Rubber Limiting Oxygen Index (LOI) Testing: Standard Methods, Procedures, and Applications

Limiting Oxygen Index (LOI) testing is one of the most widely used routine methods for evaluating the flame retardancy of rubber materials and products. This technical article explains the LOI test principle, applicable standards such as ASTM D2863, ISO 4589-2, and GB/T 2406.2, sample requirements, testing procedures, and typical industrial applications. Whether you manufacture cables, conveyor belts, seals, or flame-retardant rubber compounds, understanding LOI routine testing helps ensure compliance with international fire safety requirements.

Technical Overview

The Limiting Oxygen Index (LOI), also known as the Oxygen Index (OI) or Critical Oxygen Index (COI), 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. Because ambient air contains approximately 20.9% oxygen, materials with an LOI below 21% will generally continue to burn freely in air once ignited, while materials with an LOI above 21% tend to self-extinguish under normal atmospheric conditions. For this reason, LOI is widely regarded as a key indicator of the intrinsic flame-retardant performance of rubber compounds.

Routine LOI testing is typically performed in an oxygen index apparatus consisting of a heat-resistant glass chimney, a precision gas flow control system for oxygen and nitrogen, a sample holder, and a small propane or butane ignition burner. The test is relatively simple, reproducible, and quantitative, making it suitable for quality control, compound development, verification of flame-retardant formulations, and compliance screening. It should be noted that LOI is a small-scale laboratory screening test; for regulatory assessment of finished products, it is often combined with other fire performance tests such as vertical/horizontal burning tests, smoke density, and cone calorimetry.

LOI testing for rubber and plastic materials is governed by several internationally recognized standards, including ASTM D2863, ISO 4589-2, GB/T 2406.2, NES 714, and JIS K 7201-2. These standards are technically harmonized in most respects, differing mainly in specimen dimensions, conditioning requirements, and certain verification details. Accredited third-party laboratories routinely conduct LOI tests according to one or more of these methods depending on the end market and specification requirements.

Testing Samples

Vulcanized Rubber Compounds and Sheets

The most common test specimens are prepared from vulcanized (cured) rubber sheets with standard dimensions of 80–150 mm in length, 10 ± 0.5 mm in width, and 10 ± 0.5 mm in thickness. Where the actual product thickness is less than 10 mm, alternative thicknesses such as 6.5 mm, 3 mm, or thinner film specimens may be tested, but the reported LOI value must state the specimen thickness because thickness significantly influences the result. Specimens should be free of bubbles, contamination, surface defects, and visible porosity, as these irregularities distort burning behavior.

Finished Rubber Products

Cut specimens from finished items such as cable sheathing and insulation, conveyor belt covers, hose covers, sealing strips, gaskets, and protective rubber sheets may be tested provided they can be cut to the required dimensions. For products with complex geometry, curved or reinforced sections, the laboratory will assess specimen suitability and, where necessary, prepare molded plaques from the same compound as a substitute, clearly noting this in the test report.

Sample Quantity and Conditioning

At least 15 specimens are generally required per test to complete the concentration bracketing procedure, and laboratories commonly request 20–30 specimens to allow for repeat determinations. Prior to testing, specimens are conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours (or as specified by the applicable standard or product specification), since moisture content can measurably affect the ignition and burning behavior of rubber.

Testing Items

Routine LOI testing typically covers the following reported parameters:

• Limiting Oxygen Index (LOI value): the volume percentage of oxygen at the critical burning concentration, reported to one decimal place, together with the specimen dimensions and the applicable test standard.
• Burning behavior observations: whether the specimen self-extinguishes, burns upward with a stable flame, drips molten or flaming debris, exhibits afterflame and afterglow, or develops heavy smoke and soot.
• Verification of burning criteria: confirmation that the critical concentration corresponds to burning of 50 mm of specimen length or flaming for 180 seconds, whichever occurs first, as defined by the standard.
• Comparative or specification assessment: where required, judgment of compliance with a specified minimum LOI, for example LOI ≥ 27% or ≥ 30% for certain flame-retardant grades in cable, mining, rail, and marine specifications.

Testing Methods

Applicable Standards

• ASTM D2863: Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index).
• ISO 4589-2: Plastics — Determination of burning behaviour by oxygen index — Part 2: Ambient-temperature test.
• GB/T 2406.2: Chinese national standard, technically aligned with ISO 4589-2, widely specified for domestic Chinese market products.
• NES 714 and JIS K 7201-2: additional national and sector standards referenced in specific industrial and procurement specifications.

Test Procedure

The general test sequence, common to the above standards, is as follows:

• Condition the specimens in the standard atmosphere and verify apparatus calibration, including gas analyzer accuracy and total flow rate (typically 40 mm/s linear velocity through the chimney).
• Mount the specimen vertically in the holder so that at least 50 mm of specimen length is exposed above the top of the holder.
• Purge the chimney with the oxygen/nitrogen mixture at the selected initial concentration and allow the system to stabilize.
• Apply the ignition flame to the top of the specimen for up to 30 seconds, withdrawing and reapplying the burner as necessary, until the specimen is ignited.
• Observe whether burning reaches the 50 mm mark or persists for 180 seconds; record the outcome.
• Adjust the oxygen concentration upward or downward and repeat on fresh specimens, using the bracketing (interval-narrowing) procedure until the critical concentration interval is established.
• Calculate the LOI as LOI = 100 × O₂ / (O₂ + N₂) at the critical concentration, and perform the characteristic verification burns specified by the standard.

Factors Influencing Results

LOI values are sensitive to specimen thickness, curing state, filler loading, plasticizer and flame-retardant additive content, aging and post-curing history, moisture conditioning, and even test temperature. Laboratories therefore report LOI results together with specimen dimensions, conditioning details, and the exact standard edition used. For high-temperature service materials, ISO 4589-3 extends the method to evaluate temperature indices, which is beyond routine ambient-temperature testing.

Applications

LOI routine testing is applied across many rubber industry sectors, including:

• Wire and cable: evaluating flame-retardant insulation and sheathing compounds against flame retardancy requirements in building wiring, power cables, and data cables.
• Mining and conveyor systems: verifying flame-resistant conveyor belts and hoses used in underground coal mines, where strict minimum oxygen index values are commonly mandated.
• Rail transit and marine engineering: screening rubber seals, flooring materials, and vibration-damping components for compliance with fire safety specifications requiring low flammability materials.
• Automotive and aerospace: assessing hoses, grommets, insulation pads, and interior elastomeric parts against fire performance acceptance criteria.
• Research and formulation development: comparing flame-retardant packages such as halogenated, phosphorus-based, intumescent, and inorganic hydrated filler systems in rubber compounds.
• Quality control: routine incoming material inspection and batch release testing of flame-retardant masterbatches and vulcanized compounds.

FAQ

What is a good LOI value for rubber?

Common non-flame-retarded rubbers such as natural rubber, SBR, and EPDM typically show LOI values of approximately 17–21%, meaning they burn readily in air. Flame-retardant compounds, particularly those used in cables, mining products, and mass transit applications, are commonly specified with LOI values of 27% or higher; specialized formulations can exceed 35%.

How long does a routine LOI test take?

Excluding conditioning time, the actual testing of a complete specimen set typically requires one to three hours depending on how rapidly the critical concentration is bracketed. Including the standard 40-hour conditioning period, results are usually available within three to five working days from an accredited third-party laboratory.

Do ASTM D2863, ISO 4589-2, and GB/T 2406.2 give the same results?

The methods are closely aligned and generally produce comparable LOI values for identical materials. Minor differences in apparatus verification, specimen preparation, and rounding conventions can cause small variations, so the report should always identify the specific standard used and clients should confirm which standard their specification requires before testing.

Why does specimen thickness matter?

Thicker specimens conduct heat away from the combustion zone more effectively and generally yield slightly different LOI values than thin specimens, which tend to ignite and burn more readily. For this reason, LOI results must always be quoted together with the tested thickness, and comparisons between materials are only valid at the same thickness.

Can LOI testing alone prove fire safety compliance?

No. LOI is a small-scale screening test that measures ease of ignition and continued burning under controlled oxygen enrichment. Regulatory approval of finished rubber products usually requires additional large-scale or application-specific fire tests, such as vertical flame propagation, smoke density, toxicity, and heat release assessments. LOI remains, however, an efficient and reproducible first-line indicator widely used in both development and routine quality control.