The oxygen index test, also known as the Limiting Oxygen Index (LOI) test, is the primary laboratory method for evaluating the flame-retardant performance of rubber sheets used in industrial, mining, electrical, and construction applications. This article explains the test principle, applicable standards such as GB/T 10707, ISO 4589-2, and ASTM D2863, specimen preparation, the testing procedure, and how to interpret the results for rubber sheet materials.
Technical Overview
The oxygen index test measures the minimum concentration of oxygen, expressed as a volume percentage in an oxygen/nitrogen mixture, that will just support candle-like combustion of a vertically mounted rubber specimen under controlled laboratory conditions. This value is defined as the Limiting Oxygen Index (LOI) and is calculated as follows:
LOI (%) = 100 × V(O₂) / [V(O₂) + V(N₂)]
Since ambient air contains approximately 20.9% oxygen, a rubber sheet with an LOI above 21% cannot sustain continuous burning in normal air and is considered self-extinguishing. This makes the oxygen index test a simple, quantitative, and highly reproducible screening method for comparing the flame retardancy of rubber compounds. Natural rubber and most general-purpose synthetic rubbers have relatively low LOI values, while specialty and flame-retardant filled compounds achieve significantly higher values.
- LOI < 21%: readily flammable in air
- LOI 21–27%: self-extinguishing tendency in air
- LOI > 27%: generally classified as flame-retardant grade
Testing Samples
Standard Sheet Specimens
Specimens are die-cut or machined directly from the rubber sheet under evaluation. Typical dimensions are 80–150 mm in length, 10 ± 0.5 mm in width, and 3–4 mm in thickness, with smooth, defect-free edges and square-cut ends. All specimens must be conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity (in accordance with ISO 291 or GB/T 2941) for a minimum of 16 hours, and preferably 88 hours, before testing.
Typical Rubber Sheet Grades Tested
- Natural rubber (NR) and styrene-butadiene rubber (SBR) sheets: LOI typically 17–19%, highly combustible
- Nitrile rubber (NBR) oil-resistant sheets: LOI typically 18–21%
- EPDM weather-resistant sheets: LOI typically 21–25% depending on filler loading
- Polychloroprene (neoprene/CR) sheets: LOI typically 30–40%, inherently self-extinguishing due to chlorine content
- Silicone rubber sheets: LOI typically 25–35%, forming a protective ash layer
- Flame-retardant compounds with aluminum trihydrate (ATH), antimony trioxide, or halogenated additives: LOI commonly 28–40%
Sample Quantity and Preparation
A minimum of 15 specimens is recommended to complete the bracketing procedure and verification burns. Specimens must be free of blisters, cracks, impurities, and surface contamination, as these defects strongly influence ignition and burning behavior.
Testing Items
The rubber sheet oxygen index experiment typically includes the following test items:
- Determination of the room-temperature Limiting Oxygen Index (LOI) of the rubber sheet
- Compliance verification against product specifications or contractual requirements (for example, LOI ≥ 28% for flame-retardant grade sheet)
- Comparative evaluation of flame-retardant formulations, filler types, and loading levels
- LOI measurement after thermal aging or accelerated service conditioning, when required
- Verification of lot-to-lot consistency for incoming material quality control
Testing Methods
Apparatus
The test is performed on an oxygen index tester consisting of a heat-resistant glass chimney (typically 450 mm or taller with an internal diameter of 75–100 mm), a specimen holder that supports the sample vertically in the center of the chimney, a gas mixing and delivery system with precision flow control for oxygen and nitrogen, an oxygen analyzer with accuracy of 0.1% or better, and a small propane or butane flame igniter.
Test Conditions and Pass Criteria
The mixed gas is introduced into the chimney at a total flow rate producing a linear velocity of approximately 40 ± 2 mm/s. The ignition flame is applied to the top of the specimen for a maximum of 30 seconds, typically in short intervals. After removal of the ignition source, the specimen is judged as follows:
- Pass: burning time does not exceed the specified limit (typically 180 s) and the burn distance does not exceed the specified length (typically 50 mm from the top)
- Fail: the specimen continues to burn beyond either criterion
Test Procedure
- Condition and measure each specimen, then clamp it vertically in the holder with the top extending the specified distance above the chimney outlet
- Set the initial oxygen concentration at an estimated value near the expected LOI and purge the chimney with the mixed gas for at least 30 seconds
- Apply the ignition flame to the top of the specimen and observe the burning behavior after the igniter is removed
- Adjust the oxygen concentration upward or downward in small increments (typically 0.2–0.5%) and repeat on fresh specimens until the critical bracket is established between passing and failing concentrations
- Perform repeated verification burns at the critical concentration to confirm stability, then calculate the LOI value
- Report the LOI rounded to one decimal place, together with the test standard, specimen thickness, conditioning details, and any observed burning phenomena such as dripping, smoking, or char formation
Applicable Standards
- 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
- 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 adoption of the ISO 4589-2 method
Testing should be carried out by accredited third-party laboratories or in-house laboratories with calibrated oxygen index equipment and traceable gas flow control to ensure valid, comparable results.
Applications
Oxygen index testing of rubber sheets supports material selection, regulatory compliance, and quality control across many industries:
- Flame-retardant rubber sheet flooring, matting, and protective linings for public buildings, tunnels, and metro systems
- Underground coal mine conveyor components, sealing strips, and safety mats requiring verified flame-retardant performance
- Electrical insulating rubber mats and gaskets installed near energized equipment
- Marine, railway, and offshore engineering materials subject to flame-safety procurement requirements
- Research and development of flame-retardant rubber compounds, including optimization of ATH, magnesium hydroxide, and synergistic additive systems
- Incoming inspection and lot release of flame-retardant rubber sheet products by manufacturers and end users
FAQ
What is considered a good oxygen index for a rubber sheet? An LOI above 21% means the sheet will not sustain burning in normal air. For flame-retardant grades, procurement specifications commonly require an LOI of 28% or higher.
Why does natural rubber sheet show a low LOI? Natural rubber and most diene-based synthetics are hydrocarbon polymers with LOI values of approximately 17–19%, meaning they burn readily in air unless protected by flame-retardant additives.
Which factors affect the repeatability of the test? Specimen thickness, cure state, filler dispersion, surface cleanliness, conditioning history, gas flow calibration, and operator judgment of the burn endpoint all influence results, which is why standardized dimensions, controlled atmospheres, and repeated verification burns are essential.
How does oxygen index testing differ from other flammability tests? LOI is a small-scale, quantitative screening test that ranks relative flame retardancy. It does not replace large-scale fire tests, but it is fast, economical, and highly reproducible, making it ideal for compound development and quality control.
How many specimens are needed for one determination? Generally 15 to 20 specimens are sufficient to complete the concentration bracketing sequence and confirm the final LOI value with one decimal place.