Rubber Material Oxygen Concentration Determination: Limiting Oxygen Index (LOI) Testing Explained

Oxygen concentration determination, performed as the Limiting Oxygen Index (LOI) test, is the core method for quantifying the flame retardancy of rubber and elastomeric materials. By establishing the minimum oxygen volume fraction in an oxygen–nitrogen atmosphere that just sustains candle-like combustion, the test delivers decisive fire-safety data for cable compounds, conveyor belts, seals, and engineered rubber components. This article explains the measurement principle, applicable international standards, specimen preparation, the step-by-step test procedure, and typical applications of oxygen concentration determination for rubber materials.

Technical Overview

Oxygen concentration determination for rubber materials is standardized as the Limiting Oxygen Index (LOI) test. The 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 flaming combustion of a vertically mounted specimen after ignition by an external flame source. Because ambient air contains approximately 20.9% oxygen, the LOI value directly indicates whether a material will continue burning in normal air: rubbers with an LOI of 21% or below propagate flame readily, while compounds with an LOI above approximately 27% are generally classified as flame retardant and tend to self-extinguish in air.

The oxygen index is calculated from the gas mixture at the critical condition using the following relationship: 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 limiting concentration.

Generic rubbers such as natural rubber (NR), styrene-butadiene rubber (SBR), and butadiene rubber (BR) are inherently combustible, with typical LOI values between 17% and 19%. Flame retardancy is improved through halogenated systems, phosphorus-based additives, intumescent formulations, or high loadings of inorganic fillers such as aluminum trihydrate (ATH) and magnesium hydroxide (MDH), which can raise the LOI of specialty rubber compounds into the 30%–45% range.

The test is governed by several internationally recognized standards:

• ISO 4589-2 – 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 10707 – Rubber, vulcanized or thermoplastic: Determination of flammability by oxygen index
• GB/T 2406.2 – National adoption of ISO 4589-2 for plastic and elastomeric materials
• JIS K 7201-2 – Japanese oxygen index test method
• ISO 4589-3 – Elevated-temperature oxygen index test

Testing Samples

Molded Vulcanized Rubber Sheets

Standard cured plates or slabs prepared from the compound under evaluation. Test strips are cut from these sheets, ensuring that cure state, filler dispersion, and thickness are representative of production material. This is the most common sample form for formulation screening and batch quality control.

Thermoplastic Elastomers

Injection-molded or compression-molded plaques of TPE, TPV, and TPU compounds, which are increasingly used in cable sheathing and molded components where flame retardancy is required.

Wire and Cable Compounds

Insulation and sheathing compounds, including halogen-free flame-retardant (HFFR) formulations based on EPDM, EVA, or CR rubber, which rely heavily on LOI data to demonstrate improved ignition resistance.

Cellular and Foam Rubber

Sponge and closed-cell rubber specimens can be tested, although their low density and open structure typically produce lower and more variable LOI values; special attention to specimen preparation and orientation is required.

Specimens from Finished Products

Strips cut from seals, gaskets, hoses, conveyor belt covers, and other manufactured articles, used to verify that the flame-retardant performance of the finished product matches that of the base compound.

Typical specimen requirements include a length of 80–150 mm and a width of 10 ± 0.5 mm, with a thickness of 4 mm or 10 mm depending on the material form and standard invoked. A gauge line is marked 50 mm from the top of the specimen to define the burning-length criterion. Specimens must be free of bubbles, cracks, and flash, and are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours, or as specified by the relevant material standard.

Testing Items

Oxygen concentration determination for rubber materials typically covers the following test items:

• Limiting Oxygen Index (LOI) at ambient temperature, reported as volume percent oxygen
• Elevated-temperature oxygen index and temperature index in accordance with ISO 4589-3
• Verification of the pass/fail criteria: burning time after removal of the ignition source and burned length relative to the 50 mm gauge mark
• Comparison of LOI values across specimen thicknesses, orientations, and curing states
• Comparative screening of flame-retardant formulations, filler loadings, and additive packages
• Assessment of LOI retention after thermal aging, UV exposure, or immersion conditioning

Testing Methods

The test is conducted in an oxygen index apparatus consisting of a heat-resistant glass chimney of specified dimensions, a vertical specimen holder, a gas mixing and flow control system, calibrated flow meters or an oxygen analyzer with an accuracy of ±0.1 vol%, and a small propane or hydrogen diffusion flame igniter. Oxygen and nitrogen of at least 99.5% purity are supplied to the base of the chimney, producing a uniform upward gas flow past the specimen.

The general test procedure is as follows:

• Condition, measure, and mark each specimen, then clamp it vertically in the holder with the upper end extending into the chimney.
• Set an initial oxygen concentration based on experience or prior data, maintain the total gas flow at the standard rate, and allow the atmosphere to stabilize.
• Apply the ignition flame to the top of the specimen for up to 5 seconds, repeating if necessary until the specimen ignites.
• Observe the specimen after removal of the igniter: the result is recorded as supporting combustion if flaming persists for 3 minutes or longer, or if the flame front reaches or passes the 50 mm gauge mark.
• Adjust the oxygen concentration upward or downward and repeat the test on fresh specimens, using the bracketing procedure to converge on the limiting concentration, with concentration steps of 0.2% near the critical value.
• Test a minimum of five specimens at each qualifying concentration, or apply the full NT-series statistical sequence of ISO 4589-2 (typically requiring 15 or more specimens), and calculate the LOI from the final bracketing series.

The test report should include the LOI value, the governing standard, specimen dimensions and orientation, the ignition criterion used, gas flow conditions, whether any unusual burning behavior such as glowing or dripping was observed, and the identity of the testing facility. Tests are commonly performed by accredited third-party laboratories as well as in-house fire-safety and R&D laboratories.

Applications

Oxygen concentration determination is widely applied across the rubber industry for material development, compliance verification, and fire-risk engineering:

• Development and optimization of flame-retardant and halogen-free rubber formulations for wires, cables, and connectors
• Quality control and batch release of flame-retardant compounds used in cable insulation and sheathing
• Railway rolling stock components, where oxygen index data support compliance with EN 45545-2 fire-safety requirements
• Mining conveyor belts and flame-retardant rubber goods for underground installations subject to strict ignition-resistance regulations
• Automotive and aerospace elastomeric components, including seals, grommets, and interior parts
• Building and construction products such as expansion joints, weather seals, and flooring underlays
• Academic and industrial research into flame-retardant mechanisms, filler synergy, and char formation in elastomer systems

FAQ

What oxygen index value qualifies a rubber as flame retardant?

There is no single universal threshold, but by common industry convention rubbers with an LOI of 21% or below burn readily in air, values between 21% and 27% indicate slowly burning or self-extinguishing tendency, and an LOI above 27% is generally regarded as flame retardant. Many high-specification cable compounds achieve LOI values above 32%.

How does the LOI test differ from other flammability tests?

The LOI test measures the minimum oxygen concentration needed to sustain burning in a controlled, gently flowing atmosphere, making it a sensitive, repeatable bench-scale ranking tool. It differs from flame-spread, ignition, or smoke tests such as UL 94, IEC 60332, or EN 50399, which evaluate different aspects of fire behavior and should be used complementarily.

Which factors influence the measured oxygen index?

Specimen thickness, density, filler content and type, cure state, aging history, test temperature, and even gas flow uniformity can shift LOI results. For this reason, comparisons are only valid when specimens are prepared and tested under identical conditions and the dimensions are reported with the result.

Can the test be performed at elevated temperatures?

Yes. ISO 4589-3 extends the method to elevated temperatures to determine a temperature index, which is important for rubbers operating in hot environments such as engine compartments, power cables, and process plant equipment.

How many specimens are required for a valid LOI determination?

A complete determination following the ISO 4589-2 NT-series procedure typically consumes 15 or more specimens, while simplified screening protocols may use fewer. Sufficient spare material should always be submitted to the testing laboratory to allow repetition and confirmation runs.