The oxygen index, also known as the limiting oxygen index (LOI), is a fundamental flammability parameter used to characterize how readily plastic materials ignite and sustain combustion. Determined in accordance with international standards such as ISO 4589-2, ASTM D2863, GB/T 2406.2 and JIS K 7201-2, the test quantifies the minimum oxygen concentration in an oxygen-nitrogen mixture required to support candle-like burning of a polymer specimen. This article explains the testing principle, apparatus, sample requirements, operating procedure, calculation methods and typical industrial applications of plastic oxygen index determination.
Technical Overview
The oxygen index (OI) of a plastic material is defined as the minimum concentration of oxygen, expressed as a percentage by volume, in a flowing mixture of oxygen and nitrogen that will just support flaming combustion of a vertically mounted specimen after ignition with a small external flame. Because normal ambient air contains approximately 20.9 percent oxygen, a material with an oxygen index well above 21 will tend to self-extinguish once the ignition source is removed, while a material with an oxygen index below 21 will continue to burn in ordinary air.
The test principle was first developed in the 1960s and has since been standardized worldwide. It is one of the most widely used small-scale laboratory screening methods for comparing the relative flammability of polymers, evaluating flame-retardant formulations, and verifying compliance with material specifications. It should be noted that the oxygen index is a comparative ranking parameter, not a direct measure of fire hazard in real-scale fires; for full assessment it is normally combined with other flammability tests such as UL 94, IEC 60695-11-10 or cone calorimetry.
- ISO 4589-1: general guidance for oxygen index testing
- ISO 4589-2: determination of oxygen index at ambient temperature
- ISO 4589-3: elevated-temperature oxygen index determination
- ASTM D2863: measuring the minimum oxygen concentration to support candle-like combustion of plastics
- GB/T 2406.2: Chinese national standard, technically equivalent to ISO 4589-2
- JIS K 7201-2: Japanese industrial standard corresponding to ISO 4589-2
Testing Samples
Rigid and Self-Supporting Plastics
Injection-molded, compression-molded or machined bars are the most common specimen form. ISO 4589-2 specifies rods or rectangular bars 80 to 150 mm in length, typically 10 mm wide and 4 mm thick, with a 10 mm by 10 mm cross-section also permitted for certain materials. ASTM D2863 prefers specimens 70 to 150 mm long with a cross-section of 6.5 mm by 3.0 mm. The specimen geometry, thickness and direction of testing must always be reported, since these parameters directly influence the result.
Flexible Films and Thin Sheets
Thin films that cannot support themselves vertically are tested using a special support frame or rolled specimen holders described in the relevant annexes of the standards. The frame holds the film in a defined geometry without restricting flame propagation along the specimen surface.
Cellular Materials and Foams
Rigid and flexible foams are tested with dedicated holders that clamp the specimen without compressing the cellular structure. Because foam density and cell structure strongly affect the burning behavior, specimens must be free of skin damage and cut to the dimensions specified for cellular materials in the applicable standard.
Specimen Conditioning and Marking
Prior to testing, specimens are conditioned at 23 ± 2 °C and 50 ± 5 percent relative humidity in accordance with ISO 291 or ASTM D618, typically for at least 40 hours, unless the material specification states otherwise. Gauge marks are applied so that the burning distance can be assessed, with a reference mark 50 mm from the expected ignition end. A minimum of 15 to 30 specimens is normally required to complete a determination, because the oxygen concentration must be adjusted iteratively.
Testing Items
Accredited third-party laboratories and in-house quality laboratories typically perform the following oxygen index determinations:
- Ambient-temperature oxygen index (LOI) determination at 23 °C per ISO 4589-2, ASTM D2863, GB/T 2406.2 or JIS K 7201-2
- Elevated-temperature oxygen index testing per ISO 4589-3, used mainly for wire and cable materials and enclosed applications
- Verification of contractual or specification-based LOI limits, for example minimum LOI values of 27, 28, 30 or 32 required for flame-retardant compounds
- Comparative evaluation of flame-retardant formulations during research and development
- Batch-to-batch quality control of incoming resins, compounds and finished plastic components
Testing Methods
Test Apparatus
The standard apparatus consists of a heat-resistant glass chimney, typically at least 450 mm high and 75 to 100 mm in diameter, through which a controlled oxygen-nitrogen mixture flows upward. Key components include a specimen holder that supports the specimen vertically at the base of the chimney, separate oxygen and nitrogen supply lines fitted with calibrated rotameters or mass flow controllers, a mixing chamber, a small propane flame igniter, and particulate filters to protect the flow devices. The gas flow is regulated so that the linear velocity through the chimney is approximately 40 mm/s, as required by ISO 4589-2. Optional oxygen analyzers allow continuous verification of the mixture concentration.
Test Procedure
The specimen is clamped vertically with its upper end positioned below the top rim of the chimney, and the column is purged with the gas mixture for at least 30 seconds to establish a stable atmosphere. The propane igniter flame is then applied to the top of the specimen for a defined period, in short repeated applications of up to 30 seconds depending on the standard and specimen type, until the specimen is ignited. After removal of the ignition source, the operator observes whether combustion is sustained.
A specimen is considered to have sustained combustion if it burns for longer than the time criterion, typically 180 seconds in ISO 4589-2 and 3 minutes in ASTM D2863, or if the flame front consumes more than 50 mm of the specimen length between the gauge marks. The oxygen concentration is then adjusted stepwise: large steps of approximately 1 percent oxygen are used to bracket the critical concentration, followed by fine steps of 0.2 percent to converge on the final value. Testing alternates between concentrations at which the specimen passes and fails until the index is confirmed by the bracketing verification sequence defined in the standard.
Calculation of the Oxygen Index
The oxygen index is calculated from the volumetric flow rates of oxygen and nitrogen at the critical concentration using the following relationship:
LOI = 100 × V(O2) / [V(O2) + V(N2)]
where V(O2) is the volumetric flow of oxygen and V(N2) is the volumetric flow of nitrogen. The result is reported to the nearest 0.1 percent by volume, together with the standard used, specimen dimensions and thickness, orientation, conditioning history, and the type of ignition applied. Corrections may be applied for gas purity when the oxygen or nitrogen supply does not meet the specified concentration tolerances.
Factors Affecting Test Results
Numerous variables can shift the measured oxygen index, and laboratories must control them carefully:
- Specimen thickness: thicker specimens generally exhibit higher oxygen index values for many polymers
- Temperature: the oxygen index decreases significantly as ambient temperature rises, which is why elevated-temperature testing is specified for certain cable materials
- Humidity and conditioning: absorbed moisture can act as a flame retardant or plasticizer
- Material additives: flame retardants, mineral fillers, plasticizers and reinforcements alter burning behavior
- Gas calibration and flow accuracy: drift in flow meters directly biases the calculated index
- Operator judgment: correct assessment of ignition, dripping and the 50 mm burning criterion requires trained personnel
Applications
Oxygen index testing is applied across many industries in which plastic flammability performance is specified or regulated:
- Wire and cable: insulation and sheathing compounds for power, control and communication cables frequently carry minimum LOI requirements of 28 or higher
- Electrical and electronic equipment: housings, connectors, terminal blocks and engineering components evaluated alongside UL 94 ratings
- Rail and mass transit: interior and infrastructure materials assessed within flame spread and smoke hazard frameworks such as EN 45545-2
- Construction: panels, pipe materials, insulation products and decorative plastics used in buildings
- Marine, offshore and mining: materials subject to strict flame-retardancy specifications in confined or high-risk environments
- Polymer research and development: screening of halogenated, halogen-free, phosphorus-based, intumescent and mineral-filled flame-retardant systems
- Quality assurance: incoming resin inspection, compound certification and batch release testing by accredited third-party laboratories
FAQ
What does a higher oxygen index mean?
A higher oxygen index means the material requires a more oxygen-rich atmosphere to keep burning. Since air contains about 21 percent oxygen, materials with an LOI above 21 will generally stop burning once the ignition source is removed, and higher values indicate progressively better self-extinguishing behavior.
What oxygen index value is considered flame retardant?
There is no universal threshold, but in industrial practice plastics with an LOI of 27 or above are commonly regarded as flame retardant, and many cable and railway specifications demand values of 28, 30 or higher depending on the application and the associated fire safety standard.
Which standards govern oxygen index testing of plastics?
The principal standards are ISO 4589-2 and its equivalents ASTM D2863, GB/T 2406.2 and JIS K 7201-2 for ambient-temperature testing, and ISO 4589-3 for elevated-temperature testing. The choice of standard should match the specification referenced by the customer or regulation.
How is oxygen index testing different from the UL 94 test?
The oxygen index test measures the minimum oxygen concentration needed to sustain burning in a controlled flowing atmosphere, while UL 94 evaluates ignition, flame propagation and afterglow behavior of specimens ignited in normal air. The two methods are complementary, and material datasheets commonly report both the LOI value and the UL 94 rating.
Can the oxygen index predict real-scale fire performance?
No. The oxygen index is a small-scale comparative screening test conducted under controlled laboratory conditions. It ranks materials and supports formulation development and specification compliance, but full fire hazard assessment requires additional large-scale or performance-based testing appropriate to the end-use scenario.
How many specimens are needed and how long does a test take?
A complete determination typically consumes 15 to 30 specimens because the critical oxygen concentration must be approached iteratively and confirmed by bracketing. A single LOI determination, including conditioning verification and gas calibration, is usually completed within a few hours by an experienced operator.