The limiting oxygen index (LOI) test is a fundamental laboratory method for evaluating the flammability of plastic materials by determining the minimum oxygen concentration in an oxygen/nitrogen mixture that will just support candle-like combustion of a vertically mounted specimen. This article provides a comprehensive technical guide to the LOI tester experiment, covering the working principle, apparatus configuration, specimen preparation, test procedures in accordance with ISO 4589-2, ASTM D2863, and GB/T 2406, data interpretation, and typical industrial applications in flame-retardant material development and quality control.
Technical Overview
The limiting oxygen index (LOI), also known as the oxygen index (OI) or critical oxygen index (COI), 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 specimen held vertically. The LOI tester experiment is governed by internationally recognized standards, principally ISO 4589-2, ASTM D2863, and the Chinese national standard GB/T 2406.2, along with the Japanese equivalent JIS K 7201-2. These standards are broadly harmonized and produce comparable results.
The core principle of the instrument is straightforward: a small rectangular specimen is clamped vertically in a heat-resistant holder inside a transparent glass chimney. A controlled, upward-flowing gas mixture of oxygen and nitrogen is passed through the chimney at a defined velocity. The top of the specimen is ignited with a small gas pilot flame, and the oxygen concentration is systematically adjusted until the critical concentration is found at which the specimen either burns beyond a defined mark or self-extinguishes. The LOI value is calculated using the volumetric flow rates of the two gases:
- LOI = 100 × V(O2) / [V(O2) + V(N2)]
A typical LOI analyzer consists of a vertical borosilicate glass chimney (commonly 75–100 mm in internal diameter and approximately 450–500 mm in height), a specimen holder rod with a spring clamp, a precision gas mixing and flow control system with rotameters or mass flow controllers, gas pressure regulators, a propane or natural gas ignition source, a timing device, and a graduated scale marked at 50 mm for judging flame spread. Because ambient air contains approximately 20.95% oxygen, materials with an LOI below roughly 21% will continue to burn in normal air, while materials with higher LOI values are progressively more difficult to ignite and sustain combustion in air.
Testing Samples
Rigid and Self-Supporting Plastics
For moulded or sheet plastics in the self-supporting category, the standard specimen dimensions under ISO 4589-2 and GB/T 2406.2 are 80–150 mm in length, 10 mm in width, and 4 mm in thickness. Specimens are typically cut from compression-moulded or injection-moulded plaques, and edges should be smooth and free of burrs to avoid influencing ignition behavior. At least 10–15 specimens are usually required for a complete determination because the test is iterative.
Flexible Films and Thin Sheets
<Thin polymeric films that cannot stand upright are tested using a film support, commonly by rolling the film into a spiral around a mandrel and securing it so that a defined free-burning length is exposed. This adaptation, described in ISO 4589-2 and ASTM D2863 annexes, ensures that the combustion geometry remains reproducible for flexible materials such as packaging films and electrical insulation tapes.
Cellular Materials and Foams
Rigid and flexible cellular plastics, such as polyurethane and polystyrene foams, are tested with larger cross-sections (typically 10 mm × 10 mm up to 50 mm × 50 mm depending on density and cell size) to obtain representative burning behavior. Specimen orientation relative to the foam rise direction must be recorded, as it can significantly affect the measured index.
Rubbers, Textiles, and Composites
Elastomers, coated fabrics, fiber-reinforced laminates, and thermoset molding compounds can also be evaluated, provided the specimens can be mounted vertically and meet the dimensional requirements. For composites, the fiber lay-up direction and resin content should be documented as part of the test report.
Testing Items
The LOI tester experiment typically covers the following determinations:
- Ambient-temperature limiting oxygen index (23 ± 2 °C) in accordance with ISO 4589-2, ASTM D2863, or GB/T 2406.2
- Elevated-temperature oxygen index and temperature index determination in accordance with ISO 4589-3
- Afterflame time and afterglow time following removal of the ignition source
- Observation of flaming drips, molten debris, and char formation during combustion
- Comparative evaluation of flame-retardant formulations and batch-to-batch consistency checks
It should be emphasized that the LOI is a small-scale laboratory screening test. It measures ease of extinction under controlled conditions and does not by itself constitute a full fire hazard assessment; it is usually complemented by burning tests such as UL 94 vertical/horizontal burning, IEC 60695-11-10, or cone calorimetry.
Testing Methods
Specimen Conditioning
Prior to testing, specimens are conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity, following ISO 291 or ASTM D618, until moisture equilibrium is reached (commonly 88 hours minimum for many plastics, or as specified in the material standard). Conditioned specimens should be tested promptly to prevent reabsorption of moisture.
Instrument Preparation
High-purity oxygen and nitrogen (typically 99.5% or better) are connected to the instrument through pressure regulators set to the manufacturer's specified delivery pressure. The gas mixer and flowmeters are verified, and the total flow is adjusted to give a mean linear gas velocity of 40 ± 10 mm/s through the chimney, as required by ISO 4589-2. The chimney is purged with the gas mixture for at least 30 seconds before each ignition to ensure a stable, homogeneous atmosphere.
Test Procedure
- Mount the specimen vertically in the holder so that the top of the specimen is at least 100 mm below the top of the chimney opening, and lower the assembly into the chimney.
- Set an initial oxygen concentration estimated from experience or preliminary trials, purge the chimney, and apply the pilot flame to the top of the specimen in short applications (for example, 5-second cycles under ISO 4589-2, or up to 30 seconds under ASTM D2863) until the specimen ignites.
- Record whether the specimen, after removal of the ignition source, burns downward past the 50 mm reference mark or flames for longer than 180 seconds; either outcome classifies the specimen as "burning" at that concentration. If the flame extinguishes before both criteria are exceeded, the specimen is classified as "not burning."
- Adjust the oxygen concentration stepwise, using coarse steps initially and fine steps of 0.2% (v/v) as the critical region is approached, alternating between concentrations that give "burning" and "not burning" responses.
- Repeat the sequence with fresh specimens until the oxygen concentration balance point is established with the required number of confirming results.
Calculation and Reporting
The LOI is calculated from the equilibrium volumetric flow rates of oxygen and nitrogen using the formula given above, and the result is reported to one decimal place together with the specimen dimensions, thickness, orientation, conditioning history, ignition mode, and any observations of dripping or afterglow. Well-maintained instruments operated by trained personnel in accredited third-party laboratories typically achieve repeatability within approximately ±0.5% oxygen and reproducibility within roughly ±1–1.5% oxygen between laboratories.
Applications
The LOI tester experiment is widely applied across the polymer industry and research institutions:
- Flame-retardant research and development: screening halogenated, phosphorus-based, nitrogen-based, inorganic filler, and intumescent flame-retardant formulations for polyolefins, polyamides, polyesters, and engineering thermoplastics.
- Quality control: verifying the flame-retardant consistency of incoming resins, masterbatches, and finished compounds in cable insulation and sheathing, connectors, switches, and appliance housings.
- Regulatory compliance support: providing LOI data referenced by material specifications in railway rolling stock (EN 45545-2), electrical equipment, and certain building product applications.
- Material selection: comparing the relative flammability of candidate plastics for electrical, electronic, transportation, and construction components during the design stage.
- Academic study of polymer combustion: investigating char formation, melt dripping behavior, and the effects of additives, fillers, and copolymer composition on extinction properties.
FAQ
What does a higher LOI value indicate?
A higher LOI indicates that a greater oxygen enrichment is required to sustain combustion, meaning the material is more flame retardant. As a practical guide, materials with LOI below 21 burn readily in air, values between 21 and 28 indicate self-extinguishing tendency in air to varying degrees, and values above 28 are generally considered flame retardant.
Why is 21% oxygen used as a reference point?
Ambient air contains approximately 20.95% oxygen by volume. A specimen whose LOI is below this level will continue burning in ordinary air once ignited, whereas a specimen with an LOI above this level requires an oxygen-enriched atmosphere to keep burning and will therefore self-extinguish in air under the test geometry.
Are ISO 4589-2, ASTM D2863, and GB/T 2406.2 results interchangeable?
The three standards are technically aligned in principle, apparatus, and judgment criteria, with only minor differences in specimen dimensions, flow verification, and reporting details. Results are generally comparable, but for contractual or certification purposes the specific standard invoked should be stated and followed.
Which factors most strongly influence LOI results?
Specimen thickness (thicker specimens often yield different LOI values), test temperature (LOI decreases as temperature rises), moisture conditioning, filler and additive content, melt-dripping behavior, and the presence of surface skin or oxide layers all influence the measurement. Consistent specimen preparation and conditioning are therefore essential for valid comparisons.
Can LOI alone certify a material as safe?
No. The LOI is a comparative, small-scale flammability indicator. A complete fire safety assessment normally combines LOI with burning-rate tests, smoke and toxicity evaluations, and larger-scale fire tests appropriate to the end-use application.
