Rubber Oxygen Index Testing: Determination Methods, Standards, and Data Interpretation

The oxygen index (limiting oxygen index, LOI) test is one of the most widely accepted laboratory methods for quantifying the flammability and flame-retardant performance of rubber and elastomeric materials. This article explains the principle of rubber oxygen index determination, the applicable international and national standards, sample preparation requirements, test procedures, data interpretation, and typical industrial applications, providing engineers and quality professionals with a practical reference for obtaining accurate and reproducible combustion data.

Technical Overview

The oxygen index (OI), commonly known as the limiting oxygen index (LOI), is defined as the minimum concentration of oxygen in a flowing mixture of oxygen and nitrogen, expressed as a percentage by volume, that will just support candle-like flaming combustion of a vertically mounted test specimen under specified test conditions. Because ambient air contains approximately 20.9 percent oxygen, rubber materials with an oxygen index well above this level tend to extinguish after removal of the ignition source, indicating a higher degree of intrinsic flame retardancy.

For rubber compounds, oxygen index determination is a rapid, repeatable, small-scale laboratory screening test. It is widely used in formulation development, incoming material verification, and batch quality control. The method does not by itself describe the behavior of a product in a full-scale fire, so results are frequently combined with other fire performance tests such as UL 94 horizontal and vertical burning tests, ISO 5660 cone calorimetry, or smoke density measurements when a comprehensive fire hazard assessment is required.

The principal governing standards include ISO 4589-1 (general guidance), ISO 4589-2 (ambient-temperature test), ISO 4589-3 (elevated-temperature test), ASTM D2863, GB/T 2406.2, and GB/T 10707, which is dedicated to the determination of the burning behavior of vulcanized or thermoplastic rubber by the oxygen index method. These tests are routinely performed by accredited third-party laboratories for certification and compliance purposes.

Testing Samples

Any self-supporting solid rubber or thermoplastic elastomer that can be prepared as a rigid strip specimen is suitable for oxygen index testing. Typical sample types submitted to laboratories include the following.

Vulcanized or Molded Rubber Sheets

Standard laboratory-cured or production-cured sheets, usually 2 mm to 10 mm thick, from which strip specimens are cut. Sheet testing is the preferred approach because it eliminates the influence of part geometry and allows direct comparison between formulations.

Finished Rubber Components

Seals, gaskets, O-rings cut open, hose lining, conveyor belt cover compounds, footwear components, and cable insulation or sheathing materials can be tested, provided specimens of the required dimensions can be cut from the part without damaging the surface.

Development Compounds and Formulations

Experimental batches prepared to evaluate flame-retardant fillers such as aluminum trihydroxide (ATH), magnesium hydroxide (MDH), halogen–antimony synergistic systems, intumescent agents, or phosphorus-based additives, and to optimize their loading levels.

Cellular and Special Materials

Closed-cell sponge and foam rubbers may be tested using modified procedures and careful specimen handling. The method is not suitable for materials that shrink away from the ignition flame, melt and drip excessively, or cannot support their own weight in the vertical position.

Specimen geometry is defined by the governing standard. Under ISO 4589-2, typical specimens are strips 80 mm to 150 mm in length and 10 mm in width, with thicknesses of 4 mm or 10 mm; GB/T 10707 specifies corresponding dimensions for rubber. Specimens must be smooth, free of defects, and conditioned in a standard laboratory atmosphere, typically 23 °C ± 2 °C and 50 % ± 5 % relative humidity, before testing.

Testing Items

Common oxygen index test items requested for rubber materials include:

• Ambient-temperature oxygen index (OI/LOI) determination at 23 °C in accordance with ISO 4589-2, ASTM D2863, GB/T 2406.2, or GB/T 10707.
• Elevated-temperature oxygen index testing and temperature-index determination in accordance with ISO 4589-3.
• Flame retardancy classification and pass/fail screening against customer or product specification limits.
• Oxygen index measurement before and after heat aging, oil or fuel immersion, ozone exposure, or UV weathering to evaluate the retention of flame-retardant performance.
• Comparative evaluation of flame-retardant filler types and loading levels during compound development.
• Routine batch release and incoming material quality control testing for flame-retardant rubber compounds.

Testing Methods

Test Apparatus

A dedicated oxygen index tester is used. The main components of the system are:

• A heat-resistant glass combustion chimney, typically 450 mm to 500 mm high, mounted on a base with a gas mixing chamber and distributor.
• A vertical specimen holder that clamps the specimen in the center of the chimney.
• Precise gas flow control devices, such as rotameters or mass flow controllers, for high-purity oxygen and nitrogen, with a combined flow producing a linear gas velocity through the chimney of approximately 40 mm/s.
• An oxygen analyzer for verifying the oxygen concentration of the delivered mixture.
• A small propane or hydrogen igniter with an adjustable flame, typically 10 mm to 20 mm in length.

Specimen Preparation and Conditioning

Specimens are cut, marked with a reference line 50 mm below the top edge where required by the standard, measured for dimensions, and conditioned in the specified standard atmosphere. A minimum of 10 to 15 specimens is generally required for a single determination because the test uses an iterative approach.

Test Procedure

The determination is performed using the bracketing method with the following general sequence:

• Mount the specimen vertically in the holder and center it within the chimney.
• Purge the chimney with the oxygen/nitrogen mixture at the selected concentration for approximately 30 seconds to ensure a stable atmosphere.
• Apply the ignition flame to the top of the specimen, typically for intervals of up to 5 seconds, removing and reapplying the flame if ignition does not occur immediately.
• Observe the burning behavior after ignition and record whether the specimen burns for more than the specified observation time or burns downward past the reference mark.
• If the burning criterion is met, reduce the oxygen concentration; if the specimen extinguishes, increase the oxygen concentration.
• Repeat the adjustments using progressively smaller increments until the difference in oxygen concentration between a burning result and a non-burning result is within the small step specified by the standard.
• Confirm the final value with additional specimens as required, then report the resulting oxygen concentration as the oxygen index.

Calculation and Reporting

The oxygen index is calculated as the volume fraction of oxygen in the test gas mixture, that is, the oxygen flow divided by the sum of the oxygen and nitrogen flows multiplied by 100, and is verified by the oxygen analyzer reading. The test report states the standard used, specimen dimensions and orientation, conditioning details, the oxygen index value normally reported to the nearest 0.1 %, and any observations such as afterflame time, dripping, or charring behavior.

Typical Oxygen Index Values for Common Rubbers

Indicative oxygen index ranges for unfilled or conventionally filled gum elastomers are approximately as follows, and values rise substantially when flame-retardant fillers are incorporated:

• Natural rubber (NR): approximately 17 to 18, highly flammable in air.
• Styrene-butadiene rubber (SBR): approximately 18 to 19.
• Nitrile butadiene rubber (NBR): approximately 19 to 21.
• Ethylene propylene diene rubber (EPDM): approximately 21 to 24.
• Chloroprene rubber (CR, neoprene): approximately 26 to 35.
• Silicone rubber (VMQ): approximately 25 to 30, higher for ceramifying grades.
• Fluoroelastomer (FKM): typically above 55.
• Flame-retardant compounded rubbers with ATH, MDH, or halogen-antimony systems: commonly 28 to 45.

Factors Affecting Accuracy and Repeatability

• Specimen geometry, thickness, and density, since thicker specimens generally burn more slowly and can yield slightly different indices.
• Gas purity, flow meter calibration, and the accuracy of the oxygen analyzer.
• Ignition technique, including flame application time and the skill of the operator in judging the burning criterion.
• Material state, including the degree of cure, plasticizer content, moisture uptake, and prior aging.
• Test temperature, as the oxygen index of most rubbers decreases noticeably at elevated temperatures.

Applications

Oxygen index data for rubber supports material selection, product qualification, and regulatory compliance across many industries:

• Wire and cable: evaluation of insulation and sheathing compounds for building wire, power cable, and data cable applications.
• Automotive and electric vehicles: screening of seals, grommets, battery module cushions, and thermal barrier components for flammability performance.
• Rail transit and aerospace: qualification support for elastomeric sealing and vibration isolation parts used in rolling stock and aircraft interiors.
• Mining and bulk handling: flammability assessment of conveyor belt cover and skirting compounds.
• Construction: performance verification of glazing gaskets, expansion joint seals, and waterproofing membranes.
• Personal protective equipment: development of flame-resistant gloves, boots, and padding materials.
• Research and quality control: formulation optimization, supplier comparison, failure analysis, and ongoing batch consistency monitoring.

Many specifications reference the oxygen index alongside complementary fire tests. For example, a compound may need to achieve a specified minimum oxygen index while also passing a UL 94 V-0 rating or meeting the heat release requirements of cone calorimetry under the applicable EN, GB, or ISO product standards.

FAQ

What oxygen index value is considered flame retardant for rubber?

A widely used rule of thumb classifies materials with an oxygen index below 21 as flammable in normal air, 21 to 27 as slowly burning, and above 27 as self-extinguishing. In practice, many industry specifications for flame-retardant rubber require an oxygen index of at least 28 or 30, and the exact limit depends on the application and the applicable product standard.

How many specimens are needed for one determination?

Because the bracketing procedure requires successive trials at different oxygen concentrations, a typical determination consumes 10 to 15 specimens. Borderline or heterogeneous materials may require additional specimens to establish a stable value.

Does a high oxygen index guarantee fire safety of the finished product?

No. The oxygen index is a small-scale screening test on standard strip specimens. Real fire performance also depends on part thickness, geometry, installation conditions, heat flux, and ventilation. Full-scale or application-specific fire tests should be used for final fire risk assessment.

What is the difference between the oxygen index test and the UL 94 test?

The oxygen index test quantifies the minimum oxygen concentration needed to sustain burning in a controlled gas mixture, producing a numerical value. UL 94 tests rate the burning behavior of a specimen ignited in normal air, producing classifications such as V-0, V-1, V-2, or HB. The two methods are complementary and are frequently specified together.

Can rubber be tested at elevated temperatures?

Yes. ISO 4589-3 extends the oxygen index method to elevated temperatures, allowing the determination of a temperature index that describes how the flame resistance of a rubber degrades as the ambient temperature rises, which is valuable for engine compartment and industrial applications.

What is included in a typical test report?

A report from an accredited third-party laboratory normally identifies the test standard, specimen description and dimensions, conditioning history, the determined oxygen index value, ignition details, and observations such as dripping, afterflame, or char formation, together with a statement of measurement uncertainty where applicable.