Rubber Oxygen Index Testing Under Controlled Temperature and Humidity Conditions

Rubber oxygen index (limiting oxygen index, LOI) testing under controlled temperature and humidity conditions determines the minimum oxygen concentration required to sustain combustion of rubber materials, providing a quantitative measure of flame retardancy. Because rubber compounds are sensitive to ambient moisture and temperature, standardized conditioning and testing environments are essential for accurate, reproducible results. This article outlines the technical principles, applicable standards, sample requirements, test methods, and typical applications of rubber oxygen index testing with temperature and humidity conditioning.

Technical Overview

The oxygen index, also known 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 candle-like combustion of a vertically mounted test specimen for a specified time or over a specified length. Since normal air contains approximately 20.9 percent oxygen, rubber compounds with an LOI above roughly 28 percent are generally classified as flame-retardant and tend to self-extinguish in ambient air, while compounds with lower values will continue to burn once ignited.

Temperature and humidity play a decisive role in the measured oxygen index of rubber. Rubber is a viscoelastic and mildly hygroscopic material: absorbed moisture absorbs heat during evaporation and dilutes combustible pyrolysis gases, while elevated temperatures accelerate thermal decomposition and reduce the oxygen concentration needed to sustain burning. For this reason, oxygen index testing must be preceded by conditioning of specimens in a standard atmosphere, and, when required, additional evaluations must be performed after defined humidity exposure or at elevated test temperatures to characterize real-world performance.

The principal standards governing this test field include:

• GB/T 10707 — Rubber, vulcanized or thermoplastic — Determination of flammability by oxygen index
• ISO 4589-2 — Plastics — Determination of burning behaviour by oxygen index — Part 2: Ambient-temperature test
• ISO 4589-3 — Determination of burning behaviour by oxygen index — Part 3: Elevated-temperature test
• ASTM D2863 — Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index)
• ISO 291 and GB/T 2941 (equivalent to ISO 23529) — Standard atmospheres for conditioning and testing of plastics and rubber

Testing Samples

Vulcanized Rubber Compounds

Compression-molded or extruded sheets of vulcanized rubbers such as natural rubber (NR), styrene-butadiene rubber (SBR), nitrile butadiene rubber (NBR), ethylene-propylene-diene rubber (EPDM), chloroprene rubber (CR), silicone rubber (VMQ), and fluoroelastomers (FKM). Compounds are fully cured according to their designated vulcanization schedules before specimens are cut.

Thermoplastic Elastomers

Injection-molded or extruded plaques of thermoplastic elastomers, including TPS, TPO, TPV, and TPU grades, which are frequently used in cable jacketing, grips, and sealing profiles where flame-retardant performance is specified.

Finished and Semi-Finished Products

Specimens may be die-cut directly from cable sheathing and insulation, conveyor belt cover rubber, gaskets, seals, hose covers, and molded components. When testing finished goods, the specimen geometry and any reinforcing elements or anisotropic effects must be documented, as fabric inserts and orientation can influence burning behaviour.

Specimen Preparation and Dimensions

Specimens are typically cut to a length of 80 to 150 mm and a width of 6.5 to 10 mm, with a thickness of approximately 3 to 4 mm, in accordance with the specific requirements of the governing standard. Edges must be smooth and free of cracks, bubbles, flash, and other defects. A sufficient quantity of specimens, commonly 15 or more per test condition, is prepared so that the critical oxygen concentration can be bracketed through incremental concentration steps.

Testing Items

Typical test items evaluated within a rubber oxygen index temperature and humidity testing program include:

• Limiting oxygen index (LOI) after standard conditioning at 23 °C ± 2 °C and 50 percent ± 10 percent relative humidity, per GB/T 10707, ISO 4589-2, or ASTM D2863
• Oxygen index at elevated test temperatures, per ISO 4589-3, to assess retention of flame retardancy under thermal load
• Oxygen index after humidity conditioning, for example extended exposure at elevated temperature and high relative humidity, to quantify the effect of moisture absorption on flammability
• Oxygen index comparison before and after thermal-oxidative aging or humid aging of the compound
• Burning behaviour observations, including afterflame time, dripping, smoke release, and whether combustion propagates beyond the defined pass/fail criteria

Testing Methods

Specimen Conditioning

Before testing, specimens are conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50 percent relative humidity in accordance with ISO 291 or GB/T 2941. Vulcanized rubber specimens are typically conditioned for a minimum of 16 hours after vulcanization or after any mechanical preparation, and longer periods may be required by product specifications or for thicker cross-sections. For humidity-influence studies, a second specimen set is exposed to a defined humid environment, such as extended holding at elevated temperature and high relative humidity, until moisture equilibrium is approached; the oxygen index of this set is then compared with that of the standard-conditioned set.

Test Apparatus

The test is performed with an oxygen index apparatus comprising a heat-resistant glass chimney of standardized height and internal diameter, a specimen holder that supports the specimen vertically in the center of the chimney, precision flowmeters or mass-flow controllers for oxygen and nitrogen, a small hydrogen or hydrocarbon flame igniter, and a calibrated oxygen concentration measurement or control system. Conditioning and humidity studies additionally require a temperature-and-humidity controlled chamber, and elevated-temperature testing per ISO 4589-3 requires an electrically heated chimney assembly with precise temperature regulation.

Test Procedure

• Verify gas purity, flowmeter calibration, and chimney cleanliness; mount the conditioned specimen vertically in the holder.
• Set an initial oxygen concentration and purge the chimney with the gas mixture for a defined period to establish a stable atmosphere.
• Apply the top-ignition flame to the specimen for the specified ignition time, then withdraw the igniter and start timing.
• Record whether the specimen burns for the defined duration or consumes the defined burning length; adjust the oxygen concentration upward or downward in small increments.
• Repeat the sequence with fresh specimens until the critical oxygen concentration is bracketed between a burning and a non-burning result in the required pattern.
• Calculate the oxygen index as 100 × O₂ / (O₂ + N₂) based on the final gas mixture at the bracketed concentration.

Expression of Results

The oxygen index is reported to one decimal place as a volume percentage, together with the governing standard, specimen dimensions and orientation, conditioning atmosphere and duration, ignition method, and any observations on burning behaviour such as dripping, smoke, or char formation. For temperature and humidity programs, results for each conditioning or test-temperature condition are reported side by side so that the sensitivity of the compound to environmental exposure is clearly documented.

Influence of Temperature and Humidity

In general, absorbed moisture increases the measured oxygen index because water evaporation cools the pyrolysis zone and dilutes flammable volatiles, whereas drying or elevated test temperatures tend to lower the oxygen index by accelerating decomposition. Quantifying these effects through controlled preconditioning is essential for compounds containing hygroscopic mineral fillers, phosphorus-based or nitrogen-based flame retardants, or hydrophilic additives, and for products intended for service in hot, humid, or cyclic environments.

Applications

Rubber oxygen index testing with temperature and humidity conditioning is applied across many industries and product categories:

• Wire and cable — qualification and quality control of flame-retardant insulation and sheathing compounds, where procurement specifications frequently require LOI values of 28 percent or higher
• Mining and bulk handling — flame-retardancy evaluation of conveyor belt cover rubber and cable compounds used in underground operations
• Automotive and rail transit — flammability screening of seals, grommets, hoses, and interior elastomeric components
• Building and construction — flame-retardant rubber membranes, expansion joints, and sealing profiles
• Research and development — comparison of halogenated and halogen-free flame-retardant formulations, verification of filler and synergist effects, and validation of compound changes
• Regulatory compliance — demonstration of conformance to national and international flammability requirements using test reports issued by accredited third-party laboratories

FAQ

What oxygen index value qualifies rubber as flame-retardant?

Because air contains approximately 21 percent oxygen, any compound with an LOI above that level will theoretically not sustain combustion in still air. In practice, most industrial specifications classify rubber as flame-retardant when the LOI is 28 percent or higher, and highly flame-retardant compounds such as chloroprene and special silicone grades can exceed 30 to 35 percent.

Why are temperature and humidity conditioning required before testing?

Rubber properties vary measurably with ambient temperature and absorbed moisture, and both factors influence ignition and burning behaviour. Standardized conditioning ensures that all specimens enter the test in a defined, reproducible environmental state, which is a precondition for comparable results between laboratories and between production batches.

Which standards govern the test?

GB/T 10707 is the principal standard for rubber oxygen index testing and is technically aligned with ISO 4589-2 and ASTM D2863. Conditioning atmospheres are specified in ISO 291 and GB/T 2941, and elevated-temperature oxygen index testing follows ISO 4589-3. The applicable standard should be agreed between the client and the testing laboratory based on the product specification and target market.

How does humidity exposure change the test result?

Moisture absorbed by the rubber typically raises the measured oxygen index, since evaporating water removes heat from the flame zone and dilutes combustible gases. Comparing the LOI of standard-conditioned specimens with that of humidity-exposed specimens quantifies this effect and is particularly important for compounds with hygroscopic fillers or flame retardants.

Can finished rubber products be tested directly?

Yes. Specimens can usually be die-cut from finished articles provided the resulting dimensions meet the requirements of the governing standard. The laboratory report should state the sample origin, orientation, and any reinforcements present, because these factors can influence the measured oxygen index.

How long does a typical test take?

The combustion measurement itself is short, generally completed within one to two hours per specimen set. However, mandatory conditioning, optional humidity or thermal exposure, and report preparation must be included in the schedule, so the overall turnaround is typically several working days depending on the selected test program.