Plastic limiting oxygen index (LOI) testing measures the minimum oxygen concentration required to sustain candle-like combustion of a plastic specimen, providing a quantitative indicator of material flammability and flame-retardant performance. This article explains the LOI test principle, applicable standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, sample requirements, testing procedures, and typical industrial applications of plastic combustion oxygen index testing.
Technical Overview
The limiting oxygen index (LOI), also known as the oxygen index (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 flaming combustion of a vertically mounted plastic specimen. Because ambient air contains approximately 21% oxygen, the LOI value serves as a convenient benchmark for judging whether a material will continue to burn in normal atmospheric conditions:
- LOI below 21%: the material burns readily in air and is considered flammable.
- LOI between 21% and 27%: the material burns in air but with moderate difficulty and is generally classified as slow-burning or self-extinguishing to a limited degree.
- LOI of 27% and above: the material is difficult to ignite in air and is commonly regarded as flame retardant.
The oxygen index is calculated as follows: LOI (%) = 100 × V(O₂) / [V(O₂) + V(N₂)], where V(O₂) and V(N₂) are the volumetric flow rates of oxygen and nitrogen at the critical concentration. The test is a small-scale laboratory screening method widely specified in material specifications, research and development, and quality control. The principal standards governing the method include ISO 4589-2 and EN ISO 4589-2 (Plastics — Determination of burning behaviour by oxygen index), ASTM D2863 (Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics), GB/T 2406.2 in China, and JIS K 7201-2 in Japan. Results from different standards are broadly comparable, but specimen dimensions and pass/fail criteria differ slightly, so the applied standard must always be stated in the test report.
Testing Samples
Oxygen index testing can be performed on most rigid, flexible, and cellular plastic materials. Specimens must be representative of the material as supplied and should be free from surface contamination, edge defects, and moisture. Typical conditioning is 23 °C and 50% relative humidity for at least 88 hours unless otherwise specified.
Rigid and Molded Thermoplastics
Injection-molded or machined bars of materials such as PE, PP, PS, ABS, PVC, PA (nylon), PC, POM, PMMA, PBT, and modified or flame-retardant grades. Under ISO 4589-2, the standard specimen is typically 80–150 mm long, 10 mm wide, and 4 mm or 10 mm thick; under ASTM D2863, specimens are commonly 70–150 mm long, 6.5 mm wide, and 3 mm thick.
Thermosetting Plastics and Laminates
Phenolic, epoxy, unsaturated polyester, melamine, and polyurethane materials, as well as laminated sheets and copper-clad laminates used in electrical and electronic applications, can be tested as cut bars in accordance with the relevant standard.
Plastic Films and Thin Sheets
Thin films tend to deform or flutter in the gas flow, so they are tested using a special film support or frame attachment that holds the specimen upright in a rolled or folded configuration while preserving the vertical burning geometry.
Cellular Plastics and Foams
Rigid and flexible polyurethane foams, polystyrene foams, and polyolefin foams are tested using oversized specimens (for example, up to 150 mm × 50 mm × 10 mm) with appropriate support so that the burning behavior of the cellular structure is accurately assessed.
Fiber-Reinforced Plastics and Composites
Glass-fiber or carbon-fiber reinforced laminates and molded parts used in transportation, electrical equipment, and construction can be evaluated, with the specimen orientation (length direction relative to reinforcement) recorded in the report because it may influence the result.
A minimum of 15 specimens per material is generally recommended, since the bracketing procedure requires repeated pass/fail trials at different oxygen concentrations; in practice, 15 to 30 specimens should be prepared to ensure a valid result.
Testing Items
The primary deliverable of the test is the limiting oxygen index value of the material, determined at ambient temperature (23 °C). Typical reporting items include:
- LOI value in percent oxygen, rounded to 0.1% or 0.2% depending on the standard.
- The burning criterion outcome: whether flaming combustion reached a mark 50 mm below the top of the specimen, or whether burning time exceeded 180 seconds after ignition.
- Specimen description: dimensions, thickness, density, color, and any conditioning performed.
- Ignition method (top surface ignition, top edge ignition, or flame application to the whole specimen top) and gas flow rates.
- Optional related determinations: oxygen index at elevated temperatures in accordance with ISO 4589-3, and the temperature index at which the LOI falls to a specified level, which is used for thermal endurance assessments.
Testing Methods
The test apparatus consists of a heat-resistant glass chimney (typically at least 450 mm high and 75–100 mm in diameter), a vertical specimen holder, a precision oxygen and nitrogen mixing and metering system, and a small propane or butane ignition torch. Testing is normally performed by accredited third-party laboratories or in-house laboratories with calibrated equipment.
The general procedure is as follows:
- Condition the specimens and measure their dimensions; mark a reference line 50 mm below the top of each specimen.
- Mount the specimen vertically in the holder so that at least the specified length of specimen is exposed inside the chimney.
- Set an initial oxygen concentration based on the expected flammability of the material, purge the chimney with the gas mixture at the standard total flow rate for at least 30 seconds to ensure a stable atmosphere.
- Apply the ignition flame to the top of the specimen for up to 5 seconds, repeating as allowed by the standard, until the specimen ignites.
- Observe the burning behavior: the trial is recorded as a "pass" if flaming combustion extinguishes before reaching the 50 mm mark and within the time limit, and a "fail" if the specimen burns beyond the criterion.
- Adjust the oxygen concentration upward or downward and test fresh specimens until the critical range is bracketed, then determine the LOI either by averaging the limiting concentrations or by the calculation method given in ISO 4589-2.
- Verify instrument calibration periodically; for example, ISO 4589-2 recommends checking the apparatus with a reference material such as PMMA, which has an established nominal oxygen index of approximately 17%.
Key factors affecting accuracy include gas flow rate stability, chimney draught, specimen thickness and geometry, moisture content, and the operator's ignition technique. Reporting must reference the exact standard used, because ISO 4589-2, ASTM D2863, and GB/T 2406.2 prescribe slightly different specimen sizes and evaluation criteria.
Applications
Oxygen index testing is one of the most widely used flammability screening tools for polymeric materials. Typical applications include:
- Wire and cable insulation and sheathing compounds, where flame-retardant formulations are commonly required to achieve LOI values of 28% or higher.
- Railway applications, where EN 45545-2 fire safety requirements reference EN ISO 4589-2 oxygen index limits for certain product categories at defined hazard levels.
- Electrical and electronic equipment enclosures, connectors, and printed circuit board laminates as part of material qualification and incoming inspection.
- Construction products such as pipes, ducting, panels, and thermal insulation materials, where LOI data support material selection and compliance documentation.
- Research and development of halogen-free flame retardants, polymer modification, and smoke or char optimization, where LOI provides a rapid comparative metric.
- Quality control in plastics compounding, allowing batch-to-batch consistency of flame-retardant masterbatches and compounds to be monitored.
FAQ
What LOI value is considered flame retardant?
Although classification thresholds vary by product standard, materials with an LOI of 27% or greater are generally regarded as flame retardant because they will not continue to burn in normal air containing approximately 21% oxygen. Many industry specifications require LOI values of 28%, 30%, or 32% for critical applications.
How is LOI testing different from the UL 94 test?
LOI testing quantifies the minimum oxygen concentration needed to sustain burning and yields a numerical value, whereas UL 94 (such as the V-0, V-1, and V-2 classifications) evaluates vertical or horizontal burning behavior in normal air against defined afterflame and ignition criteria. The two methods are complementary, and material specifications frequently require both.
How many samples are required for a test?
At least 15 specimens are normally needed because the bracketing procedure consumes multiple specimens at different oxygen concentrations. Preparing 15 to 30 specimens of the correct dimensions is recommended for a valid determination.
Can the oxygen index be measured at elevated temperature?
Yes. ISO 4589-3 extends the method to elevated temperatures, allowing determination of the oxygen index as a function of temperature and derivation of the temperature index, which is useful for assessing long-term thermal endurance of insulation materials.
Is a high LOI sufficient proof of fire safety?
No. The LOI is a small-scale laboratory screening result and does not by itself characterize real-scale fire hazard, which also depends on heat release, smoke production, flame spread, and toxicity. Full fire safety assessment should combine LOI data with appropriate reaction-to-fire and end-product fire tests specified in the relevant product standards.
