Limiting Oxygen Index (LOI) testing determines the minimum oxygen concentration, in an oxygen and nitrogen mixture, required to sustain candle-like combustion of chemically active substances such as polymers, reactive resins, elastomers, and flame-retardant compounds. Accredited third-party laboratories perform this test in accordance with ISO 4589-2, ASTM D2863, and GB/T 2406.2, delivering quantitative flammability data that supports flame-retardant formulation development, quality control, material qualification, and regulatory compliance across the electrical, rail, automotive, and construction industries.
Technical Overview
The limiting oxygen index (LOI), also known 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 flaming combustion of a vertically mounted specimen. Because normal ambient air contains approximately 20.9 percent oxygen, a material with an LOI below 21 percent will generally continue to burn in air once ignited, whereas materials with higher LOI values tend to self-extinguish under ordinary atmospheric conditions.
For chemically active substances, including polymers that depolymerize on heating, thermosetting resins, halogenated or phosphorus-based compounds, intumescent systems, and materials that release flammable or inert gaseous decomposition products, the LOI method provides a sensitive and highly repeatable small-scale means of ranking relative flammability. The combustion behavior of these materials is governed not only by heat and mass transfer but also by their decomposition chemistry, such as volatile release, char formation, and the generation of diluting or radical-quenching gases. LOI testing captures the net effect of these chemical mechanisms in a single numerical result.
While the LOI value is not, by itself, a direct measure of full-scale fire hazard, it remains one of the most reproducible bench-scale flammability indicators available. It is widely used for screening flame-retardant formulations, demonstrating lot-to-lot consistency, populating material datasheets, and verifying compliance with product safety specifications in the electrical and electronics, rail, marine, automotive, mining, and construction sectors.
Testing Samples
Rigid and Semi-Rigid Polymeric Materials
Typical specimens include engineering thermoplastics, thermosetting moldings, laminates, and filled or reinforced compounds. Under ISO 4589-2 and GB/T 2406.2, specimens are generally 80 to 150 mm in length and 10 mm in width with a preferred thickness of 4 mm. Under ASTM D2863, the standard cross-section is 6.5 mm in width and 3 mm in thickness, with a length of 70 to 150 mm. Edges should be smooth and free of burrs, cracks, and flash that could influence ignition behavior.
Flexible Materials, Films, and Textiles
Thin films, sheets, coated fabrics, and textile fabrics may be tested using supported or rolled specimen configurations to maintain a reproducible vertical geometry. Textile materials are commonly evaluated under GB/T 5454 or equivalent methods, with specimens prepared to the dimensions specified by the applicable standard so that ignition and flame spread behavior remain comparable between laboratories.
Cellular and Foamed Materials
Rigid and flexible foams, including polyurethane, polystyrene, polyolefin, and phenolic foams, can be evaluated using specimen geometries defined for cellular materials in the relevant annexes of ISO 4589-2. Because foam density and cell structure strongly influence burning behavior, samples should be cut cleanly and should be representative of the manufactured product.
Rubber and Elastomeric Materials
Elastomeric compounds, including insulation and sheathing materials for wire and cable, seals, and conveyor belt covers, may be tested under GB/T 10707 or under the general plastics oxygen index procedures. Specimens are typically die-cut from cured sheets of controlled thickness.
Reactive Resins, Adhesives, Coatings, and Chemically Modified Systems
Epoxy, unsaturated polyester, vinyl ester, polyurethane, and acrylic systems, as well as intumescent coatings, structural adhesives, and chemically modified or flame-retardant-treated compounds, should be fully cast, cured, and conditioned before testing. Free specimens of the specified dimensions are preferred; where a substrate is integral to the product, the test configuration should be agreed in advance. Chemically active or reactive samples require documented pre-screening to confirm they are stable under the test conditions.
Testing Items
Core and extended test offerings for chemically active substances include the following:
- Ambient-temperature limiting oxygen index (LOI) determination in accordance with ISO 4589-2, EN ISO 4589-2, GB/T 2406.2, ASTM D2863, or JIS K 7201-2.
- Elevated-temperature LOI determination and temperature index (TI) determination in accordance with ISO 4589-3, identifying the temperature at which the LOI falls to the oxygen concentration of air.
- Comparative LOI ranking of candidate formulations, filler loadings, or flame-retardant packages during research and development.
- Verification of LOI values against specification limits, for example minimum requirements of 26, 28, or 30 percent oxygen commonly cited in industry material specifications.
- Qualitative burning behavior observations, including afterflame and afterglow time, melt dripping, char formation, smoke release, and intumescence.
- Aging studies in which LOI is measured before and after thermal, humidity, or UV exposure to assess retention of flame-retardant performance.
Testing Methods
Applicable Standards
- ISO 4589-1, which provides general guidance on oxygen index test methods and result interpretation.
- ISO 4589-2 and its adoptions as EN ISO 4589-2 and GB/T 2406.2, covering the ambient-temperature determination for plastics.
- ISO 4589-3, covering LOI determination at elevated temperatures for temperature index derivation.
- ASTM D2863, measuring the minimum oxygen concentration to support candle-like combustion of plastics.
- GB/T 5454 for textile fabrics and GB/T 10707 for rubber and elastomeric materials.
- JIS K 7201-2 as the Japanese national adoption for plastic materials.
Test Apparatus
The test is conducted in an oxygen index apparatus consisting of a heat-resistant glass chimney of specified dimensions, a specimen holder that supports the specimen vertically in the center of the chimney, precision gas metering and mixing controls, and a small premixed gas flame igniter applied at the top of the specimen. High-purity oxygen and nitrogen are supplied at a controlled total flow rate that produces the linear gas velocity through the chimney required by the standard, and the oxygen concentration of the mixture is verified using a calibrated oxygen analyzer, typically of the paramagnetic type. Timing devices and graduated marks on the specimen are used to assess the burning response.
Test Procedure
- Specimens are conditioned prior to testing, typically at 23 ± 2 °C and 50 ± 5 percent relative humidity, unless the governing standard or the client specification requires a different conditioning regime.
- Specimen dimensions are verified, and the specimen is mounted vertically so that its top is at the prescribed position below the chimney outlet.
- The chimney is purged with the oxygen and nitrogen mixture at the specified flow rate for at least 30 seconds before ignition to ensure a stable atmosphere.
- The igniter flame is applied briefly to the top of the specimen, with reapplication as permitted by the standard, until the specimen sustains flaming combustion.
- The specimen response is recorded. A specimen is normally considered to support combustion if flaming persists beyond a defined period, typically 3 minutes, or if the flame front reaches a reference mark set 50 mm below the point of ignition.
- The oxygen concentration is adjusted stepwise, initially in coarse increments for scouting and then in increments of 0.2 percent, to bracket the critical concentration at which the response changes from burning to self-extinguishing.
- Confirmation specimens are tested at the bracketing concentrations, and the final LOI value is calculated using the standard formula, typically LOI = cF + k·d, where cF is a reference concentration, d is the oxygen concentration step, and k is a factor determined from the number of specimens that burned in the confirmation series.
- The result is reported to the nearest 0.1 percent oxygen, together with the specimen geometry, conditioning, standard applied, and any observed burning behavior.
Safety Considerations for Chemically Active Samples
Because the test involves an open flame and the thermal decomposition of chemically active substances, samples should undergo documented pre-screening, such as calorimetric or thermogravimetric analysis, to exclude pyrophoric, explosive, or otherwise unstable materials from open-flame testing. Testing is performed in small specimen quantities under appropriate ventilation with exhaust gas management, and reactive systems must be fully cured, degassed, and dimensionally stable before specimens are cut.
Interpretation of Results
- LOI below 21 percent: the material generally continues to burn in normal air and is regarded as flammable.
- LOI of 21 to 24 percent: marginal performance, with self-extinguishing behavior possible in still air.
- LOI of 25 to 28 percent: generally classified as flame-retardant under ambient conditions.
- LOI above 28 percent: considered highly flame-retardant and difficult to ignite in air; many industry specifications set minimum requirements in this region.
Applications
LOI testing of chemically active substances serves a broad range of engineering, regulatory, and commercial purposes:
- Flame-retardant research and development, including the optimization of halogen-free, phosphorus-based, nitrogen-based, mineral filler, and intumescent systems, where LOI provides rapid comparative feedback on formulation changes.
- Incoming material and lot release quality control for resins, compounds, textiles, foams, and elastomers, ensuring consistent flammability performance in production.
- Railway applications, where EN ISO 4589-2 data support fire safety compliance of materials and components under the EN 45545-2 framework.
- Electrical and electronic products, where LOI complements ignition source tests from the IEC 60695 series and material classifications such as UL 94, and is widely specified for enclosure, connector, and insulating materials.
- Wire and cable materials, including insulation and sheathing compounds for which minimum LOI values are commonly written into purchase specifications.
- Mining conveyor belts and industrial belting, where statutory flame-resistance requirements are frequently expressed as minimum oxygen index values.
- Construction products, furniture, mattresses, and protective textiles, supporting material selection and conformity documentation.
- Academic research and failure analysis, providing a quantitative parameter for studying the effects of chemical structure, additives, aging, and environmental exposure on combustibility.
FAQ
What is considered a good LOI value?
Any value above 21 percent indicates that the material will not continue to burn in ordinary air once the ignition source is removed. In practice, most industrial specifications treat values of 26 to 30 percent or higher as evidence of effective flame-retardant performance, and the required threshold depends on the application and the governing product standard.
How does LOI testing differ from UL 94 or glow-wire testing?
LOI quantifies the oxygen requirement for sustained combustion and yields a continuous numerical value, which makes it ideal for ranking formulations and monitoring small changes in composition. UL 94 vertical and horizontal burn tests and IEC 60695 glow-wire tests evaluate the response of a finished configuration to a specific ignition source. The methods are complementary, and many qualification programs require both LOI data and a burn classification.
How many specimens are required for one determination?
A single LOI determination typically consumes 10 to 20 specimens, including preliminary scouting runs at coarse oxygen concentration increments and the confirmation series at fine increments of 0.2 percent oxygen. Additional specimens should be supplied when elevated-temperature testing or replicate determinations are requested.
Can LOI be measured at elevated temperatures?
Yes. ISO 4589-3 defines the procedure for determining LOI at elevated temperatures, from which the temperature index is derived, meaning the temperature at which the LOI of the material falls to 21 percent. This parameter is valuable for chemically active polymers whose flame-retardant performance degrades significantly as temperature increases.
Is the LOI value a direct measure of fire hazard?
No. LOI is a bench-scale ranking parameter measured under controlled, gentle combustion conditions. It does not by itself predict behavior in a fully developed fire, where factors such as heat flux, oxygen vitiation, geometry, and scale dominate. LOI should be combined with reaction-to-fire tests such as cone calorimetry and application-specific fire performance standards for complete hazard assessment.
What precautions apply to chemically active or reactive samples?
Reactive samples must be fully cured and conditioned before specimen preparation, and all chemically active substances should be pre-screened by thermal analysis to confirm they can be safely subjected to open-flame ignition. Testing is performed on small specimens with appropriate ventilation and exhaust gas handling, and any known hazardous decomposition products should be declared to the laboratory in advance.