point, after which the increment is reduced, commonly to 0.2%, to converge on the narrowest concentration band in which some specimens just satisfy the criterion and others just fail it. Each concentration trial consumes one specimen, which is why an adequate sample population is essential.

Calculation and Expression of Results

The limiting oxygen index is calculated from the volume flow rates of the two gases at the critical concentration:

LOI = 100 × V(O2) / [V(O2) + V(N2)], where V(O2) and V(N2) are the volumetric flow rates of oxygen and nitrogen respectively. The result is reported as a percentage to the nearest 0.1%, for example LOI = 28.5%. Under well-controlled conditions, within-laboratory repeatability is typically on the order of ±0.2% to ±0.5% oxygen, while between-laboratory reproducibility is somewhat wider due to differences in apparatus, gas calibration, and operator judgment of the pass criterion.

Interpretation of LOI Values

Although there is no universal pass/fail threshold, the following classification is widely applied as an engineering guideline:

• LOI below 21%: the material continues to burn in ordinary air and is considered easily flammable
• LOI between 21% and 27%: the material burns slowly in air and shows some tendency toward self-extinguishing
• LOI above 27%: the material will generally not sustain combustion in air once the ignition source is removed and is commonly regarded as flame retardant

Engineering polymers such as PEEK, PVC, PTFE, and polyimides exhibit LOI values well above 30%, whereas commodity plastics such as PE, PP, PS, and ABS typically fall in the range of 17% to 21% without flame-retardant additives.

Applications

Oxygen index testing is applied across many industries in which plastics must demonstrate controlled flammability performance:

• Wire and cable: quality control of insulation and sheathing compounds, where LOI is often specified alongside vertical flame tests
• Electrical and electronic equipment: screening of enclosure and component materials, frequently complemented by UL 94 or IEC 60695-11-10 burning tests
• Railway and mass transit: EN 45545-2 fire safety requirement sets that incorporate oxygen index among the accepted test methods for certain components
• Construction products: evaluation of pipes, panels, flooring, and interior finish materials, usually combined with reaction-to-fire classifications such as EN 13501-1
• Automotive interior materials: comparative assessment of polymers used in cabins, in conjunction with standards such as FMVSS 302-type horizontal burn requirements
• Aerospace and shipbuilding: material qualification programs where self-extinguishing behavior in air is a baseline expectation
• Flame-retardant R&D: ranking of halogenated, phosphorus-based, intumescent, and mineral-filled formulations, and verification of loading levels of additives such as aluminum hydroxide and magnesium hydroxide
• Academic and regulatory studies: fundamental research into combustion mechanisms and polymer degradation

In practice, LOI data are most powerful when used comparatively, for example benchmarking a candidate material against a proven incumbent or tracking the consistency of a flame-retardant formulation over production batches.

FAQ

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

As a general rule, an LOI of 27% or higher indicates that a material will self-extinguish in normal air and is commonly accepted as flame retardant. However, actual acceptance limits depend on the end-use specification; some rail and cable applications require LOI values of 30% or more, while other specifications accept lower values when supported by additional flame tests.

How does the oxygen index test differ from UL 94 testing?

LOI testing measures the minimum oxygen concentration needed to sustain burning in a controlled atmosphere, producing a continuous numerical value. UL 94 testing instead assesses burning behavior, afterflame time, and flaming drips in ambient air and assigns a classification such as V-0, V-2, or 5VA. The two methods are complementary: LOI is excellent for screening and comparison, while UL 94 directly addresses specific end-product hazard scenarios.

How many specimens are needed for one oxygen index determination?

Standards generally recommend a minimum of 15 specimens, since each trial at a given oxygen concentration consumes one specimen and the bracketing sequence requires multiple adjustments to converge on the critical value. Additional specimens may be needed for materials with erratic burning behavior.

Why can LOI results differ between laboratories?

Differences typically arise from gas purity and flow calibration, chimney geometry, operator judgment of the pass criterion, and variations in specimen preparation, thickness, and conditioning. Interlaboratory reproducibility is usually within about ±1% oxygen when all parties follow the same standard with calibrated equipment.

Does a high LOI guarantee safety in a real fire?

No. The oxygen index test is a small-scale screening method that does not account for factors such as high heat flux, flame spread across large surfaces, smoke production, or toxic gas evolution. LOI results should always be considered together with reaction-to-fire and end-product flammability tests relevant to the actual application.

Can the test be performed at elevated temperatures?

Yes. ISO 4589-3 defines an elevated-temperature oxygen index procedure in which the chimney atmosphere and specimen are preheated. Because LOI decreases as temperature rises, this method is valuable for materials used near heat sources, such as cable insulation in energized equipment, and supports determination of a temperature index at which the LOI drops to a critical level such as 21%.