ISO 4589 Oxygen Index Test: Principle, Specimens, Procedure, and Applications

The ISO 4589 oxygen index test is the internationally recognized laboratory method for measuring the minimum oxygen concentration required to sustain candle-like combustion of plastics, rubber, textiles, and other polymeric materials. As a quantitative flammability screening test, it is widely specified for quality control, material certification, and flame-retardant research across the rail, automotive, aerospace, cable, and electronics industries. This article explains the principle of ISO 4589, specimen requirements, tested items, the step-by-step procedure, typical result interpretation, and common applications.

Technical Overview

The ISO 4589 standard series defines the determination of the limiting oxygen index (LOI), sometimes called the oxygen index or critical oxygen index, of polymeric materials. The LOI is the minimum concentration of oxygen, expressed as a volume percentage in a flowing mixture of oxygen and nitrogen, that will just support combustion of a vertically mounted specimen after it has been ignited from its top end. Because normal air contains approximately 20.9% oxygen, the LOI immediately indicates whether a material will continue to burn in air under the test conditions.

The ISO 4589 series consists of three parts:

• ISO 4589-1 – General guidance on the oxygen index concept, apparatus requirements, and factors influencing results.
• ISO 4589-2 – Ambient-temperature test, the most frequently specified method, which determines the LOI at 23°C.
• ISO 4589-3 – Elevated-temperature test, which determines the oxygen-temperature relationship for applications where materials operate hot, such as electrical equipment.

The core formula of the test is straightforward: LOI equals the volumetric flow of oxygen divided by the combined volumetric flow of oxygen and nitrogen, multiplied by 100. The test is performed in a vertical borosilicate glass chimney of typically 450 mm height and 75 to 100 mm internal diameter, in which the gas mixture flows upward at a standardized linear velocity. A small propane or butane diffusion flame is used as the ignition source, and calibrated mass flow controllers or analyzers ensure the oxygen concentration is known accurately.

As a rule of thumb, materials with an LOI below 21% burn readily in air, values between 21% and 27% indicate slow-burning or self-extinguishing behavior, and values above approximately 27% are generally regarded as flame retardant. The test is closely related to ASTM D2863 in North America and GB/T 2406.2 in China, and it is referenced directly in fire safety specifications such as EN 45545-2 for railway applications.

Testing Samples

ISO 4589-2 defines five specimen types to accommodate rigid plastics, sheets, cellular materials, thin sections, and flexible films. At least 15 specimens of the selected type are normally required, and specimens should be free from bubbles, cracks, burrs, and surface contamination. Unless otherwise specified, specimens are conditioned at 23 ± 2°C and 50 ± 5% relative humidity for a minimum of 88 hours before testing.

Type I to Type III Rigid Specimens

Type I specimens measure 80 to 150 mm in length with a nominal 10 mm by 10 mm cross-section and are intended for dense rigid materials prepared by molding or machining. Type II specimens use the same length and width but a reduced 4 mm thickness, suiting sheet products. Type III specimens, also nominally 10 mm by 10 mm in cross-section, are intended for cellular or foamed materials with an apparent density below approximately 100 kg/m³, whose larger section compensates for the reduced fuel load per unit area.

Type IV Specimens

Type IV specimens measure 70 to 150 mm in length with a nominal 6.5 mm width and 3 mm thickness. They are used when it is impractical to prepare Type I, II, or III specimens, for example with materials of limited thickness, small parts, or certain thermoset laminates. Results from Type IV specimens must not be directly compared with those from other specimen types.

Type V Flexible Film Specimens

Type V specimens measure 140 ± 5 mm by 52 ± 0.5 mm with a thickness up to 10.5 mm and are intended for flexible films and thin sheets. Because such materials may deform under the gas flow or shrink away from the igniter, a support frame may be required to hold the specimen in position while still allowing the top edge to be ignited.

Specimen Marking and Preparation

Before testing, reference marks are applied so that the damaged length can be assessed: for most rigid specimen types a reference line is positioned 50 mm below the top of the specimen. Specimens may also be pre-dried when specified by the material standard, since moisture content can measurably influence the oxygen index of hygroscopic polymers such as polyamides.

Testing Items

Testing performed in accordance with ISO 4589 typically covers the following items, which are reported together with full specimen and conditioning details:

• Determination of the limiting oxygen index (LOI) at ambient temperature (23°C) according to ISO 4589-2, expressed to the nearest 0.1% oxygen.
• Verification testing at a specified oxygen concentration, in which the laboratory confirms whether a material meets or exceeds a contractual or regulatory LOI threshold.
• Elevated-temperature oxygen index determination according to ISO 4589-3, including derivation of the temperature index for electrical and appliance applications.
• Supplementary observations: afterflame time, afterglow time, dripping of flaming particles, char formation, smoking, and deformation behavior.
• Comparative evaluation of flame-retardant formulations, filler loadings, and base polymer grades for R&D purposes.

Testing Methods

The ambient-temperature procedure of ISO 4589-2 is performed as a controlled incremental, or staircase, sequence. The general steps are as follows:

• Mounting: The conditioned specimen is clamped vertically in the holder so that the top of the specimen is at least 100 mm below the top rim of the glass chimney.
• Gas flow setting: Oxygen and nitrogen flows are mixed and adjusted to give the selected oxygen concentration at the standardized total linear flow velocity through the column, and the system is allowed to purge and stabilize for at least 30 seconds.
• Ignition: A small gas flame is applied to the top of the specimen for up to 5 seconds. If stable ignition, evidenced by a flame persisting for at least 5 seconds after removal of the igniter, is not achieved, the application is repeated within the limits defined in the standard.
• Response recording: The result is recorded as response "O" if the specimen extinguishes before the flame front reaches the reference mark 50 mm from the top, or within the applicable time criterion; otherwise the response is recorded as "X". Afterflame and afterglow times and any dripping are noted.
• Concentration adjustment: Based on the response, the oxygen concentration is increased after an "X" response and decreased after an "O" response, first in coarse steps to bracket the transition and then in fine steps, typically 0.2% oxygen, to converge on the limiting value.
• Calculation: The LOI is calculated from the final series of O and X responses using the statistical up-and-down formula specified in ISO 4589-2, and the result is reported together with specimen type, dimensions, orientation, conditioning, and the ignition method used.

ISO 4589-2 also describes an alternative propagating-ignition procedure in which the flame is applied to the top surface in a way intended to establish a spreading flame front, which is used for materials whose behavior under top-edge ignition does not reflect real fire performance. The report must always state which procedure was used, because the numerical LOI can differ between the two methods.

Typical limiting oxygen index values for reference materials include the following:

• PTFE: approximately 95% oxygen – essentially non-combustible in this test.
• Rigid PVC: approximately 45% – inherently flame retardant due to chlorine content.
• Phenolic resins and polyimides: approximately 30 to 40% – charring, high-temperature polymers.
• Polycarbonate: approximately 25 to 28% – self-extinguishing tendency in air.
• Polyamide 66 (conditioned): approximately 24 to 28%, falling when wet or plasticized.
• PE, PP, PS, and PMMA: approximately 17 to 18% – burn readily in normal air.

Applications

Because the oxygen index is reproducible, numerically comparable, and requires only small specimens, ISO 4589 testing is used across many sectors:

• Railway engineering: EN 45545-2 for European rolling stock specifies minimum oxygen index values obtained by ISO 4589-2 for certain components and materials used in train interiors and cabling.
• Wire and cable: insulation and sheathing compounds, including halogen-free and low-smoke formulations, are routinely qualified by LOI alongside smoke and toxicity tests.
• Electrical and electronics: housings, connectors, circuit-breaker parts, and appliance components where ISO 4589-3 elevated-temperature data support thermal endurance assessments.
• Aerospace and marine: interior panels, seating foams, tubing, and coatings, where specifications require demonstrated resistance to burning in ambient air.
• Automotive and mining: fuel-line materials, conveyor belts, battery components, and under-hood plastics.
• Research and quality control: screening flame-retardant additives, comparing resin grades, and verifying incoming material consistency in production plants.

FAQ

What exactly does the ISO 4589 oxygen index measure?

It measures the minimum oxygen concentration in an oxygen and nitrogen mixture, expressed as a volume percentage, that will just support flaming combustion of a vertically held specimen after top ignition at 23°C. It ranks relative flammability; it does not directly predict behavior in a fully developed fire.

How is ISO 4589-2 related to ASTM D2863 and GB/T 2406.2?

All three standards describe the same fundamental limiting oxygen index technique. GB/T 2406.2 is the Chinese standard adopted from ISO 4589-2, and ASTM D2863 is technically similar, with differences in some apparatus details, specimen handling, and calculation conventions. Results are usually comparable but must be reported with the standard used.

What LOI value is considered flame retardant?

Since air contains about 21% oxygen, materials with an LOI above 21% will not easily sustain burning in still air. In industrial practice, an LOI of 27% or higher is commonly treated as the threshold for classifying a material as flame retardant, while values above 32% indicate highly flame-resistant compounds.

Can oxygen index results from different specimen types be compared?

No. LOI values depend on specimen type, thickness, and orientation. A 4 mm specimen and a 10 mm specimen of the same polymer can yield different values, so comparisons are valid only within the same specimen type and test conditions, which must always be stated in the report.

Is ISO 4589 a pass or fail regulatory test?

On its own it is a screening and characterization method. However, many specifications, such as railway standard EN 45545-2 and various cable and military material specifications, assign minimum LOI acceptance limits to it, which turns the measurement into a compliance criterion within those schemes.

What factors can cause inaccurate LOI results?

Common sources of error include inaccurate gas mixing or uncalibrated flow meters, incorrect gas flow velocity, unstable chimney environment from drafts, improper igniter position and flame size, poor specimen conditioning, and incorrect response judgment near the transition point. Regular calibration and strict adherence to the standard minimize these effects.