The Limiting Oxygen Index (LOI) test is a fundamental laboratory method for quantifying the flammability of plastic materials by determining the minimum oxygen concentration, in a flowing oxygen and nitrogen mixture, that will just sustain candle-like combustion of a vertically mounted specimen. Rooted in internationally recognized standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, the method delivers a reproducible numerical index widely used for flame-retardant formulation development, quality control, and material selection. This article explains the underlying combustion principle, apparatus, sample requirements, test procedure, calculation, and practical applications of oxygen index determination for plastics.
Technical Overview
The oxygen index, more formally known as the Limiting Oxygen Index (LOI), is defined as the minimum concentration of oxygen, expressed as a volume percentage in a mixture of oxygen and nitrogen, that will just support candle-like combustion of a material under specified test conditions. The core principle of oxygen index determination is conceptually simple: every polymer requires a certain partial pressure of oxygen to sustain a propagating flame front. By systematically varying the oxygen content of a controlled atmosphere surrounding a burning specimen, the operator locates the critical boundary between sustained burning and self-extinguishment, and expresses that boundary as a single, highly reproducible number.
The Combustion Principle Behind the Test
Polymer combustion is essentially a gas-phase process. Heat from an ignition source decomposes the surface of the plastic into flammable volatile products, which mix with oxygen from the surrounding air and burn in the flame zone. The flame, in turn, feeds heat back to the polymer surface, generating more fuel and sustaining the cycle. If the oxygen supply is progressively diluted with an inert gas such as nitrogen, the heat release rate in the flame zone eventually becomes insufficient to sustain this feedback loop, and the specimen extinguishes. The oxygen index identifies the exact dilution point at which combustion is only marginally sustained.
Interpretation Against Ambient Air
Because ordinary air contains approximately 20.9 percent oxygen by volume, the LOI value has an intuitive reference point. Materials with an LOI above 21 generally cannot sustain candle-like combustion in still air at room temperature and tend to self-extinguish after removal of the ignition source. Indicative categories frequently used in engineering practice include:
- LOI below 21: readily combustible in air, for example polyethylene, polypropylene, polystyrene, and PMMA, which typically measure around 17 to 18.
- LOI of about 21 to 27: marginal to improved ignition resistance, typical of materials such as polyamide 66 and polycarbonate.
- LOI above 27: generally regarded as flame-retardant or self-extinguishing, for example rigid PVC (approximately 37 to 45), polyphenylene sulfide, polyetherimide, and fluoropolymers (often well above 45).
Testing Samples
The validity of an oxygen index result depends strongly on specimen geometry, preparation quality, and conditioning. Standards prescribe different specimen configurations for different material categories, and the dimensions used must always be stated in the test report because LOI values are not directly comparable across different thicknesses.
Self-Supporting Rigid Plastics
The most common configuration is a rectangular strip of 80 to 150 mm in length and 10 mm in width, with a preferred thickness of 4 mm or 10 mm. Specimens must be cut cleanly, free from flash, bubbles, cracks, and surface contamination, and should represent the material in its finished or as-molded state.
Flexible Plastics and Thin Films
Limp materials that cannot stand upright on their own require modified preparation, such as rolling the film spirally around a thin support rod or using a supported-specimen technique described in the relevant annexes of the standards. Consistency in preparation is essential, since curling during the test can distort the burning behavior.
Cellular (Foamed) Materials
Cellular plastics are tested with larger cross sections, typically 10 mm by 10 mm or greater, to reflect realistic burning behavior. The skin and core structure of the foam should be preserved as far as possible during specimen cutting.
Specimen Conditioning
Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5 percent relative humidity for a defined period, commonly at least 40 hours, before testing. Moisture content can measurably affect results, particularly for hygroscopic polymers such as polyamides.
Testing Items
A typical oxygen index determination program conducted by accredited third-party laboratories includes the following items:
- The Limiting Oxygen Index (LOI) value, reported as volume percent oxygen to one decimal place.
- Qualitative observations of burning behavior, including flame spread, dripping of flaming particles, glowing combustion, smoking, and char formation.
- Pass or fail screening at a specified oxygen concentration, often requested for batch conformity checks.
- Comparative flammability ranking of competing formulations, additive levels, or production batches.
- Optional extensions such as oxygen index at elevated temperature in accordance with ISO 4589-3.
Testing Methods
The test method follows a consistent principle across ISO 4589-2, ASTM D2863, and GB/T 2406.2: a vertically held specimen is ignited at its top inside a glass chimney through which a precisely metered upward flow of oxygen and nitrogen passes, and the oxygen concentration is adjusted until the limiting condition is found.
Test Apparatus
- A vertical heat-resistant glass chimney, typically several hundred millimeters high, which shields the specimen from draughts and defines the gas flow path.
- A specimen holder that clamps the lower end of the sample so that the top portion protrudes freely into the chimney.
- Cylinder supplies of oxygen and nitrogen with two-stage pressure regulators.
- Precision flow measurement devices, such as calibrated rotameters or mass flow controllers, followed by a mixing chamber that homogenizes the gases.
- A small propane or butane diffusion-flame igniter with a controllable flame.
- A timing device and a graduated scale or pre-marked reference lines for measuring damaged length.
Standard Test Procedure
- Condition, measure, and mark each specimen, including a reference line 50 mm below the top edge.
- Mount the specimen vertically and set the initial oxygen and nitrogen flow rates to achieve the specified total flow and linear gas velocity through the chimney.
- Allow the gas mixture to purge the chimney for at least 30 seconds to establish a stable atmosphere.
- Apply the ignition flame to the top surface of the specimen, typically in short applications not exceeding a cumulative ignition time of about 30 seconds, until the entire top is ignited.
- Withdraw the igniter and start timing; record the burning time and the damaged length down the specimen.
- Judge the response as sustained combustion if the specimen burns for 180 seconds or more, or if the damaged length reaches 50 mm or more from the top edge, which is the standard criterion for rigid plastics.
- Adjust the oxygen concentration upward if the specimen extinguishes before the criterion, or downward if it exceeds it, using progressively smaller steps, for example 1 percent increments initially and 0.2 percent increments near the limit.
- Repeat with fresh specimens until the limiting concentration is tightly bracketed, then compute the LOI in accordance with the calculation rules of the standard.
Calculation of the Oxygen Index
The oxygen concentration at any flow setting is calculated directly from the volumetric flow rates of the two gases: LOI equals 100 multiplied by the oxygen flow rate divided by the sum of the oxygen and nitrogen flow rates. When rotameters are used, readings may require correction for local temperature and pressure, whereas mass flow controllers report the mixture directly. The final LOI is determined from the oxygen concentration series using the bracketing results, typically confirmed with additional replicate specimens, and is reported to one decimal place together with the standard used, specimen dimensions, and conditioning state.
Relevant Standards
- ISO 4589-2, covering the ambient-temperature oxygen index determination for plastics.
- ISO 4589-3, extending the same principle to elevated-temperature testing.
- ASTM D2863, measuring the minimum oxygen concentration to support candle-like combustion of plastics.
- GB/T 2406.2, the Chinese national standard technically aligned with ISO 4589-2.
- JIS K 7201-2, the corresponding Japanese industrial standard.
Factors Influencing Results
- Specimen thickness and geometry, since thicker sections act as heat sinks and generally yield different LOI values for the same polymer.
- Test temperature, because LOI decreases as ambient temperature rises, which is why the standard test is performed at 23 °C.
- Moisture content and conditioning history, especially for hygroscopic materials.
- Formulation variables such as fillers, plasticizers, and flame-retardant additives, as well as melt-dripping behavior during burning.
- Operator ignition technique and the calibration accuracy of the gas flow measurement system.
Applications
Oxygen index determination is one of the most widely applied flammability screening tools in the plastics industry. Typical applications include:
- Research and development of flame-retardant formulations based on halogenated, phosphorus-based, intumescent, or mineral-filled systems such as aluminum hydroxide and magnesium hydroxide.
- Incoming material inspection and batch-to-batch quality control of flame-retardant compounds.
- Material screening for electrical and electronic products, where LOI data commonly support qualification programs toward UL 94 ratings.
- Documentation of fire-performance characteristics for construction products, railway components, aircraft interiors, and cable insulation and sheathing materials.
- Comparative academic research, supplier benchmarking, and failure analysis support.
It should be emphasized that the LOI is a laboratory screening index. It complements, but does not replace, reaction-to-fire assessments such as UL 94 burning tests, IEC 60695-based methods, or cone calorimetry, which evaluate behavior under conditions closer to real fire scenarios.
FAQ
What does an oxygen index value of exactly 21 mean?
An LOI of 21 places the material right at the oxygen concentration of ordinary air, which is about 20.9 percent. Such a material is at the borderline of sustaining combustion in still air, so values below 21 indicate ready combustibility while values above 21 indicate a tendency to self-extinguish after the ignition source is removed.
What LOI value is considered flame retardant?
Classification thresholds vary by industry and specification, but a widely used engineering convention treats LOI values of 27 and above as flame-retardant or self-extinguishing. Many procurement specifications for engineering plastics and cable compounds require LOI values of at least 28, or even 32 for demanding applications.
Are ISO 4589-2, ASTM D2863, and GB/T 2406.2 results equivalent?
The three standards are harmonized in principle, using a vertical specimen, top ignition, a flowing oxygen and nitrogen mixture, and the 180-second or 50-millimeter response criteria. Minor differences in chimney dimensions, flow velocity requirements, and data treatment can cause small discrepancies between results, so the specific standard applied should always be stated in the test report.
Why is nitrogen used as the diluent gas?
Nitrogen is chemically inert with respect to combustion, inexpensive, and readily available in high purity. Replacing a portion of the oxygen with nitrogen reduces the oxidizing power of the atmosphere without introducing any chemical side effects, allowing the oxygen concentration to be tuned precisely and reproducibly.
Does a high LOI guarantee fire safety in a real application?
No. The oxygen index measures candle-like combustion of a small specimen under controlled laboratory conditions. Real fires involve high radiant fluxes, elevated temperatures, complex geometries, and interactions between components. LOI data should therefore be combined with results from UL 94, cone calorimeter, smoke density, and other reaction-to-fire tests for a complete fire-hazard assessment.
How many specimens are needed for one determination?
A single LOI determination typically consumes roughly 10 to 15 specimens, because a fresh specimen must be used at every oxygen concentration step as the bracketing procedure converges on the limiting value. Accredited third-party laboratories therefore usually request 15 to 20 specimens per material to complete the determination and confirmatory replicates.