Standardized Oxygen Index Testing of Rubber: LOI Principles, Methods, and Applications

The oxygen index test (Limiting Oxygen Index, LOI) is one of the most widely used small-scale methods for evaluating the flame retardance of rubber and elastomeric compounds. Standardized under ISO 4589-2, IEC 61144, ASTM D2863, and GB/T 2406.2, it quantifies the minimum oxygen concentration required to sustain candle-like combustion of a vertically mounted specimen. This article explains the technical principles, specimen requirements, test procedures, applicable standards, and typical industrial applications of standardized oxygen index testing for rubber materials.

Technical Overview

The Limiting Oxygen Index (LOI) is defined as the minimum concentration of oxygen, expressed as a volume percentage in an oxygen/nitrogen mixture, that will just support flaming combustion of a vertically mounted specimen under specified laboratory conditions. Because ambient air contains approximately 20.95% oxygen by volume, a material with an LOI of 21 or higher will not continue to burn in a normal air atmosphere under the geometry of the test, while materials with lower values will propagate flame once ignited.

Most unfilled elastomers, such as natural rubber (NR), styrene-butadiene rubber (SBR), nitrile rubber (NBR), and ethylene-propylene rubber (EPDM), are inherently flammable, with LOI values typically between 17% and 21%. Flame-retardant formulations, including chloroprene rubber (CR), chlorosulfonated polyethylene (CSM), halogen-free compounds filled with aluminum trihydrate (ATH) or magnesium hydroxide (MDH), and certain silicone rubbers, can achieve LOI values of 28% to 45% or higher. The oxygen index test therefore provides a sensitive, repeatable, and quantitative ranking tool for comparing flame-retardant efficiency between compound variants.

It should be emphasized that the oxygen index is a small-scale laboratory screening and quality-control measurement. It is used for material comparison and specification compliance, but it does not by itself constitute a full assessment of fire hazard in end-use conditions, which normally requires additional large-scale or application-specific fire tests.

Testing Samples

Molded Vulcanized Rubber Sheets

The most common sample form is a strip cut from compression-molded or vulcanized rubber plaques. Standard specimen dimensions per ISO 4589-2 are 80–150 mm in length, 10 mm in width, and 4 mm or 10 mm in thickness, with smooth, defect-free edges. Edges should be cut cleanly, since irregular surfaces can influence ignition and flame spread behavior.

Cable Insulation and Sheath Compounds

Strips taken from wire and cable insulation or sheathing compounds are frequently tested for flame-retardance qualification. Specimens may be prepared from molded plaques of the cable compound or, where agreed, cut directly from the cable component and tested as flat strips of standard dimensions.

Flexible Sheets and Thin Rubber Materials

Thin or flexible rubber sheets that distort when mounted vertically are supported on a specimen frame or holder as specified in the standard, ensuring that the specimen remains upright and the test geometry is reproducible.

Special Forms

Cellular rubber, rubber-coated fabrics, and rubber linings may also be evaluated, but the test report must clearly state the specimen form, thickness, and any support used, because these factors directly affect the measured oxygen index.

Specimen Quantity and Conditioning

A minimum of 15 specimens is typically required for a single LOI determination, since the bracketing procedure involves a series of pass/fail trials. Specimens should be conditioned in a standard laboratory atmosphere, typically 23 ± 2 °C and 50 ± 5% relative humidity, to moisture equilibrium (commonly a minimum of 40 hours) unless otherwise specified by the product standard or client requirement.

Testing Items

Standardized oxygen index testing of rubber typically includes the following determination items:

• Limiting Oxygen Index at ambient temperature (23 ± 2 °C), the primary test item for flame-retardance grading.
• Oxygen index at elevated temperatures (temperature-index) per ISO 4589-3 or equivalent, to characterize how flame retardance degrades as temperature rises.
• Oxygen index after thermal or air-oven aging, to verify that flame-retardant performance is retained over the service life of the compound.
• Oxygen index after immersion or special conditioning (for example, water or oil exposure), often required for cable and automotive compounds.
• Comparison testing across compound variants, filler loadings, or suppliers for formulation development and incoming material control.
• Recording of auxiliary observations such as afterflame and afterglow behavior, dripping of flaming particles, sooting, and char formation.

Testing Methods

Applicable Standards

The oxygen index test for rubber is carried out under internationally and nationally recognized standards, including:

• ISO 4589-2 — Plastics: Determination of burning behaviour by oxygen index, Part 2: Ambient-temperature test; widely applied to rubber compounds.
• IEC 61144 — Determination of the oxygen index of rubbers at 23 °C, specific to vulcanized and thermoplastic rubbers.
• 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 equivalent to ISO 4589-2.
• JIS K 7201-2 — Japanese industrial standard for oxygen index determination, commonly referenced in Asian supply chains.

Test Apparatus

The apparatus consists of a vertical heat-resistant glass column with a minimum height of approximately 450 mm and an internal diameter of 75–100 mm, a specimen holder that clamps the lower part of the specimen so that it hangs vertically, a metered oxygen/nitrogen gas supply with precision flow control, and a small pilot-flame igniter. Oxygen and nitrogen of at least 98% purity are used, and the mixed gas flow through the column is regulated to a linear velocity of 40 ± 10 mm/s. Periodic verification of the apparatus using a certified reference material, such as polymethyl methacrylate with a known oxygen index, is good laboratory practice.

Test Procedure

The general test sequence is as follows:

• Condition the specimens and allow them to reach thermal equilibrium in the test laboratory.
• Mount the specimen vertically in the holder and position the assembly in the center of the glass column.
• Purge the column with the oxygen/nitrogen mixture at the selected starting concentration for at least 30 seconds before ignition.
• Apply the pilot flame to the top of the specimen, following the specified top-ignition or propagation-ignition procedure, with flame application controlled in short intervals and not exceeding the maximum contact time stated in the standard.
• Record whether the specimen burns for more than 180 seconds or the flame front consumes more than 50 mm after removal of the ignition source; this defines the pass/fail criterion for that trial.
• Adjust the oxygen concentration upward or downward using the bracketing method, typically in steps of 0.2% oxygen, and repeat trials on fresh specimens until the limiting concentration is bracketed.
• Calculate and report the LOI value, rounded to 0.1% oxygen, together with specimen type, dimensions, orientation, ignition procedure, and observed burning behavior.

Interpretation of Results

Although acceptance limits are always defined by the relevant product specification, the following interpretations are commonly applied in the rubber industry:

• LOI below 21%: the material continues to burn in normal air and is considered flammable.
• LOI between 21% and 27%: the material tends to self-extinguish in air but offers limited flame-retardant margin.
• LOI of 28% and above: the material is generally classified as flame retardant.
• LOI of 32% and above: the material is regarded as highly flame retardant, typical of premium cable sheathing and mining compounds.

Within a single laboratory, repeatability of approximately ±0.5% absolute oxygen is normally achievable with well-prepared specimens; interlaboratory reproducibility is somewhat wider, which is why round-robin verification and reference-material checks are recommended for accredited third-party laboratories.

Applications

Standardized oxygen index testing supports a broad range of rubber industry activities:

• Flame-retardant compound development, including halogenated systems (CR, CSM) and halogen-free systems (EPDM/EPR and silicone filled with ATH or MDH).
• Wire and cable manufacturing, where compound oxygen index specifications support compliance with vertical flame propagation tests on finished cables under the IEC 60332 series and related national cable standards.
• Mining and tunnel engineering, including fire-resistant conveyor belt compounds, cable compounds for underground use, and elastomeric seals with demanding flame-safety requirements.
• Rail and mass-transit vehicles, where elastomeric components such as seals, hoses, and buffering elements are screened for flame performance during material qualification.
• Automotive engineering, covering fuel and coolant hoses, sealing profiles, underhood components, and interior elastomeric parts.
• Quality control of incoming raw materials and outgoing compounds, providing a fast, quantitative check on batch-to-batch consistency of flame-retardant formulations.
• Regulatory and certification support, where LOI data generated by accredited third-party laboratories form part of technical files and customer specifications.

FAQ

What oxygen index value is required for rubber to be considered flame retardant?

While acceptance criteria are set by the applicable product specification, industry practice generally treats an LOI of 28% or higher as flame retardant. Values above 32% are considered highly flame retardant and are frequently specified for mining cables and specialty safety compounds.

Which standards govern rubber oxygen index testing?

The principal standards are ISO 4589-2 and GB/T 2406.2 for ambient-temperature testing, IEC 61144 which is specific to rubbers, and ASTM D2863 in North American practice. ISO 4589-3 covers elevated-temperature (temperature-index) determinations.

How does the oxygen index test differ from UL 94 burning tests?

The oxygen index test quantifies the minimum oxygen concentration needed to sustain combustion in a controlled atmosphere, giving a continuous numerical result. UL 94 tests rate burning behavior in normal air, including afterflame time, dripping, and ignition of cotton, and assign ratings such as V-0 or HB. The two methods are complementary and are often both specified for elastomeric materials.

How many specimens are needed for one determination?

At least 15 specimens are typically consumed by the bracketing procedure for a single LOI value. Additional specimens should be prepared if the material shows borderline or erratic burning behavior.

Does a high LOI guarantee real-world fire safety?

No. The oxygen index is a small-scale screening and quality-control method. It ranks materials and verifies formulation consistency, but full fire-safety assessment requires application-specific tests such as finished-cable flame tests, conveyor belt drum friction and flame tests, or reaction-to-fire testing on the end product.

Can oxygen index testing be performed after aging?

Yes. Measuring the oxygen index after thermal aging, immersion, or environmental conditioning is a common requirement for cable, automotive, and infrastructure compounds, demonstrating that flame retardance is retained throughout the design life of the rubber component.