Rubber oxygen index testing, also known as limiting oxygen index (LOI) testing, determines the minimum concentration of oxygen in an oxygen–nitrogen atmosphere that will just sustain candle-like combustion of a rubber specimen. As a core small-scale indicator of flame retardancy, the oxygen index value is widely specified for rubber compounds used in cables, seals, hoses, conveyor belts, and elastomeric components, and is measured according to recognized methods such as ISO 4589-2, ASTM D2863, and GB/T 10707. This article explains the test principle, sample requirements, test procedure, calculation, typical LOI values for common rubbers, and the main industrial applications of rubber oxygen index testing.
Technical Overview
The limiting oxygen index (LOI, often simply called the oxygen index or OI) 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 combustion of a vertically mounted specimen after ignition from its top edge. Below this concentration, the atmosphere is too oxygen-poor for the specimen to keep burning and the flame self-extinguishes; above it, burning is sustained.
In the test, a conditioned rubber specimen is clamped vertically in a heat-resistant glass chimney while a precisely controlled oxygen–nitrogen mixture flows upward around it. A small igniting flame is applied to the top of the specimen, and the burning behavior is observed. The oxygen concentration is raised or lowered step by step until the critical concentration that just supports combustion is bracketed, and the LOI is calculated from the gas flows.
Because ambient air contains approximately 21% oxygen by volume, the LOI value provides an immediate practical benchmark: a rubber with an LOI above 21 will not easily sustain burning in normal air, while a rubber with an LOI below 21 will continue to burn once ignited. Although the oxygen index method was originally developed for plastics, it is fully applicable to self-supporting rubber and elastomer specimens and is standardized for rubber materials in standards such as GB/T 10707. It should be noted that the oxygen index is a small-scale screening and comparison test; it does not by itself constitute a complete fire hazard assessment of an end product.
Testing Samples
Vulcanized Rubber Sheets and Plates
Cured (vulcanized) rubber sheets are the most common sample form. Standard specimens are typically 80–150 mm long and 10 mm wide, with a thickness of 3–4 mm, depending on the governing standard; ASTM D2863 specifies a width of 6.5 mm and a thickness of 3 mm. Specimens must be self-supporting, free of defects, cut cleanly, and representative of the compound in its final cured state.
Thermoplastic Elastomers and Rubber-Plastic Blends
Thermoplastic elastomers (TPE, TPV, TPU) and blends such as NBR/PVC are molded into plaques or strips of the required geometry and then cut into test specimens. The molding conditions must be documented, since processing history can influence combustion behavior.
Flame-Retardant Compounds and Masterbatches
During formulation development, candidate compounds containing flame retardants such as aluminum trihydrate (ATH), magnesium hydroxide, halogen–antimony systems, or phosphorus-based additives are vulcanized into test slabs specifically for oxygen index screening, allowing rapid comparison of flame-retardant efficiency.
Finished Rubber Products
Specimens can in many cases be cut directly from finished articles such as cable sheaths, hose walls, conveyor belt covers, gaskets, and diaphragms, provided the product geometry allows a self-supporting specimen of the required dimensions to be obtained without excessive trimming.
All samples should be conditioned to equilibrium at 23 ± 2 °C and 50 ± 5% relative humidity before testing, following the conditioning requirements of the applicable standard. A minimum of about 15 specimens is normally required for a single LOI determination, because the bracketing procedure consumes several specimens.
Testing Items
Typical test items offered for rubber materials by accredited third-party laboratories and in-house fire laboratories include:
- Ambient-temperature limiting oxygen index (LOI) of vulcanized or thermoplastic rubber, tested to ISO 4589-2, ASTM D2863, GB/T 10707, GB/T 2406.2, or JIS K 7201-2.
- Elevated-temperature oxygen index and derived temperature index, tested to ISO 4589-3, which evaluates how flame retardancy degrades as temperature rises.
- Comparative LOI screening of flame-retardant formulations, filler loadings, and curing systems during compound development.
- Quality-control release testing and incoming-material verification against customer or industry specifications.
- After-aging LOI evaluation, in which specimens are heat-aged or UV-aged first and then retested to confirm long-term retention of flame retardancy.
The test report normally states the LOI value in volume percent oxygen (to one decimal place), the standard used, specimen dimensions and orientation, conditioning details, the pass/fail criterion applied, and any observations such as afterflame time, afterglow, dripping, or sooting.
Testing Methods
Test Apparatus
A standard oxygen index apparatus consists of a vertical heat-resistant glass chimney (typically 70–100 mm in internal diameter and at least 450 mm high), a specimen holder that supports the specimen upright on its lower end, a gas supply and mixing system with calibrated flow control for oxygen and nitrogen, a small premixed propane or butane igniting burner, a timer, and a barometer. The total gas flow is regulated to give a nominal linear velocity of about 40 mm/s through the chimney, corresponding to roughly 10 L/min for a standard column. An oxygen analyzer may be used to verify the mixed-gas concentration.
Test Procedure
The general procedure, common to ISO 4589-2, ASTM D2863, and GB/T 10707, is as follows:
- Condition the specimens, measure their dimensions, and mark a reference line 50 mm below the top edge where required by the standard.
- Set an estimated initial oxygen concentration based on experience or a preliminary trial, and establish the specified total gas flow through the chimney.
- Purge the chimney with the gas mixture for at least 30 seconds to stabilize the atmosphere.
- Mount the specimen vertically and apply the igniting flame to its top edge in the prescribed on–off cycle, then withdraw the burner.
- Observe and record whether the specimen sustains combustion beyond the criterion — typically burning for more than 180 seconds or the flame front crossing the 50 mm mark constitutes "burning".
- If the specimen extinguishes before the criterion, increase the oxygen concentration; if it burns beyond the criterion, decrease it. Use progressively smaller increments (for example 5%, 2%, 1%, and finally 0.2%) as the critical concentration is approached.
- Repeat until the pass/fail boundary is bracketed by a sufficient number of determinations as required by the standard.
Calculation of the Oxygen Index
The oxygen index is calculated from the volume fraction of oxygen in the mixture: LOI = 100 × V(O2) / [V(O2) + V(N2)]. The final reported value is derived from the series of bracketing determinations using the statistical correction algorithm defined in the standard (a final-concentration term adjusted by a factor k multiplied by the step size d). Where required, the result is corrected for barometric pressure to a reference pressure of 101.3 kPa, and the LOI is reported to one decimal place as a percentage.
Pass Criteria and Interpretation
A specimen is considered to have "burned" if, after removal of the ignition source, it continues flaming beyond the observation period (commonly 180 seconds) or if the flame front reaches the mark 50 mm below the ignited edge. As a rule of thumb, rubbers with LOI below 21 are readily combustible, values of 21–24 indicate slow-burning or self-extinguishing tendency, values of 24–28 are generally regarded as flame-retardant, and values above 28 indicate highly flame-retardant behavior. Many industrial specifications for flame-retardant rubber set minimum LOI requirements in the 27–35 range.
Factors Influencing Results
LOI results are sensitive to specimen thickness (thicker specimens often show higher LOI), filler type and loading, plasticizer and oil content, degree of cure, thermal or oxidative aging history, test temperature, gas purity, flow calibration, and operator technique. Interlaboratory reproducibility is typically on the order of ±1% oxygen, so small differences between materials should be interpreted with caution and confirmed statistically.
Applications
Rubber oxygen index testing is used throughout the rubber and elastomer value chain, from compound development to certification of finished products:
- Wire and cable: screening and qualification of insulation and sheathing compounds, often combined with UL 94 and vertical flame propagation tests.
- Railway: EN 45545-2 material requirement sets reference oxygen index testing to EN ISO 4589-2 for selected rubber components.
- Automotive and electric vehicles: flame-retardant seals, gaskets, hoses, and battery-pack elastomeric parts evaluated against OEM specifications.
- Mining: flame-resistant conveyor belt covers and fire-retardant rubber products, where minimum LOI values are frequently specified.
- Construction and infrastructure: expansion joints, waterproofing membranes, door and window seals, and fire-stop elastomers.
- Consumer and industrial goods: appliance gaskets, footwear components, and protective equipment.
- Research and failure analysis: benchmarking halogen-free flame-retardant systems, studying filler synergy, and investigating field failures involving combustion.
Because the LOI is a screening method, it is commonly paired with complementary fire-performance tests such as UL 94, cone calorimetry to ISO 5660, and smoke density measurements to build a complete flammability profile of the material or product.
FAQ
What is the oxygen index value of a rubber?
It is the minimum volume percentage of oxygen in an oxygen–nitrogen mixture that will just keep a vertically burning rubber specimen alight in the candle-like burning mode. The higher the value, the more oxygen-rich the atmosphere must be for combustion to continue, and therefore the better the flame retardancy of the rubber.
What LOI value is considered flame retardant for rubber?
Since normal air contains about 21% oxygen, any rubber with an LOI above 21 will tend to self-extinguish in air. In practice, LOI values of 27 and above are widely accepted as indicating flame-retardant behavior, and many specifications for cable sheathing, conveyor belting, and rail components require LOI values between 28 and 35.
What are typical LOI values for common rubber types?
Indicative ranges for unfilled or standard compounds are as follows; actual values depend strongly on formulation, filler loading, and cure state:
- Natural rubber (NR): approximately 17–19
- Styrene-butadiene rubber (SBR): approximately 17–19
- Nitrile rubber (NBR): approximately 18–21
- Butyl rubber (IIR): approximately 18–20
- Ethylene-propylene rubber (EPDM): approximately 21–25
- Polychloroprene rubber (CR): approximately 26–32
- Silicone rubber (VMQ): approximately 24–30
- Fluoroelastomer (FKM): typically above 40
Do ISO 4589-2, ASTM D2863, and GB/T 10707 give the same results?
The three methods share the same principle, apparatus concept, and pass/fail logic, and results are generally comparable. However, they differ in specimen dimensions, ignition details, and calculation procedures, so results obtained under different standards should not be treated as strictly interchangeable, and comparisons should always reference the standard actually used.
How much sample is needed for a test?
At least 15 specimens are normally required for one LOI determination. As a practical guide, a cured sheet of approximately 150 mm × 100 mm × 3–4 mm provides sufficient material to cut the required specimens with margin for retests.
Does a high oxygen index guarantee fire safety of the finished product?
No. The oxygen index is a small-scale laboratory screening test on a material specimen. Real fire performance also depends on product geometry, thickness, installation, ignition scenario, heat release, smoke production, and toxicity. LOI data should therefore be combined with application-specific fire tests and regulatory assessments when demonstrating overall fire safety.