Limiting Oxygen Index (LOI) testing is the core method for quantifying the flammability of chemical fibers such as polyester, polyamide, acrylic, and aramid. By measuring the minimum oxygen concentration required to sustain combustion, manufacturers, researchers, and accredited third-party laboratories can rank, certify, and optimize flame-retardant fiber performance in accordance with ISO 4589-2, ASTM D2863, and GB/T 5454. This article explains the test principle, sample preparation, procedure, result interpretation, and typical industrial applications.
Technical Overview
The Limiting Oxygen Index (LOI), also called the oxygen index or critical oxygen index, is defined as the minimum volumetric concentration of oxygen in a flowing oxygen–nitrogen mixture that will just support candle-like combustion of a vertically mounted specimen. Because ambient air contains approximately 20.95% oxygen, a fiber with an LOI above 21% tends to self-extinguish in normal air, while a fiber with an LOI below 21% will continue to burn once ignited.
LOI is expressed as a percentage and calculated as follows:
- LOI (%) = 100 × V(O₂) / [V(O₂) + V(N₂)], where V denotes the volumetric flow rates of oxygen and nitrogen at the critical concentration.
For chemical fibers, LOI testing provides a quantitative, reproducible, and sample-efficient way to evaluate flame retardancy, complementing qualitative burning tests such as vertical flame spread (e.g., ISO 15025, GB/T 5455, ASTM D6413). The method is widely used for polyester, polyamide, acrylic, polypropylene, viscose, spandex, modacrylic, aramid, polyphenylene sulfide (PPS), and polybenzimidazole (PBI) fibers.
Applicable standards include:
- ISO 4589-2 / EN ISO 4589-2 — Determination of burning behaviour by oxygen index, ambient-temperature test (polymeric materials).
- ASTM D2863 — Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics.
- GB/T 2406.2 — Chinese equivalent for plastics, technically aligned with ISO 4589-2.
- GB/T 5454 — Textiles: determination of burning behaviour by the oxygen index method, commonly applied to textile forms of chemical fibers.
- JIS K 7201-2 — Japanese industrial standard for the oxygen index of polymeric materials.
Testing Samples
Fabric Specimens
Woven, knitted, or nonwoven fabrics made from the target chemical fiber are cut into rectangular strips (typically 150 mm × 50 mm for GB/T 5454). Fabric testing is the most representative form because fabric structure, density, and finishing strongly influence LOI. Specimens are usually tested in both warp and weft directions and may be tested before and after a specified number of laundering cycles to assess the durability of flame-retardant finishes.
Yarn and Filament Specimens
Continuous filaments or spun yarns can be wrapped onto frames or wound into bundles of the required length and linear density. This form is suitable for fiber producers who need to evaluate flame retardancy at the yarn stage before weaving or knitting.
Staple Fiber and Fiber Assemblies
Loose staple fibers are generally not tested directly because of uncontrolled packing density. They are typically processed into standardized nonwoven mats or carded webs with specified areal density, or they may be evaluated in special holders where the standard permits. Powder and granule methods referenced in ISO 4589-2 are generally reserved for polymer resins rather than textile fibers.
Specimen Conditioning
- Standard atmosphere conditioning: typically 23 ± 2 °C and 50 ± 5% relative humidity for a defined period before testing.
- Oven conditioning (e.g., 105 ± 2 °C) may be required for certain standards to remove absorbed moisture from hydrophilic fibers such as viscose.
- Specimens must be free from creases, contamination, and finishing residues unless the finish itself is the object of evaluation.
Testing Items
A standard LOI test campaign for chemical fibers typically reports the following items:
- LOI value (percentage by volume) determined at ambient temperature, reported to the nearest 0.1% or 0.2% depending on the standard.
- Ignition behaviour: whether the specimen ignites within the specified ignition time (commonly up to 30 seconds of flame application).
- Afterflame time and afterglow time observed at the critical concentration.
- Burned or damaged length relative to the pass criterion (for example, burning that does not reach a defined 50 mm gauge mark within 3 minutes).
- Melting, sputtering, dripping, and char formation behaviour, which is particularly relevant for thermoplastic fibers such as PET and polyamide.
- Flammability classification based on LOI thresholds, together with before/after laundering comparison when durability is evaluated.
Testing Methods
Apparatus
The oxygen index apparatus consists of a heat-resistant borosilicate glass chimney (typically about 500 mm high) mounted on a base with a gas distribution screen, a vertical specimen holder, precision flow meters or mass flow controllers for oxygen and nitrogen, an oxygen analyzer, a small propane or butane ignition burner, and timing devices. The total gas flow is calibrated to the velocity specified by the applicable standard so that results from different laboratories remain comparable.
Procedure
- Condition the specimens and verify gas purity, flow calibration, and ambient laboratory conditions.
- Clamp the specimen vertically so that its top extends a defined distance above the top of the chimney.
- Set an initial oxygen concentration based on prior knowledge of the material class.
- Purge the chimney with the oxygen–nitrogen mixture for at least 30 seconds before ignition.
- Apply the ignition flame to the top of the specimen; ignition is confirmed when the flame catches and sustains for the period defined by the standard.
- Observe whether combustion exceeds the criterion (burning time or burned length). Increase the oxygen concentration if the specimen fails to meet the criterion and decrease it if combustion is sustained.
- Use the bracketing method to narrow the oxygen concentration interval (typically down to 0.2% steps near the critical point) until the LOI is bracketed.
- Verify the result with replicate specimens as required by the standard (often ten or more per condition) and calculate the reported LOI.
Result Interpretation
Although classification thresholds vary slightly between product standards and regulations, the following ranges are widely referenced in the fiber and textile industry:
- LOI < 21%: readily flammable in air; the fiber supports its own combustion (typical of polypropylene at ~17–18%, acrylic at ~18%, cotton at ~18–19%, and viscose at ~19%).
- LOI 21–26%: slow-burning or marginally combustible; ordinary polyester (~20–22%) and polyamide (~20–23%) fall in this band.
- LOI ≥ 26–28%: generally recognized as flame-retardant or self-extinguishing in air; typical values include modacrylic (~27–33%), meta-aramid (~28–32%), PPS (~34), and PBI (~41 or higher).
LOI is a ranking tool for material comparison, not a direct prediction of full-scale fire hazard. For regulatory compliance, LOI results are usually combined with flame spread, afterflame, and melt-dripping tests performed to the applicable product standard.
Applications
- Flame-retardant fiber development: screening copolymer modifications, halogen-free phosphorus/nitrogen additives, and intrinsic flame-retardant polymer chemistries during R&D.
- Quality control: incoming inspection of flame-retardant polyester, modacrylic, and aramid fibers for spinning mills, weavers, and nonwoven producers.
- Durability assessment: comparing LOI before and after repeated laundering to distinguish durable (fiber-inherent) flame retardancy from non-durable surface finishes.
- Protective clothing: performance verification of fibers used in flame-retardant workwear, firefighter garments, military apparel, and arc-rated clothing aligned with standards such as GB 8965, EN ISO 11612, EN ISO 11611, and NFPA 2112.
- Public occupancy and transportation interiors: curtains, seat coverings, carpets, and wall linings for hotels, hospitals, cinemas, aircraft, and railway vehicles, where LOI supports compliance with flame-retardance requirements.
- Home and technical textiles: certification support for bedding, upholstery, sound-absorbing panels, insulation batts, and filtration media.
- Academic research: quantitative evaluation of char formation, thermal degradation, and synergistic flame-retardant systems in polymer science studies.
FAQ
What LOI value qualifies a chemical fiber as flame retardant?
In most industrial and regulatory contexts, a fiber with an LOI of 26–28% or higher is considered flame retardant or self-extinguishing in normal air. The exact threshold depends on the governing product standard, so the applicable specification should always be consulted.
Which standard should be used for textiles versus plastics?
For textile forms of chemical fibers, GB/T 5454 is the dedicated oxygen index method. For fiber-form polymers tested as molded bars or with special holders, ISO 4589-2, ASTM D2863, GB/T 2406.2, and JIS K 7201-2 apply. All methods share the same fundamental principle but differ in specimen geometry and pass criteria.
Can loose staple fibers be tested directly?
Not usually. Packing density of loose fibers is difficult to control, so staple fibers are converted into standardized yarn, nonwoven, or fabric forms before testing. This ensures reproducible and comparable LOI values.
Why is conditioning so important for LOI testing?
Moisture content significantly affects combustion behaviour, especially for hydrophilic fibers such as viscose and flame-retardant rayon. Standardized conditioning in a controlled atmosphere (or oven drying where specified) is essential for valid, repeatable results.
How is LOI testing different from a vertical flame test?
LOI produces a continuous numerical value that is ideal for ranking materials and tracking incremental formulation improvements. Vertical flame tests such as ISO 15025, GB/T 5455, and ASTM D6413 simulate a real ignition scenario and evaluate afterflame time, char length, and dripping. The two methods are complementary and are frequently both required for full compliance.
How many specimens are required?
The number varies by standard, but a complete determination typically consumes ten to twenty specimens because the critical oxygen concentration must be bracketed and verified with replicates. Laboratories usually request additional sample quantity to allow repeat testing and laundering durability studies.