The limiting oxygen index (LOI) test is a fundamental laboratory method for quantifying the relative flammability of plastic materials by measuring the minimum oxygen concentration required to sustain candle-like combustion. This article provides a complete technical overview of the plastic oxygen index determination experiment, covering the test principle, applicable standards such as ISO 4589-2, ASTM D2863, GB/T 2406.2 and JIS K 7201-2, specimen requirements, operating procedures, and typical applications in flame-retardant development and quality control.
Technical Overview
The oxygen index (OI), 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 flaming combustion of a vertically mounted specimen under specified test conditions. Because ambient air contains approximately 20.9 percent oxygen, a plastic with an LOI above 21 will generally stop burning once the ignition source is removed, and such materials are commonly described as self-extinguishing in air. The higher the oxygen index, the greater the relative flame resistance of the polymer.
The oxygen index determination is a small-scale laboratory screening test. It provides a reproducible, quantitative ranking of the burning behavior of plastics and is widely used to compare base polymers, evaluate the effectiveness of flame retardants, verify compound quality, and demonstrate compliance with material specifications. It should be emphasized that the LOI is a relative measure obtained under strictly controlled conditions and must not be interpreted as a direct prediction of behavior in full-scale fires.
Plastic oxygen index determination experiments are performed according to internationally recognized standards, including:
- ISO 4589-1 and ISO 4589-2 (also adopted as EN ISO 4589-2) for the ambient-temperature test
- ISO 4589-3 for the elevated-temperature test
- ASTM D2863, Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics
- GB/T 2406.1 and GB/T 2406.2
- JIS K 7201-2
Testing Samples
Rigid Plastics (Molded and Sheet Materials)
Standard specimens are typically 80 to 150 mm long, 10 ± 0.5 mm wide, and 4 ± 0.25 mm thick, cut from compression-molded or injection-molded plaques, sheets, or finished parts. Specimens must be free of cracks, bubbles, and burrs, with smooth edges, and a reference mark is drawn 50 mm below the top edge to define the burn-distance criterion.
Flexible Films and Tapes
Thin flexible materials cannot support themselves vertically. Film specimens are therefore wound onto a rod or held in a supporting frame in accordance with the relevant annexes of ISO 4589-2 and ASTM D2863 so that a defined vertical geometry is maintained during ignition and burning.
Cellular (Foam) Materials
For rigid cellular plastics, the standard specimen thickness is 10 ± 0.5 mm. Low-density foams may require a support so that the specimen remains vertical without significant deformation throughout the test.
Specimen Quantity and Conditioning
At least 15 specimens are required per material or condition, and 20 or more are recommended because the bracketing procedure consumes additional specimens. Unless otherwise specified, specimens are conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity for at least 88 hours in accordance with ISO 291, and dimensions are verified at several points before testing.
Testing Items
Common test items requested for plastic oxygen index determination include:
- Ambient-temperature limiting oxygen index (LOI) per ISO 4589-2, ASTM D2863, GB/T 2406.2, or JIS K 7201-2
- Elevated-temperature oxygen index per ISO 4589-3
- Temperature index, defined as the temperature at which the oxygen index falls to the oxygen content of air
- Comparative oxygen index measurements before and after heat aging, UV exposure, or humidity conditioning
- Supplementary observations, including afterflame time, flaming drips, char formation, shrinkage, and smoke release
Testing Methods
Test Principle
A rectangular specimen is clamped vertically in the center of a heat-resistant glass chimney. A metered mixture of oxygen and nitrogen flows upward through the column at a controlled total rate corresponding to a linear gas velocity of 40 ± 10 mm/s. The top of the specimen is ignited with a small gas diffusion flame, and the oxygen concentration is progressively adjusted until the critical concentration is found at which the specimen just sustains combustion. The pass criterion is defined as burning for at least 180 seconds after removal of the ignition source or burning downward past the mark located 50 mm from the top of the specimen. The minimum oxygen concentration satisfying this criterion, established over replicate specimens, is reported as the LOI.
Apparatus and Test Conditions
- Vertical glass chimney (burning column) mounted on a gas-mixing chamber containing a diffusion screen of glass beads
- Specimen holder that positions the specimen centrally in the column
- High-purity oxygen and nitrogen supplies (typically at least 99.5 percent) with pressure regulators, needle valves, calibrated flow meters, and an oxygen analyzer for concentration verification
- Ignition source, typically a small propane diffusion burner
- Timer, marking gauge, measuring instruments, and a conditioning chamber
For elevated-temperature testing, a heated chimney assembly conforming to ISO 4589-3 is used, allowing the oxygen index to be determined at controlled temperatures above ambient.
Operating Procedure
- Verify gas purity, calibrate the flow system and oxygen analyzer, and confirm that the total flow gives the required linear velocity
- Select an initial oxygen concentration based on experience or a trial specimen
- Mount the specimen vertically and align its top edge at the height defined by the standard holder
- Purge the column with the gas mixture for at least 30 seconds before ignition
- Apply ignition using top-surface ignition (flame applied to the specimen top for a maximum of 5 seconds, repeated until burning persists for at least 5 seconds) or propagating ignition (flame applied across the top edge and down the upper face for approximately 5 seconds)
- Record the burning time and whether the flame front crosses the 50 mm mark, classifying each result as burning or not burning according to the criterion
- Adjust the oxygen concentration upward or downward using the bracketing intervals defined in the standard, and confirm the critical concentration with replicate specimens
- Complete the calculation, repeat confirmation runs with fresh specimens, and document all observations
Calculation and Expression of Results
The oxygen index is expressed as the volume fraction of oxygen in the test mixture, calculated as LOI = 100 × VO2 / (VO2 + VN2), and is normally reported to the nearest 0.1 percent. The test report should identify the standard used, specimen dimensions and orientation, the criterion applied, the calculated oxygen index, and any observed phenomena such as dripping, charring, or smoking. With properly calibrated equipment, within-laboratory repeatability is typically about 0.5 percentage points and between-laboratory reproducibility is generally within approximately 1 to 1.5 percentage points.
Applications
- Flame-retardant research and development, including screening of halogen-free phosphorus, nitrogen, intumescent, and mineral filler systems in polyolefins, polyamides, PC/ABS, epoxies, and polyurethanes
- Quality control of incoming resins, masterbatches, and flame-retardant compounds against minimum LOI specifications
- Wire and cable industry evaluation of insulation and sheathing compounds in combination with vertical flame tests
- Electrical and electronics applications, including enclosures, connectors, and printed circuit board laminates
- Railway and transportation materials, where EN ISO 4589-2 is referenced within EN 45545-2 requirement sets
- Building and construction screening of flame-retardant sheets, pipes, and foam insulation alongside regulatory reaction-to-fire tests
- Textile applications, where the oxygen index of fibers and fabrics is determined per GB/T 5454
- Academic research, teaching laboratories, and generation of comparative flammability data for material datasheets
FAQ
What LOI value indicates a flame-retardant plastic?
A material with an LOI above 21 will typically self-extinguish in ambient air. In practice, many industry specifications require an LOI of at least 26 or 28 for a plastic to be classified as flame retardant, and higher values indicate greater relative flame resistance.
What are typical oxygen index values for common plastics?
- POM approximately 15; PP and PE approximately 17 to 18; PMMA approximately 17 to 18; PS approximately 18; ABS approximately 18 to 19
- PA66 approximately 22 to 25; PC approximately 25 to 27; phenolic laminates approximately 30 or higher
- Rigid PVC approximately 45; PVDF approximately 44; PTFE greater than 95
How does the oxygen index test differ from UL 94 testing?
The LOI test determines a threshold oxygen concentration for sustained burning in a controlled atmosphere, while UL 94 tests such as V-0, V-2, and 5VA grade the afterflame and ignition behavior of specimens burned in air. The two methods are correlated to some degree but independent, and material specifications frequently require both.
Does specimen thickness affect the result?
Yes. The oxygen index depends on specimen geometry, and thicker specimens frequently show higher oxygen indices due to heat-sink effects. This is why the standards fix specimen dimensions and why results are only comparable between specimens of the same thickness and form.
Does temperature affect the oxygen index?
Yes. The oxygen index of most plastics decreases as temperature increases. ISO 4589-3 provides the elevated-temperature method, and the temperature at which the LOI drops to 21, known as the temperature index, is a useful measure of flame resistance under service conditions.
Can LOI results predict real fire behavior?
No. The oxygen index is a small-scale comparative test under controlled gas flow and geometry. Real fires involve different heat fluxes, ventilation, and sample configurations, so LOI data should be used together with methods such as UL 94, cone calorimetry, and applicable regulatory reaction-to-fire tests for a complete fire hazard assessment. For formal certification, testing should be carried out by accredited third-party laboratories.