Rubber Oxygen Index (LOI) Combustion Testing: Principle, Standards, and Procedure

The oxygen index combustion test, also known as the Limiting Oxygen Index (LOI) test, is a standardized laboratory method used to evaluate the flammability of rubber and elastomeric materials by determining the minimum oxygen concentration required to sustain candle-like combustion. Performed in accordance with recognized standards such as ISO 4589-2, ASTM D2863, and GB/T 10707, the test delivers a quantitative and highly repeatable measure of flame retardancy that supports material development, quality control, and regulatory compliance. This article explains the technical principle, sample requirements, test procedure, calculation method, and typical applications of oxygen index testing for rubber materials.

Technical Overview

The oxygen index, or Limiting Oxygen Index (LOI), 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 test specimen under specified conditions. Because normal ambient air contains approximately 20.9% oxygen, an LOI value above 21% indicates that the material is unlikely to continue burning in air once the ignition source is removed, while values below 21% indicate that the material will readily sustain combustion.

For rubber materials, the test is typically conducted in accordance with GB/T 10707, while the equivalent international frameworks are provided by ISO 4589-2 and ASTM D2863. Elevated-temperature oxygen index determinations are covered by ISO 4589-3. The LOI value is calculated from the volumetric flow rates of the two gases at the critical concentration, using the relationship: LOI (%) = 100 × V(O2) / [V(O2) + V(N2)].

As an indicative classification commonly used in industry:

• LOI below 21% — readily combustible; continues to burn in normal air
• LOI of 21% to 24% — slow-burning; tends to self-extinguish
• LOI of 25% to 31% — generally considered flame-retardant
• LOI above 32% — highly flame-retardant, difficult to ignite

These thresholds are indicative only; actual acceptance limits depend on the product specification, end-use regulation, or contractual agreement applicable to the material being tested.

Testing Samples

Vulcanized and Molded Rubber Compounds

Flat sheets or molded plaques of natural rubber, SBR, EPDM, NBR, CR, silicone rubber, or fluoroelastomer compounds are the most common test materials. Specimens are cut from compression-molded or vulcanized sheets so that surfaces are smooth, uniform, and free of defects such as blisters, pores, or contamination, since surface irregularities can significantly affect burning behavior.

Finished and Semi-Finished Products

Specimens may be taken from cable insulation and sheathing, sealing gaskets, O-rings, hose liners, conveyor belt covers, and automotive components. Where the product geometry does not permit standard-size specimens, representative strips are cut and their actual dimensions are recorded in the test report.

Foam and Cellular Rubber

Cellular materials can also be evaluated, but results are strongly influenced by cell structure and density. Test reports for foamed rubbers should always state the density and thickness of the specimens, as these parameters are not directly comparable with solid rubber results.

Specimen Dimensions and Conditioning

Typical specimen dimensions are 70 mm to 150 mm in length, approximately 6.5 mm in width, and approximately 3 mm in thickness, although exact dimensions and tolerances are specified by the governing standard. Prior to testing, specimens are conditioned in a standard laboratory atmosphere, commonly 23 ± 2 °C and 50 ± 5% relative humidity, until moisture equilibrium is reached. The number of specimens required is typically 10 to 15, because the bracketing procedure consumes several samples.

Testing Items

Core and optional test items offered for rubber oxygen index evaluation include:

• Determination of the Limiting Oxygen Index (LOI) at ambient temperature (23 °C)
• Determination of the oxygen index at elevated temperatures in accordance with ISO 4589-3
• Pass/fail verification against a specified LOI limit, such as LOI ≥ 27% or LOI ≥ 30%
• Comparative evaluation of flame-retardant formulations before and after aging
• Influence assessment of specimen thickness on the measured LOI value

The test report normally includes the LOI value, specimen dimensions and orientation, the combustion criterion applied, the ignition method, the gas flow conditions, and any deviations or observations such as dripping, glowing combustion, or afterflame behavior.

Testing Methods

Apparatus

The test is performed in a dedicated oxygen index apparatus consisting of a vertical heat-resistant glass chimney, typically 450 mm to 500 mm in height with an internal diameter of 75 mm to 100 mm, equipped with a specimen holder that supports the specimen vertically in the center of the column. The instrument also comprises regulated oxygen and nitrogen supplies, precision flow meters or an oxygen analyzer for measuring the gas mixture, a gas mixing chamber, and a small propane or butane ignition burner.

Test Procedure

The general procedure is as follows:

• Measure and condition the specimens, then mount one vertically in the holder with the marked upper end extending the distance specified by the standard above the top of the chimney.
• Set the initial oxygen concentration based on experience or preliminary trials, and establish the mixed gas flow through the column at a linear velocity of approximately 40 mm/s, allowing the system to purge for a defined stabilization period.
• Apply the ignition flame to the top of the specimen in short intervals — per ASTM D2863, the flame is applied for up to 5 seconds at a time and removed to check ignition, with a total application not exceeding 30 seconds.
• Observe the burning behavior after removal of the igniter. Combustion is regarded as sustained if the specimen burns for 3 minutes or if it burns for a length of 50 mm from the top.
• Using the bracketing method, adjust the oxygen concentration in suitable increments: if the specimen sustains burning, lower the oxygen concentration; if it extinguishes prematurely, raise it. Near the critical point, the concentration step is refined to approximately 0.2% oxygen.
• Repeat until the critical concentration is bracketed, then verify repeatability with additional specimens at the final concentration.

Result Determination and Calculation

The LOI is reported as the volume percentage of oxygen in the nitrogen/oxygen mixture at the critical concentration, calculated from the gas flow rates: LOI = 100 × V(O2) / [V(O2) + V(N2)]. Results are reported to one decimal place together with the specimen geometry, since LOI values are thickness-dependent and comparisons are only valid for specimens of similar dimensions tested under identical conditions.

Applications

Oxygen index testing of rubber is widely applied across industries in which flammability performance is a safety-critical or contractual requirement:

• Wire and cable — evaluating insulation and sheathing compounds for flame-retardant grades
• Automotive and rail vehicles — qualifying seals, gaskets, hoses, and interior elastomeric components
• Mining and bulk handling — flame-resistance assessment of conveyor belt cover compounds
• Aerospace and marine — verification of low-flammability elastomers for seals and insulation
• Building and construction — screening of sealing strips, expansion joints, and membrane materials
• Research and development — screening flame-retardant filler systems, such as aluminum hydroxide, magnesium hydroxide, and halogenated or intumescent packages, and optimizing loading levels
• Quality control — incoming material verification and batch-to-batch consistency monitoring in accredited third-party laboratories and in-house facilities

FAQ

What does a high oxygen index value mean?

A high LOI means the material requires a more oxygen-rich atmosphere than normal air to keep burning, which indicates better flame retardancy and a greater tendency to self-extinguish after the ignition source is removed.

Which standards apply to rubber oxygen index testing?

The principal standards are GB/T 10707 for rubber materials, ISO 4589-2 as the international plastics and elastomer method, ASTM D2863 as the North American equivalent, and ISO 4589-3 for elevated-temperature determinations. The applicable standard should be agreed between the parties before testing.

How many specimens are required?

Because the bracketing method progressively narrows the oxygen concentration, a complete determination typically consumes 10 to 15 specimens, and laboratories should be supplied with sufficient material accordingly.

How does LOI testing differ from the UL 94 test?

LOI testing quantifies the minimum oxygen concentration needed to sustain burning and yields a numerical value, whereas UL 94 is a pass/fail burning characteristics classification based on specimen ignition, afterflame time, and dripping behavior in still air. The two methods are complementary and are frequently specified together.

What factors can influence the result?

Specimen thickness, density, filler content and type, degree of cure, moisture conditioning, gas flow stability, and operator ignition technique all affect the measured value. Consistent specimen preparation and strict adherence to the specified conditioning and flow parameters are therefore essential for reproducible results.

Can LOI results from different laboratories be compared directly?

Yes, provided the specimens have identical dimensions, conditioning history, and are tested to the same standard by competent personnel. Interlaboratory comparisons are only meaningful when all test parameters are matched, as specified in the standards.