The limiting oxygen index (LOI) test is one of the most widely used laboratory methods for evaluating the flammability of rubber materials, quantifying the minimum oxygen concentration required to sustain combustion. This article provides a comprehensive technical overview of LOI determination for rubber samples, covering test principles, applicable international standards, specimen requirements, test procedures, and typical industrial applications. Understanding this method is essential for flame-retardant compound development, quality control, and compliance verification with fire safety specifications.
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. Since ambient air contains approximately 20.9% oxygen, a rubber material with an LOI below 21% will generally continue to burn in air once ignited, while materials with higher LOI values tend to self-extinguish under normal atmospheric conditions.
For rubber materials, LOI testing serves as a sensitive and reproducible small-scale screening method for comparing the relative flammability of different formulations. Unfilled natural rubber typically exhibits an LOI of approximately 17–19%, indicating high flammability, whereas compounds incorporating flame-retardant fillers, halogenated systems, or intumescent additives can achieve LOI values above 30%. The test result is influenced by material composition, vulcanization state, specimen geometry, and test conditions, which is why standardized procedures and strict conditioning requirements are essential for obtaining comparable data.
LOI results are primarily intended for quality control, material comparison, and research purposes. They complement, but do not replace, larger-scale fire performance tests such as flame spread, heat release, or smoke density evaluations required for many end-use applications.
Testing Samples
Vulcanized Rubber Sheets
Standard test plaques vulcanized under controlled conditions represent the most common sample type. Sheets are cut into rectangular strips of specified dimensions, typically 80–150 mm in length and 10 mm in width, with a preferred thickness of 4 mm as specified in ISO 4589-2, or 70–150 mm long, 6.5 mm wide, and 3 mm thick according to ASTM D2863. Specimens must be free from surface defects, bubbles, and contamination.
Compounded and Raw Rubber
Unvulcanized compounds are usually press-cured or molded into standard plaques before testing. Since the degree of cure significantly affects burning behavior, vulcanization parameters must be documented and held constant when comparing formulations.
Cellular and Foam Rubber
Sponge and foam rubbers can be tested using the same general procedure, but their lower density and open-cell structure typically result in lower LOI values. Specimen dimensions and orientation relative to the rise direction should be reported because of their strong influence on results.
Finished Rubber Components
Products such as cable sheathing, conveyor belt cover rubber, gaskets, seals, and hose linings may be sectioned into test specimens, provided the material can be cut into the required strip geometry without delamination or structural damage.
Testing Items
The primary test item is the limiting oxygen index of the rubber material at ambient temperature (23°C), expressed as a percentage by volume. Depending on customer or specification requirements, the following related determinations may also be included:
- LOI at ambient temperature per ISO 4589-2, ASTM D2863, or GB/T 10707
- LOI at elevated temperatures per ISO 4589-3, where applicable
- Ignition observations, including afterflame time and afterglow time
- Recording of whether combustion reaches the extinction criteria or specimen support marks
- Burning phenomena such as dripping, smoking, or char formation
Testing Methods
Applicable Standards
LOI determination for rubber is most commonly performed in accordance with the following standards, and the specific method should be agreed upon before testing:
- ISO 4589-1 and ISO 4589-2 — Determination of burning behaviour by oxygen index (general guidance and ambient-temperature test)
- ASTM D2863 — Measuring the minimum oxygen concentration to support candle-like combustion of plastics (oxygen index method)
- GB/T 10707 — Rubber vulcanized or thermoplastic; determination of flammability by oxygen index
- GB/T 2406.2 — Plastics; determination of burning behaviour by oxygen index, Part 2
- JIS K 7201-2 — Japanese equivalent oxygen index method
Test Apparatus
The test is conducted in a dedicated oxygen index apparatus comprising a heat-resistant glass chimney of specified dimensions, a specimen holder that supports the strip vertically, a gas mixing and flow control system with calibrated flowmeters, a supply of high-purity oxygen and nitrogen, and a small ignition burner fed by hydrocarbon gas. Many modern instruments also integrate a paramagnetic or zirconia-based oxygen analyzer to verify the actual oxygen concentration delivered to the chimney.
Specimen Preparation and Conditioning
Specimens are cut cleanly from cured plaques or products, edges are smoothed to remove flash and burrs, and reference marks are applied to define the measurement length. Unless otherwise specified, specimens are conditioned at 23 ± 2°C and 50 ± 5% relative humidity for a minimum of 88 hours in accordance with ISO 291 or the equivalent conditioning clause of the selected test standard. A minimum of 15 specimens is generally recommended so that the bracketing procedure can be completed.
Test Procedure
A conditioned specimen is clamped vertically in the holder and inserted into the chimney. A controlled flow of oxygen and nitrogen is established at the total flow rate specified by the standard, and the gas mixture is allowed to purge the chimney. The top of the specimen is then ignited with the burner so that the upper surface burns as a candle flame. The operator observes whether ignition is followed by burning or glowing that reaches the extinction criteria within the specified time or length limits:
- Burning or glowing that persists for more than a defined period after removal of the ignition source
- Burning that extends beyond the upper reference mark or reaches the lower mark
Based on the outcome, the oxygen concentration is systematically adjusted upward or downward, and fresh specimens are tested until the critical oxygen concentration is bracketed between a concentration at which the specimen just passes the criteria and one at which it just fails. The procedure is repeated at closely spaced concentration intervals to confirm the result.
Calculation and Expression of Results
The limiting oxygen index is calculated from the volume flow rates of oxygen and nitrogen at the critical concentration, expressed as:
LOI = 100 × VO2 / (VO2 + VN2)
The result is reported as a percentage by volume, rounded to one decimal place, together with the test standard used, specimen dimensions and thickness, conditioning details, and any observed burning behavior such as dripping, smoking, or char formation. Because thickness strongly affects LOI, the tested dimension must always accompany the reported value.
Applications
LOI testing of rubber is applied across many industries in which elastomeric components are exposed to potential ignition sources:
- Wire and cable insulation and sheathing, where LOI screening supports compliance with flame retardancy requirements in building and railway applications
- Mining conveyor belts and transmission belts, for which elevated LOI values contribute to fire safety approvals
- Automotive and aerospace seals, hoses, and mounts requiring enhanced flame resistance
- Building construction gaskets, expansion joints, and waterproofing membranes
- Research and development of flame-retardant rubber formulations, including halogen-free and mineral-filled systems
- Incoming material quality control and batch-to-batch consistency verification in accredited third-party laboratories
Manufacturers frequently use LOI data in combination with other fire performance tests, such as vertical or horizontal burn tests and smoke density measurements, to build a complete flammability profile for regulatory submissions and product certification.
FAQ
What is considered a good LOI value for rubber?
As general guidance, rubbers with an LOI below 21% burn readily in air, values between 21% and 28% indicate slow-burning or self-extinguishing tendencies, and values above 28% are generally regarded as flame retardant. Acceptance thresholds, however, are always defined by the relevant product specification or end-user requirement.
Which standard should be selected for rubber LOI testing?
ISO 4589-2 and ASTM D2863 are the most widely referenced international methods, while GB/T 10707 is specific to vulcanized rubber and commonly requested in the Chinese market. The applicable standard is normally dictated by the customer specification, purchase agreement, or regulatory framework governing the end product.
How many specimens are required, and how long does testing take?
A typical determination consumes 15 to 20 specimens to complete the bracketing and confirmation sequence. Including conditioning, the test itself is usually completed within one working day once conditioned specimens are available.
Does a high LOI guarantee fire safety of the final product?
No. LOI is a small-scale screening test that measures the ease of ignition and sustained combustion under controlled laboratory conditions. Actual fire performance depends on geometry, mass, heat exposure, and the environment, so LOI results must be supplemented by application-specific fire tests defined in the relevant product standards.