Oxygen index testing, also known as Limiting Oxygen Index (LOI) testing, is a fundamental laboratory method for evaluating the flammability characteristics of rubber materials. This test determines the minimum concentration of oxygen in an oxygen-nitrogen mixture required to sustain candle-like combustion of a rubber specimen under controlled conditions. This article provides a comprehensive technical overview of rubber oxygen index testing, covering applicable standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, sample requirements, test procedures, and practical applications across the rubber industry.
Technical Overview
The Oxygen Index (OI), also referred to as the Limiting Oxygen Index (LOI), is defined as the minimum volumetric concentration of oxygen in a flowing mixture of oxygen and nitrogen that will just support flaming combustion of a vertically mounted test specimen. Since ambient air contains approximately 20.9% oxygen by volume, materials with an LOI below 21% will generally continue to burn freely in air, while materials with higher LOI values will self-extinguish once an external ignition source is removed. The LOI value is expressed as a dimensionless percentage and is calculated as the volume fraction of oxygen in the total gas mixture.
For rubber materials, oxygen index testing serves as a widely accepted screening method for flammability assessment and flame-retardant formulation development. Most unfilled hydrocarbon rubbers, such as natural rubber and styrene-butadiene rubber, exhibit LOI values in the range of 17 to 21 percent, indicating inherent flammability. Halogenated rubbers, silicone rubbers, and compounds incorporating flame-retardant additives such as aluminum hydroxide, magnesium hydroxide, antimony trioxide, or halogenated and phosphorus-based agents can achieve LOI values of 27 percent or higher, which is commonly recognized as the threshold for flame-retardant classification in many specifications.
Oxygen index testing of rubber is primarily conducted in accordance with the following standards, all of which define the apparatus, specimen preparation, ignition procedure, and calculation methodology:
- ISO 4589-2 – Plastics – Determination of burning behaviour by oxygen index – Part 2: Ambient-temperature 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 based on ISO 4589-2
- ISO 4589-3 – Elevated-temperature oxygen index determination
Testing Samples
Oxygen index testing is applicable to virtually all vulcanized thermoset rubber compounds and thermoplastic elastomers, whether in the form of molded plaques, sheets, extruded profiles, or finished product cuttings. Specimens are typically prepared in rectangular bar form, with common dimensions of 80 to 150 mm in length, 10 mm in width, and 3 to 4 mm in thickness for molded materials, in accordance with the requirements of ISO 4589-2 and ASTM D2863. Prior to testing, specimens must be conditioned in a controlled atmosphere, typically at 23 ± 2 °C and 50 ± 5% relative humidity, for a minimum of 40 to 88 hours depending on the governing standard. At least 15 specimens are generally required for a complete determination.
Natural Rubber (NR) and Polyisoprene (IR)
Natural rubber and synthetic polyisoprene compounds exhibit low intrinsic LOI values, typically between 17% and 19%, due to their hydrocarbon structure and high combustible content. Testing of these materials is essential to quantify the effectiveness of added flame retardants and to verify compliance where flame retardancy is specified.
Styrene-Butadiene Rubber (SBR) and Butadiene Rubbers
SBR and solution-polymerized butadiene rubbers, widely used in tires, conveyor belts, and molded goods, generally show LOI values of approximately 18% to 21%. Highly carbon-black-filled or mineral-filled compounds can achieve moderate improvements in oxygen index performance.
Nitrile Butadiene Rubber (NBR) and Hydrogenated NBR
The acrylonitrile content of NBR provides slightly better flame resistance than pure hydrocarbon rubbers, with LOI values typically ranging from 19% to 22%. Hydrogenated grades (HNBR) are tested to evaluate performance in demanding oil-field and automotive sealing applications.
Ethylene Propylene Diene Rubber (EPDM)
EPDM compounds, extensively used in wire and cable applications, roofing membranes, and automotive components, usually display LOI values of 21% to 28% depending on filler loading. LOI testing is a key quality control measure for flame-retardant EPDM insulation and sheathing compounds.
Chloroprene Rubber (CR) and Other Halogenated Rubbers
The chlorine content in chloroprene rubber imparts inherent flame resistance, yielding LOI values typically in the range of 26% to 32%. Chlorosulfonated polyethylene (CSM) and fluoroelastomers (FKM) achieve even higher values, with FKM compounds often exceeding 40% oxygen index.
Silicone Rubber
Silicone rubbers, particularly those formulated with platinum-catalyzed flame retardant systems, demonstrate favorable burning behavior with LOI values commonly between 25% and 35%. Testing is frequently performed for silicone materials used in rail transit, aerospace, and cable applications.
Flame-Retardant Rubber Compounds and Finished Products
Finished articles such as cable insulation, conveyor belt covers, fire-resistant seals, protective sheeting, and gaskets can be sampled directly for oxygen index verification. Test results are used to confirm that production batches meet specified minimum LOI requirements in purchase specifications and regulatory frameworks.
Testing Items
The primary deliverable of an oxygen index test is the Limiting Oxygen Index value of the rubber material, reported to the nearest 0.1%. A typical testing program may include the following items:
- Determination of LOI at ambient temperature (23 °C) per ISO 4589-2, ASTM D2863, or GB/T 2406.2
- Elevated-temperature oxygen index determination per ISO 4589-3, where required by end-use specifications
- Comparison of measured LOI values against contractual or regulatory minimum thresholds (for example, LOI ≥ 27% or ≥ 28%)
- Assessment of afterflame time, afterglow, flaming drops, and burning characteristics as observational data
- Evaluation of the influence of specimen geometry, thickness, and orientation on the measured index
- Comparative LOI screening during flame-retardant formulation development and raw material qualification
Testing Methods
The test is performed using a vertical glass chimney apparatus consisting of a heat-resistant glass column of specified dimensions, a specimen holder that supports the specimen vertically in the center of the column, calibrated mass flow controllers or rotameters for oxygen and nitrogen supply, a gas mixing chamber, and an ignition source in the form of a small propane or butane burner. A top cover with an outlet port is placed on the column during testing to stabilize the gas flow.
The general test procedure is as follows:
- Condition the specimens in the standard atmosphere and verify dimensions and freedom from defects.
- Mount the specimen vertically in the holder so that at least 50 mm of specimen length is exposed above the top of the holder.
- Purge the column with nitrogen, then establish the initial oxygen-nitrogen mixture at the specified total flow rate, calibrated to produce a linear gas velocity consistent with the standard (typically 40 mm/s in ISO 4589-2).
- Apply the ignition flame to the top of the specimen for up to 5 seconds, repeating as necessary within the permitted ignition period, until the specimen ignites and sustains flaming combustion.
- Record whether the specimen burns for at least 3 minutes or over a consumed or charred length of at least 50 mm after removal of the ignition source. This outcome is defined as the pass criterion for sustained combustion.
- Adjust the oxygen concentration upward or downward using the bracketing or interval procedure defined in the standard, progressively narrowing the interval to 0.2% oxygen, until the LOI value is established with the required statistical confidence.
- Calculate the oxygen index as: LOI = 100 × V(O₂) / [V(O₂) + V(N₂)], where V denotes volumetric flow rates of oxygen and nitrogen.
- Verify the result by additional confirmation tests, then report the LOI value together with the standard used, specimen dimensions, conditioning details, and any observed burning behavior.
Several factors can influence the reproducibility of oxygen index results and must be controlled during testing. These include specimen thickness and density, aging and conditioning history, ambient temperature, gas flow calibration accuracy, filler dispersion and compounding uniformity, and the presence of plasticizers or volatile additives. Accredited third-party laboratories perform instrument calibration, gas purity verification, and flow rate validation at defined intervals to ensure the reliability and comparability of reported LOI values.
Applications
Oxygen index testing of rubber materials supports material selection, product certification, and quality control across numerous industries:
- Wire and cable – Qualification of EPDM, CR, CSM, and silicone-based insulation and sheathing compounds against flame retardancy requirements.
- Rail transit and aerospace – Verification of fire performance for rubber seals, flooring components, and interior materials within applicable fire safety engineering frameworks.
- Automotive industry – Evaluation of fuel and coolant hose materials, sealing systems, and under-hood components requiring reduced flammability.
- Mining and bulk material handling – Compliance testing of flame-resistant conveyor belt cover compounds.
- Building and construction – Assessment of elastomeric seals, gaskets, expansion joints, and waterproofing membranes.
- Research and development – Screening of halogen-free and mineral-filled flame-retardant rubber formulations during compound development.
- Regulatory compliance and procurement – Demonstration of conformance to specification limits requiring minimum LOI values for supplied rubber goods.
FAQ
What is considered a good oxygen index value for rubber?
Generally, rubber materials with an LOI of 27% or above are regarded as flame retardant, since such materials will not continue to burn in normal air containing about 21% oxygen. LOI values between 21% and 26% indicate slow-burning or self-extinguishing tendencies, while values below 21% indicate that the material will burn readily in air.
How many specimens are required for an LOI test?
A complete oxygen index determination normally requires a minimum of 15 specimens, as the standard statistical procedure involves successive adjustments of oxygen concentration to bracket the critical value. Additional specimens may be necessary for materials exhibiting variable burning behavior.
What are the differences between ISO 4589-2, ASTM D2863, and GB/T 2406.2?
All three standards measure the same fundamental property using a vertical chimney apparatus. ISO 4589-2 and GB/T 2406.2 are technically harmonized, while ASTM D2863 differs slightly in certain procedural details such as specimen dimensions and flow verification requirements. Results are generally comparable, but the specific standard used must always be stated in the test report.
How does oxygen index testing compare with other flammability tests?
LOI testing is a small-scale laboratory screening method that provides a numerical, reproducible measure of ease of extinction. It complements, but does not replace, larger-scale reaction-to-fire tests such as flame spread, smoke density, or vertical/horizontal burning tests. Many specifications require LOI testing in combination with additional fire performance evaluations.
Does temperature affect oxygen index results?
Yes. LOI values decrease as ambient temperature increases, which is why the standard test is performed at 23 °C. For applications involving elevated service temperatures, ISO 4589-3 provides a method for determining the oxygen index at higher temperatures and the so-called temperature index, which identifies the temperature at which the LOI equals 20.9%.
How can the LOI of a rubber compound be improved?
Common strategies include incorporating mineral fillers such as aluminum hydroxide or magnesium hydroxide, adding halogen-based flame retardants often synergized with antimony trioxide, using phosphorus-containing or intumescent additives, optimizing the crosslink density, and selecting inherently flame-resistant base polymers such as chloroprene or fluoroelastomers.
Can finished rubber products be tested directly?
Yes, provided that specimens of the required dimensions can be cut from the product without destroying the material structure. Where the product geometry does not permit standard specimens, the test can be performed on compression-molded plaques of the same compound, and this deviation must be documented in the report.
