Determination of the Oxygen Index of Rubber: Test Methods, Standards, and Procedures

The oxygen index (also known as the limiting oxygen index, LOI) is a key quantitative indicator of the flammability characteristics of rubber materials, defining the minimum oxygen concentration required to sustain candle-like combustion. This article provides a comprehensive overview of the rubber oxygen index determination method, covering applicable standards such as GB/T 10707, ISO 4589-2, and ASTM D2863, specimen preparation, apparatus, test procedures, and result evaluation. It serves as a practical technical reference for rubber product manufacturers, quality control engineers, flame-retardant compound developers, and laboratory professionals.

Technical Overview

The oxygen index (OI), commonly referred to as the 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 flaming combustion of a vertically mounted test specimen under controlled laboratory conditions. Because ambient air contains approximately 20.9% oxygen, the LOI value provides a direct and intuitive measure of the relative flammability of a rubber material: materials with an LOI below 21% will generally continue to burn in normal air, while materials with an LOI above approximately 26–27% are considered self-extinguishing or flame retardant in air.

For rubber materials, the oxygen index method is one of the most widely used small-scale flammability tests because it is quantitative, reproducible, and highly sensitive to the influence of flame retardants, fillers, and polymer composition. Unlike pass/fail tests, the LOI yields a numerical result that can be used to rank materials, compare formulations, verify flame-retardant treatment efficiency, and support regulatory compliance. It should be noted, however, that the oxygen index is a laboratory screening value and should not be used as the sole basis for assessing fire hazard in real-world end-use conditions.

The principal standards governing rubber oxygen index determination include:

• GB/T 10707 – Rubber vulcanized or thermoplastic – Determination of flammability by oxygen index (the Chinese national standard specifically written for rubber).
• ISO 4589-2 – Plastics – Determination of burning behaviour by oxygen index – Part 2: Ambient-temperature test, widely applied to elastomeric and polymeric materials.
• ASTM D2863 – Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index Method).
• GB/T 2406.2 – The Chinese equivalent of ISO 4589-2, applicable to plastics and rubber-like materials.
• ISO 4589-3 – Elevated-temperature oxygen index determination, used when the material must be evaluated at temperatures above ambient.

These standards describe essentially the same principle: a vertically held specimen is ignited at its top edge in a glass chimney through which a controlled oxygen/nitrogen mixture flows upward, and the oxygen concentration is adjusted stepwise until the critical concentration that just sustains burning for a specified time or over a specified damaged length is established.

Testing Samples

Sample Form and Dimensions

Oxygen index testing of rubber is normally performed on rectangular strip specimens cut from vulcanized sheets, molded plaques, or finished products. Typical dimensional requirements are:

• Length: 80 mm to 150 mm (commonly 100 mm), with the longer axis oriented in the direction of interest.
• Width: 10 mm (typically 10 ± 0.5 mm).
• Thickness: 4 mm (typically 4 ± 0.25 mm) for moldings; thinner specimens (e.g., 1 mm to 3 mm) may be tested for film, sheet, or tape products, provided the actual thickness is reported.
• Free length above the specimen holder: normally at least 50 mm to 60 mm so that the ignition zone and the measured burned length are not affected by the holder.

Where the material is anisotropic, specimens should be cut in both the machine direction and the transverse direction, since filler orientation, calendering, and fiber reinforcement can produce direction-dependent LOI values.

Specimen Quantity and Conditioning

A complete determination typically requires at least 15 specimens, because the bracketing procedure consumes several specimens to bracket the critical oxygen concentration and additional specimens to confirm the boundary. Specimens must be visually free from pores, cracks, flash, and surface contamination. Standard conditioning prior to testing is:

• At least 16 hours (GB/T 10707 requires a minimum conditioning period as specified, commonly 16 h to 88 h) at 23 ± 2 °C and 50 ± 5% relative humidity.
• Testing should be carried out at the same ambient conditions, or the conditioning state should be reported.
• Specimens that have undergone thermal aging, chemical exposure, or surface treatment should be identified, since these factors measurably alter the LOI of rubber.

Special Sample Cases

• Cellular rubber: cellular materials require modified criteria and are tested per the applicable clauses of the standards; results are not directly comparable with dense rubber.
• Soft or flexible rubber: may require a support frame or a specimen holder with a center support rod to keep the strip vertical without bending into the flame path.
• Tubing, cable insulation, and profiles: sections may be tested in their delivered geometry with the free length and orientation documented.

Testing Items

The core measured parameter is the oxygen index (LOI) of the rubber, expressed as a dimensionless volume percentage:

LOI = 100 × V(O2) / [V(O2) + V(N2)]

where V(O2) and V(N2) are the volume flow rates of oxygen and nitrogen in the critical gas mixture. Related items and derived evaluations commonly reported alongside the LOI include:

• Ignition behavior: ease of ignition, ignition time, and whether the specimen ignites at the top surface only or across the full width.
• Burning characteristics: afterflame time, afterglow, flaming drips, and whether combustion proceeds by surface flame or with charring.
• Damaged (burned) length over the specified observation period.
• Flame retardance classification: LOI ≥ 27 is generally regarded as indicating a flame-retardant (self-extinguishing in air) rubber, while LOI ≤ 21 indicates a readily combustible rubber.
• Comparative formulation studies: the effect of flame retardants (e.g., halogenated systems, aluminum hydroxide, magnesium hydroxide, phosphorus/nitrogen synergists, intumescent agents) on the LOI value.
• Verification of compliance with product specifications for items such as flame-retardant conveyor belts, cable sheathing, and sealing components.

Testing Methods

Apparatus

A standard oxygen index apparatus consists of the following principal components:

• A heat-resistant glass chimney (typically 450–500 mm high with an internal diameter of 75–100 mm) mounted on a vertical supporting frame, through which the gas mixture flows upward.
• A specimen holder that grips the lower end of the specimen and positions it vertically along the chimney axis, with the top of the specimen a defined distance (commonly at least 100 mm) below the chimney top.
• A gas supply and mixing system delivering oxygen and nitrogen of at least 99.5% purity, with pressure regulators and calibrated flowmeters or a mass-flow controller permitting total flow rates typically in the range of 10–12 L/min (or as specified).
• An ignition source: a small propane or butane burner with a 10–15 mm flame, or an electric igniter, applied downward onto the top of the specimen from above the chimney.
• A timer and scale for measuring afterflame time and burned length, and optionally an oxygen analyzer to verify the mixture concentration at the chimney base.

Test Procedure

The general procedure specified by GB/T 10707, ISO 4589-2, and ASTM D2863 is as follows:

• Condition the specimens, then measure and record their dimensions; mark reference lines (e.g., 50 mm from the top) to assess the burned length.
• Mount the specimen vertically in the holder and position it centrally in the chimney.
• Set an initial oxygen concentration based on prior knowledge of the material, purge the chimney with the mixture for at least 30 s at the test flow rate to establish a stable atmosphere.
• Apply the ignition flame to the top of the specimen for up to 30 s, removing it intermittently (e.g., 5 s on / 5 s off cycles) until the specimen is ignited; ignition is achieved when flaming combustion of the top surface is sustained for at least 15 s after removing the igniter.
• Record whether the specimen burns for 3 min or more, or over a damaged length of 50 mm or more. If either criterion is met, the specimen "passes" at that concentration (burning is too vigorous); if the flame extinguishes before both limits are reached, the specimen "fails" (does not support burning).
• Adjust the oxygen concentration using the incremental step-down / step-up (bracketing) method: reduce the oxygen concentration after a "pass" result and increase it after a "fail" result, progressively narrowing the interval until the difference between the lowest concentration at which burning is supported and the highest concentration at which it is not supported is within the specified increment (typically ≤ 0.5% or the d-value defined in the standards).
• Verify the boundary with repeated confirmatory specimens as required by the standard.
• Calculate the LOI from the critical volume flow rates of oxygen and nitrogen and report the result rounded to 0.1% or 0.5% as required, together with specimen dimensions, orientation, conditioning, and observed burning behavior.

Calculation and Expression of Results

The oxygen index is calculated as:

LOI (%) = 100 × Q(O2) / [Q(O2) + Q(N2)]

where Q(O2) is the volume flow of oxygen and Q(N2) is the volume flow of nitrogen in the critical mixture. When flowmeters operating under different pressures are used, corrections for temperature and pressure are applied as described in the standards, or the concentration is confirmed with a calibrated oxygen analyzer. The final report should state the standard used, specimen thickness, whether the result refers to the top-surface ignition procedure, and any deviations such as the use of a support for flexible rubber.

Factors Affecting Test Results

• Specimen thickness: thicker rubbers generally show higher LOI values; comparisons are only valid at equal thickness.
• Fillers and flame retardants: inorganic hydrated fillers, halogen–antimony synergists, and intumescent systems raise the LOI substantially, while plasticizing oils tend to lower it.
• Temperature: LOI decreases with increasing ambient temperature; ISO 4589-3 is used for elevated-temperature determinations.
• Humidity and conditioning: absorbed moisture and post-vulcanization changes can shift results.
• Gas purity, flow stability, and flowmeter calibration: directly influence the accuracy of the reported concentration.
• Operator technique in ignition: inconsistent ignition timing is a major source of scatter between laboratories.

Measurement Uncertainty and Precision

Repeatability within a single laboratory is typically on the order of ±0.5% oxygen or better when the apparatus is well controlled, while reproducibility between laboratories is commonly in the range of ±1% to ±1.5% oxygen. Regular verification of flowmeter calibration, gas purity, chimney condition, and igniter flame size is essential. Testing conducted by accredited third-party laboratories in accordance with GB/T 10707, ISO 4589-2, or ASTM D2863 provides the highest confidence in cross-laboratory comparability.

Applications

Oxygen index determination is applied throughout the rubber industry for material development, quality control, and compliance verification. Typical application areas include:

• Wire and cable: LOI testing of insulation and sheathing compounds for building wire, power cables, and rolling stock cables, often as an input to compliance with flame propagation standards such as IEC 60332 series requirements.
• Mining and bulk handling: flame-retardant conveyor belts and fire-resistant rubber components for underground coal mining, where minimum LOI values are frequently specified.
• Automotive and rail: evaluation of seals, gaskets, hoses, and interior elastomeric parts for reduced flammability in support of rail and vehicle fire-safety requirements.
• Construction and infrastructure: expansion joints, waterproofing membranes, sealing strips, and bridge bearing elastomers where fire performance must be documented.
• Flame-retardant compound development: ranking of halogen-free and halogenated flame-retardant formulations, optimization of filler loading, and verification of synergy between additives.
• Quality control: incoming inspection of raw compounds, batch release testing, and monitoring of the effect of aging, migration of plasticizers, or loss of flame retardant on long-term fire performance.
• Research and failure analysis: academic studies of combustion mechanisms and root-cause investigation of materials involved in fire incidents.

FAQ

What is a good oxygen index value for rubber?

A rubber with an LOI of 21% or less will burn readily in normal air. Values between 22% and 26% indicate improved but not fully self-extinguishing behavior, and values of 27% or above are generally classified as flame-retardant rubbers that will not continue to burn in air under the test conditions. Many high-specification applications, such as mining conveyor belts and cable sheathing, specify minimum LOI values in the range of 28% to 38% depending on the product standard.

Which standard should be used for rubber: GB/T 10707, ISO 4589-2, or ASTM D2863?

GB/T 10707 is written specifically for vulcanized and thermoplastic rubber and is the preferred standard in the Chinese market. ISO 4589-2 and ASTM D2863 are internationally recognized and technically similar in principle; the choice depends on the specification invoked by the customer, the end product standard, or the target market. All three methods produce an LOI value expressed in volume percent oxygen, but direct comparison should always be made within the same standard and at the same specimen thickness.

How many specimens are required for one determination?

At least 15 specimens are normally required: several are consumed in locating the approximate critical concentration and the remainder in bracketing and confirming the boundary. Additional specimens are needed if the material shows high variability or if testing is performed in two orientations.

Is a high LOI sufficient to prove fire safety of a rubber product?

No. The oxygen index is a small-scale laboratory screening test that ranks relative flammability under a specific gas flow condition. Product fire safety must be assessed with the relevant full-scale or application-specific tests, such as flame propagation, smoke density, and toxicity tests required by the applicable product standards. LOI results are best used for formulation comparison, quality control, and as supporting evidence within a broader fire-safety assessment.

Can the oxygen index be tested at elevated temperatures?

Yes. ISO 4589-3 defines the elevated-temperature oxygen index procedure, which is important for rubbers whose LOI drops significantly as temperature rises, such as in engine compartments or electrical equipment environments. The result is reported as the oxygen index at the specified test temperature.

How long does a typical oxygen index test take?

For a well-characterized material, a complete determination including conditioning, bracketing, and confirmation can typically be completed within one working day. Conditioning time, number of specimens, and operator experience are the main determinants of the total turnaround time.