Limiting oxygen index (LOI) analysis is one of the most widely used small-scale flammability tests for evaluating the flame-retardant performance of rubber compounds and vulcanizates. This technical article explains the principles of LOI testing for rubber materials, covering international standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, along with sample requirements, testing procedures, result interpretation, and typical industrial applications.
Technical Overview
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. Because ambient air contains approximately 20.9% oxygen, materials with an LOI below 21% will continue to burn freely in air once ignited, while materials with higher LOI values are increasingly difficult to ignite and sustain combustion. In practical terms, rubber compounds with an LOI above 26–28% are generally regarded as flame retardant, and formulations exceeding 32% are often classified as self-extinguishing in many industrial specifications.
Rubber compounds exhibit a wide range of intrinsic flammability depending on polymer chemistry. Hydrocarbon-based rubbers such as natural rubber (NR), styrene-butadiene rubber (SBR), butadiene rubber (BR), and ethylene-propylene-diene rubber (EPDM) typically show LOI values of only 17–22%, reflecting their high hydrocarbon content and substantial heat of combustion. Halogen-containing elastomers such as chloroprene rubber (CR, LOI approximately 26–32%) and fluorocarbon rubber (FKM, LOI often above 40%) offer inherently better flame resistance, while silicone rubber (VMQ) forms a protective silica ash that can be further enhanced with flame-retardant fillers. LOI analysis therefore serves as a fundamental screening and quality-control tool for comparing base polymers, optimizing flame-retardant packages, and verifying batch-to-batch consistency of flame-retardant rubber compounds.
LOI testing is a relatively simple, reproducible, and material-specific bench-scale method. It does not replace large-scale fire performance tests, but it provides an essential ranking parameter for research and development, incoming material inspection, and compliance demonstration in flame-retardancy specifications.
Testing Samples
Vulcanized Rubber Sheets and Plaques
The most common sample form is a standard vulcanized rubber sheet, compression-molded or cured under production conditions. Specimens are cut to the standard dimensions of 80–150 mm in length and 10 ± 0.5 mm in width, with preferred thicknesses of 6.5 ± 0.5 mm or 3.0 ± 0.5 mm depending on the end-use requirement. Sheet specimens are suitable for NR, SBR, NBR, EPDM, CR, FKM, and silicone-based compounds.
Rubber Products and Finished Components
Specimens may also be prepared from finished goods such as cable insulation and sheathing, hose covers, conveyor belt covers, sealing profiles, gaskets, diaphragms, and protective linings. When the product geometry does not permit cutting a flat strip of the standard width, the dimensions are recorded and reported, since LOI values can vary with specimen thickness and geometry.
Cellular and Foamed Rubber
Sponge and expanded rubber materials can be tested using cut strips, provided that the cell structure is uniform and the specimen edges are cut cleanly without tearing or crushing. The apparent density of cellular specimens must be reported alongside the LOI value because of its strong influence on combustion behavior.
Rubber-Coated Fabrics and Thin Films
Coated fabrics and thin rubber layers may be tested using the methods for flexible thin materials, in which the specimen is supported or rolled on a mandrel to maintain the required geometry during vertical burning.
Sample Conditioning
- Specimens should be free of surface contamination, release agents, and sharp burrs that could influence ignition behavior.
- Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours before testing, in accordance with ISO 291 or ASTM D618.
- A minimum of 5 to 10 specimens per material is normally required to complete the oxygen-concentration bracketing procedure and establish the LOI with statistical confidence.
Testing Items
Accredited third-party laboratories typically perform the following evaluations within the scope of rubber LOI analysis:
- Determination of the limiting oxygen index (LOI) of rubber compounds and vulcanizates at ambient temperature (23 ± 2 °C).
- Verification of flame-retardant classification thresholds specified in customer or product standards.
- Comparative evaluation of flame-retardant formulations, filler loadings, and curing systems during research and development.
- Quality-control verification of incoming rubber compounds, masterbatches, and finished rubber components.
- Supporting data for material qualification in cable, automotive, rail, mining, and aerospace specifications that cite LOI requirements.
- Supplementary temperature-indexed LOI measurement at elevated temperatures where required by specifications.
Testing Methods
LOI testing of rubber compounds is conducted in accordance with internationally recognized standards, principally ISO 4589-2, ASTM D2863, and GB/T 2406.2, with JIS K 7201-2 providing an equivalent method in the Japanese framework. These standards are technically harmonized in most respects, although minor differences exist in ignition procedures, flow rates, and rounding conventions, so the applicable standard must be agreed before testing.
Test Apparatus
- A heat-resistant glass chimney of approximately 450–500 mm height and 75–100 mm internal diameter, mounted vertically on a base.
- A specimen holder that clamps the lower portion of the specimen and supports it vertically at the center of the chimney.
- Gas mixing and metering systems supplying oxygen and nitrogen at controlled flow rates, with a total flow typically set between 10 and 14 L/min.
- A small hydrogen or hydrocarbon gas flame igniter, adjustable in length, applied to the top of the specimen from above.
- A graduated scale or timing marks on the chimney to assess the burned length criterion (typically 50 mm).
Test Procedure
The specimen is clamped vertically in the holder so that the top of the specimen extends below the top of the chimney as specified. The chimney is purged with the oxygen/nitrogen mixture at the selected concentration and flow rate for at least 30 seconds to establish a stable atmosphere. The igniter flame is then applied to the top surface of the specimen for up to 30 seconds, with brief removal to check whether sustained ignition has occurred. The operator observes whether the specimen continues to burn for at least 3 minutes or consumes at least 50 mm of its length—the pass/fail criterion defined by the standards.
Using the bracketing (interval adjustment) method, the oxygen concentration is raised or lowered stepwise, testing fresh specimens at each level, until the critical oxygen concentration is bracketed within a defined interval—typically 0.2% oxygen or a statistically derived increment. The LOI is calculated from the volume flows of the two gases:
LOI = 100 × VO2 / (VO2 + VN2)
The result is reported as the volume percentage of oxygen, together with the test standard, specimen dimensions, conditioning history, and whether the specimen dripped, charred, or extinguished spontaneously. A verification check on the oxygen analyzer using ambient air (20.9%) and pure oxygen (100%) is normally performed before each test series.
Factors Affecting Results
- Specimen thickness and geometry—thicker specimens tend to show slightly higher LOI values for many rubbers.
- Filler content and type—aluminum trihydrate (ATH) and magnesium hydroxide (MDH) release water on decomposition and substantially raise LOI.
- Plasticizer and oil content—flammable extender oils reduce LOI and increase dripping tendency.
- Crosslink density and cure state, which influence char formation and decomposition behavior.
- Ambient temperature—LOI decreases at elevated temperatures, which is addressed by ISO 4589-3 for temperature-indexed measurements.
Applications
LOI analysis of rubber compounds supports material selection, product development, and regulatory compliance across numerous industries:
Wire and cable: LOI data for insulation and sheathing compounds support flame-retardant cable designs and complement vertical flame propagation tests such as those in IEC 60332 series standards.
Automotive and rail: fuel hose covers, sealing profiles, and interior elastomeric components are screened for flame retardancy in line with OEM and EN 45545 material requirements.
Mining: conveyor belt cover compounds are qualified against flame-retardancy specifications that cite LOI in combination with drum friction and burner tests.
Building and construction: expansion joints, waterproofing membranes, and door seals benefit from LOI verification as part of fire performance documentation.
Aerospace and defense: elastomeric seals, gaskets, and vibration mounts are qualified with LOI and related combustion parameters per program-specific material specifications.
Formulation development: LOI ranking guides the optimization of halogen-free flame-retardant systems based on ATH, MDH, phosphorus compounds, nitrogen-phosphorus synergists, intumescent agents, and nano-fillers.
FAQ
What LOI value is considered flame retardant for rubber?
Although classification thresholds depend on the applicable specification, rubbers with an LOI of 26–28% or above are commonly described as flame retardant, and those above approximately 32% as self-extinguishing. Many cable and belt specifications set minimum LOI requirements in the 28–32% range for critical compounds.
Which rubber types have the highest LOI values?
Fluorocarbon rubber (FKM) generally exhibits LOI values above 40%, followed by chloroprene rubber (CR) at approximately 26–32%. Halogen-free rubbers such as NR, SBR, EPDM, and NBR typically fall between 17% and 22% and require substantial flame-retardant loading, usually ATH or MDH, to reach flame-retardant levels.
How does LOI relate to the UL 94 test?
LOI is a quantitative measurement of the oxygen concentration required to sustain burning, whereas ANSI/UL 94 grades materials qualitatively (for example V-0, V-1, or V-2) based on afterflame time and dripping behavior in a vertical burn configuration. The two methods complement each other: LOI provides a sensitive ranking parameter for formulation work, while UL 94 addresses ignition and propagation behavior relevant to end-use scenarios.
How many specimens are required for an LOI test?
Typically 5 to 15 specimens are needed, depending on how rapidly the critical oxygen concentration is bracketed. Additional specimens are consumed when the initial estimate is far from the final value or when intermediate oxygen concentration checks are performed.
Can LOI results be compared directly between laboratories?
When testing is performed to the same standard, with identical specimen thickness and conditioning, interlaboratory reproducibility is generally within approximately 1–1.5 percentage points of oxygen. Reporting full test conditions is essential for meaningful comparison of results from different accredited laboratories.
