Limiting Oxygen Index (LOI) analysis is the most widely used small-scale laboratory method for quantifying the flammability of rubber and elastomeric materials, measuring the minimum oxygen concentration required to sustain candle-like combustion. This technical article explains the principles of rubber LOI testing, applicable international standards such as ISO 4589-2, ASTM D2863, and GB/T 10707, specimen requirements, test procedures, result interpretation, and typical applications across the cable, mining, transportation, and construction industries.
Technical Overview
The Limiting Oxygen Index (LOI), also known as the Oxygen Index (OI) or Critical Oxygen Index (COI), 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 candle-like combustion of a vertically mounted specimen. Because ambient air contains approximately 20.95% oxygen, the 21% threshold serves as a practical benchmark: rubber materials with an LOI below 21% will continue to burn in normal air once ignited, while materials with an LOI above 21% tend to self-extinguish under ambient conditions.
Most hydrocarbon-based rubbers, including natural rubber (NR), styrene-butadiene rubber (SBR), butadiene rubber (BR), and ethylene-propylene-diene rubber (EPDM), are inherently combustible, with LOI values typically ranging from 17% to 22%. Specialty elastomers containing halogen or silicon atoms in the polymer backbone, such as chloroprene rubber (CR), chlorosulfonated polyethylene (CSM), fluoroelastomers (FKM), and silicone rubber (VMQ), exhibit substantially higher LOI values due to their intrinsically flame-resistant molecular structures. Flame-retardant formulations incorporating aluminum trihydroxide (ATH), magnesium hydroxide (MDH), halogen-antimony synergists, phosphorus compounds, or intumescent additives can raise the LOI of conventional rubbers by 10 or more percentage points.
LOI analysis is a comparative, small-scale screening test rather than a direct assessment of real-scale fire hazard. It is widely applied in quality control, flame-retardant formulation development, incoming material verification, and compliance demonstration against material specifications. For a complete fire-safety evaluation, LOI results are typically combined with other flammability and fire-propagation tests specified in end-product standards.
Testing Samples
Standard Vulcanized Compounds
Rubber compounds are typically vulcanized into flat test plaques under controlled conditions of temperature, pressure, and time. Rectangular strip specimens are then die-cut or machined from the plaques. Typical specimen dimensions are 80–150 mm in length, 10 mm in width, and 3–4 mm in thickness, depending on the governing standard and product form. Specimens must be free from voids, contamination, and surface defects that could influence burning behavior.
Finished Product Samples
Specimens may be taken directly from finished rubber goods, including cable insulation and sheathing strips, conveyor belt cover and interply samples, seals and gaskets, hose covers, molded parts, and rubber sheets. Where the product geometry does not permit full-size specimens, reduced-thickness or assembled specimens may be tested in accordance with the relevant standard, with the specimen preparation method documented in the test report.
Cellular and Foam Rubbers
Sponge and foam rubber materials, such as sealing strips and thermal insulation foams, can also be evaluated, although their low density and open-cell structure generally produce lower and more variable LOI values. Special attention must be paid to specimen orientation and the preservation of the as-manufactured cellular structure during cutting.
Sample Conditioning
Prior to testing, specimens are conditioned in a controlled atmosphere, typically 23 ± 2 °C and 50 ± 5% relative humidity, for a minimum period defined by the applicable standard, unless a different conditioning state is specified. Conditioning is critical because moisture content and thermal history can measurably influence LOI results.
Testing Items
The following test items are commonly performed within a rubber LOI analysis program at accredited third-party laboratories or in-house fire-testing facilities:
- Limiting Oxygen Index (LOI) at ambient temperature: determination of the minimum oxygen volume fraction, reported as O2 / (O2 + N2) × 100%, that just sustains combustion of the rubber specimen.
- Temperature Index (TI): determination of the temperature at which the LOI falls to 21%, characterizing how flame retardancy degrades at elevated ambient temperatures, typically per ISO 4589-3.
- Burning behavior observations: afterflame time, afterglow behavior, flaming drips, specimen distortion, and char formation, which supplement the numerical LOI value.
- Comparative LOI evaluation: ranking of baseline versus flame-retarded formulations, or verification of minimum LOI requirements in material specifications and procurement documents.
Testing Methods
Applicable Standards
- ASTM D2863: Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index), widely applied to elastomeric materials.
- ISO 4589-2: Determination of burning behaviour by oxygen index — Part 2: Ambient-temperature test, the primary international method for LOI determination.
- ISO 4589-3: Determination of burning behaviour by oxygen index — Part 3: Elevated-temperature test, used for temperature index determination.
- GB/T 10707: The Chinese national standard specifically addressing the determination of flammability of vulcanized or thermoplastic rubber by the oxygen index method.
- GB/T 2406.2: The Chinese national standard for oxygen index determination, technically aligned with ISO 4589-2.
- JIS K 7201-2: The Japanese industrial standard corresponding to the ISO 4589-2 oxygen index method.
Test Apparatus
A standard oxygen index apparatus consists of a heat-resistant glass combustion chimney of specified dimensions, a vertical specimen holder with a support frame, a precision gas mixing and delivery system for oxygen and nitrogen with calibrated flow measurement, and a small propane or butane diffusion-flame igniter. The gas supply system must maintain a stable, homogeneous mixture at a controlled total flow rate, with oxygen concentration adjustable with sufficient resolution, typically to 0.1% or 0.2%, because the LOI result is calculated directly from the gas flow rates.
Test Procedure
- Mount the conditioned specimen vertically in the holder so that at least the specified length extends above the top of the chimney.
- Establish the oxygen and nitrogen flow rates to achieve the desired starting oxygen concentration at the specified total flow and linear gas velocity through the chimney.
- Allow the gas mixture to purge the chimney, then apply the igniter flame to the top of the specimen for the prescribed duration until the specimen ignites.
- Observe the burning behavior after removal of the igniter and judge whether combustion meets or exceeds the standard pass criterion, typically burning of 50 mm of specimen length or continued combustion beyond a defined time limit.
- Systematically vary the oxygen concentration, increasing it after failure to sustain burning and decreasing it after sustained burning, until the critical concentration bracketing the transition point is established within the required interval.
- Record the LOI value, along with specimen geometry, conditioning, ignition method, and any observed phenomena such as afterflame, afterglow, or dripping.
Calculation and Expression of Results
The limiting oxygen index is calculated as LOI = 100 × V(O2) / [V(O2) + V(N2)], where V(O2) and V(N2) are the volumetric flow rates of oxygen and nitrogen at the critical concentration. Results are reported to the nearest 0.1% or 0.2% oxygen, together with the specimen dimensions, test method designation, and environmental conditions. Repeatability within a single laboratory is generally good, while reproducibility between laboratories is typically on the order of one to two percentage points of oxygen, which should be considered when setting specification limits.
Interpretation of LOI Values
- LOI below 21%: the material burns readily in ambient air and is generally classified as flammable.
- LOI between 21% and 25%: the material shows slow-burning or self-extinguishing tendencies in air.
- LOI between 26% and 34%: the material is generally regarded as flame retardant and difficult to ignite in air.
- LOI above 35%: the material is considered highly flame resistant and will not sustain combustion in normal air.
Typical LOI Ranges for Common Rubbers
- Natural rubber (NR), SBR, BR: approximately 17–19%.
- Nitrile rubber (NBR): approximately 20–22%.
- EPDM: approximately 19–22%, higher in filled flame-retardant grades.
- Chloroprene rubber (CR): approximately 26–35%.
- Chlorosulfonated polyethylene (CSM): approximately 25–30%.
- Silicone rubber (VMQ): approximately 25–35%.
- Fluoroelastomer (FKM): above 35%.
Factors Influencing Results
LOI values are sensitive to specimen thickness, with thicker specimens often exhibiting different indices than thin ones; to ambient temperature, since LOI decreases as temperature rises; to humidity and prior aging of the rubber; to the degree of vulcanization; and to filler type and loading. Consequently, comparisons between materials are valid only when specimen geometry, conditioning, and test conditions are matched, and any deviation must be clearly stated in the test report.
Applications
LOI analysis supports material selection, formulation optimization, and regulatory compliance across numerous industries:
- Wire and cable: screening flame-retardant insulation and sheathing compounds, including halogen-free formulations based on ATH and MDH, in support of flame tests such as IEC 60332 series requirements.
- Mining and bulk material handling: verification of flame-resistant rubber conveyor belts and skirtboard compounds for underground and surface operations, where specifications frequently impose minimum LOI thresholds.
- Rail and mass transit: evaluation of seals, hoses, flooring underlays, and elastomeric components contributing to overall fire-safety compliance of rolling stock.
- Automotive and aerospace: qualification of engine-bay seals, grommets, fuel and hydraulic hoses, and high-performance FKM or silicone sealing systems requiring high flame resistance.
- Construction and infrastructure: assessment of expansion-joint sealants, waterproofing membranes, bridge bearing pads, and structural sealing profiles.
- Personal and industrial protection: development of heat and flame resistant gloves, protective clothing components, and firestop sealing elastomers.
- Research and development: systematic optimization of flame-retardant packages, study of synergistic additive combinations, and monitoring of compound consistency during production.
FAQ
What is a good LOI value for rubber?
A rubber with an LOI of 26% or higher is generally considered flame retardant, since it will not easily sustain burning in normal air containing approximately 21% oxygen. Values above 35% indicate highly flame-resistant materials suitable for the most demanding applications.
Why is 21% oxygen used as the reference point?
Ambient air contains approximately 20.95% oxygen by volume. A material whose LOI is below this level can sustain combustion in ordinary air, while a material whose LOI exceeds it will self-extinguish once an ignition source is removed.
Which standard should be selected for rubber LOI testing?
ASTM D2863 and ISO 4589-2 are the internationally recognized methods and are broadly applicable to elastomers, while GB/T 10707 is the dedicated Chinese national standard for rubber flammability by oxygen index. The choice depends on the market, the product specification, and the referenced end-product standard.
Does specimen thickness affect the LOI result?
Yes. LOI depends on geometry, and thicker or thinner specimens can yield different values for the same compound. Results should always be compared at identical specimen dimensions, and the tested geometry must be reported together with the LOI value.
Is LOI testing a pass/fail certification test?
LOI is fundamentally a comparative screening method. However, many material specifications and procurement documents impose minimum LOI requirements, making it a de facto acceptance criterion. It does not by itself demonstrate compliance with full-scale fire-performance requirements, which is why it is usually combined with tests such as UL 94 or IEC 60332 flame tests.
How can the LOI of rubber be improved?
The LOI of hydrocarbon rubbers can be raised substantially through the incorporation of mineral fillers such as ATH or MDH, halogen-antimony synergistic systems, phosphorus-based or intumescent flame retardants, nitrogen-containing additives, and by blending with intrinsically flame-resistant elastomers such as CR or silicone rubber, with the optimal approach balancing flame performance against mechanical and processing properties.