Plastic oxygen index testing, also known as Limiting Oxygen Index (LOI) testing, measures the minimum concentration of oxygen in an oxygen/nitrogen mixture required to sustain candle-like combustion of a plastic specimen, making it one of the most widely used bench-scale methods for evaluating polymer flammability. Conducted in accordance with ISO 4589-2, ASTM D2863, and GB/T 2406.2, the test provides a quantitative and repeatable flammability index that supports material selection, flame-retardant formulation development, and regulatory compliance in industries such as electronics, construction, wire and cable, and transportation.
Technical Overview
The Limiting Oxygen Index (LOI), also referred to as the oxygen index (OI), 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 combustion of a vertically mounted plastic specimen under controlled laboratory conditions. Because ambient air contains approximately 20.9% oxygen, a material with an LOI below this threshold will continue to burn freely in air, while a material with an LOI well above 21% tends to self-extinguish once the ignition source is removed. The plastic oxygen index test therefore provides a single, numerical measure of relative flammability that is ideal for ranking polymers, screening flame-retardant formulations, and verifying compliance with material specifications.
A widely used general interpretation of LOI results is as follows:
- LOI below 21%: the material burns readily in normal air and is classified as highly flammable.
- LOI between 21% and 24%: the material is still relatively flammable and requires caution in application.
- LOI between 24% and 27%: the material burns slowly and is considered moderately flammable.
- LOI of 27% and above: the material is difficult to ignite in air and is generally regarded as self-extinguishing or flame-retardant.
It should be noted that LOI values are influenced by polymer chemistry, flame-retardant additives, inorganic fillers, specimen geometry, moisture content, and test temperature. Materials with an LOI measured at room temperature may behave differently at elevated temperatures, which is why complementary methods such as ISO 4589-3 and UL 94 burning tests are often performed alongside the ambient-temperature oxygen index determination.
Testing Samples
Rigid Self-Supporting Plastics
Molded, extruded, or machined bars of thermoplastics and thermosets are the most common test specimens. Typical dimensions per ISO 4589-2 are 80 mm to 150 mm in length, 10 mm in width, and 4 mm to 10 mm in thickness, with 10 mm × 4 mm cross-sections being the most widely referenced geometry. ASTM D2863 specifies a similar specimen length of 70 mm to 150 mm and a width of 10 mm.
Flexible Plastics and Films
Thin films and flexible sheeting cannot stand freely in the specimen holder and are tested with appropriate support frames or rod holders as described in the relevant annexes of ISO 4589-2. Modified pass criteria are applied to account for the different burning behavior of thin, flexible materials.
Cellular Materials and Foams
Rigid and flexible polymeric foams, such as those used in insulation and upholstery, are tested as rectangular blocks, typically 10 mm × 10 mm in cross-section, using support devices that prevent specimen distortion without shielding the specimen from the gas flow.
Rubber, Elastomers, and Composite Materials
Vulcanized rubbers, thermoplastic elastomers, laminates, and fiber-reinforced composites can also be evaluated using the oxygen index method, provided specimens are cut cleanly and the material does not melt or drip away from the holder in a manner that prevents meaningful measurement.
Specimen Conditioning and Quantity
Prior to testing, specimens are conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity, typically for a minimum of 40 hours. A complete determination generally requires at least 15 specimens per material, because the test uses a bracketing procedure in which successive specimens are tested at different oxygen concentrations.
Testing Items
Accredited third-party laboratories offering plastic oxygen index testing typically provide the following test items:
- Limiting Oxygen Index (LOI) determination at ambient temperature in accordance with ISO 4589-2 / EN ISO 4589-2, ASTM D2863, GB/T 2406.2, or JIS K 7201-2.
- Oxygen index determination at elevated temperatures in accordance with ISO 4589-3, used to assess how flammability changes under thermal stress.
- Verification of minimum LOI requirements specified in customer specifications, procurement standards, or regulatory documents.
- Comparative evaluation of flame-retardant formulations, including halogenated and halogen-free flame-retardant systems.
- Quality control testing for incoming resins, compounded materials, and finished plastic components.
- Supporting data for material data sheets, technical documentation, and regulatory submissions.
Testing Methods
Applicable Standards
- ISO 4589-2 / EN 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 (Oxygen Index).
- GB/T 2406.2: Chinese national standard, technically aligned with ISO 4589-2.
- JIS K 7201-2: Japanese industrial standard equivalent for the ambient-temperature oxygen index test.
Test Apparatus
The test apparatus consists of a heat-resistant glass chimney, typically 450 mm to 500 mm in height, in which the specimen is held vertically by a specimen holder; a supply of oxygen and nitrogen with pressure regulation and calibrated flow meters or mass flow controllers; a gas mixing and distribution system that delivers a uniform upward flow through the chimney; and a small propane or butane igniter used to apply a defined flame to the top of the specimen. Optional oxygen analyzers may be used to verify the oxygen concentration of the mixed gas stream.
Test Procedure
The general procedure for the ambient-temperature oxygen index test is as follows:
- Condition, measure, and mark the specimens, including a reference mark 50 mm from the top (ignition) end.
- Mount the specimen vertically in the holder and position it centrally within the chimney.
- Purge the chimney with the oxygen/nitrogen mixture at the specified total flow rate to establish stable conditions.
- Set an initial oxygen concentration based on experience or prior knowledge of the material class.
- Apply the igniter flame to the top of the specimen until ignition occurs, then remove the igniter and start timing.
- Record whether the specimen burns for at least 180 seconds or over a distance of at least 50 mm, as required by ISO 4589-2 for self-supporting plastics.
- Adjust the oxygen concentration upward or downward in systematic increments, testing successive specimens until the limiting concentration is bracketed within the specified interval.
- Calculate the LOI using the relationship LOI (%) = 100 × V(O2) / [V(O2) + V(N2)], applying a barometric pressure correction where required.
- Report the final LOI value, typically to one decimal place, together with the test standard, specimen dimensions, conditioning history, and observed burning behavior such as dripping, charring, or afterflame time.
Factors Affecting Results
Accurate LOI results depend on strict control of gas purity, flow calibration, specimen preparation, and ambient conditions. Because LOI decreases with increasing temperature and can vary with specimen thickness, comparisons between materials are only valid when specimens are prepared and tested under identical conditions. Consistent ignition technique and correct application of pass criteria are also essential for reproducibility between laboratories.
Applications
Electrical and Electronic Products
LOI data are widely used to qualify engineering plastics for enclosures, connectors, switches, printed circuit board substrates, and other electronic components where flame-retardant performance is required, often in combination with UL 94 flammability ratings.
Wire and Cable
Insulation and sheathing compounds for power cables, communication cables, and harnesses are routinely evaluated by oxygen index testing, with specifications frequently requiring minimum LOI values of 28% or higher for flame-retardant grades.
Building and Construction
Pipes, ducting, wall coverings, insulation boards, flooring, and decorative materials are assessed for combustibility classification, supporting compliance with building codes and fire safety regulations.
Transportation
Interior materials for automotive, rail transit, marine, and aerospace applications are screened using oxygen index testing because these sectors impose stringent flammability and smoke requirements on polymeric components.
Furniture and Textiles
Flexible foams, upholstery fabrics, and coated materials are tested to evaluate their resistance to small ignition sources and to support fire safety assessments of finished furniture.
Research, Development, and Quality Control
In polymer laboratories and manufacturing plants, the oxygen index test serves as a fast, economical screening tool for developing new flame-retardant compounds, comparing additive loadings, and monitoring production consistency between batches.
FAQ
What LOI value is considered flame retardant? As a general rule, materials with an LOI of 27% or above are regarded as self-extinguishing or flame retardant, while values above 30% indicate excellent resistance to ignition in air. Specific product standards may define their own minimum thresholds.
How does oxygen index testing differ from UL 94 testing? The oxygen index test quantifies the minimum oxygen concentration needed to sustain burning, whereas UL 94 evaluates the ability of a material to extinguish after ignition by a small flame in horizontal or vertical orientations. The two methods are complementary and are frequently specified together.
How many specimens are required for a test? A full determination normally requires at least 15 specimens, because the bracketing procedure tests successive specimens at incrementally adjusted oxygen concentrations until the limiting value is established.
Does a high LOI guarantee fire safety in real applications? No. The oxygen index test is a small-scale comparative method. Real fire performance depends on geometry, heat flux, ventilation, and interaction with other materials, so LOI data should be combined with larger-scale fire tests for complete hazard assessment.
Why can LOI results vary between laboratories? Small differences in gas flow calibration, oxygen concentration control, ignition technique, specimen conditioning, and ambient temperature can shift results. Using accredited third-party laboratories operating under ISO/IEC 17025 management systems minimizes inter-laboratory variation and ensures the reliability of reported values.