The Limiting Oxygen Index (LOI) test is a fundamental laboratory method for evaluating the combustion performance of plastics by determining the minimum oxygen concentration required to sustain flaming combustion of a vertically mounted specimen. Performed in accordance with internationally recognized standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, the test delivers a quantitative and highly reproducible flammability index that supports material selection, flame-retardant formulation development, quality control, and regulatory compliance. This article provides a comprehensive technical overview of the oxygen index test, covering its principle, specimen requirements, test procedure, data interpretation, and typical industrial applications.
Technical Overview
The oxygen index test, more formally known as the Limiting Oxygen Index (LOI) test, quantifies the flammability of polymeric materials by measuring the minimum volumetric concentration of oxygen, in a flowing mixture of oxygen and nitrogen, that will just support sustained candle-like flaming combustion of a vertically mounted specimen. Because ambient air contains approximately 20.9% oxygen, the LOI value provides an immediate and intuitive indication of whether a plastic will continue to burn in normal atmospheric conditions.
The result is expressed as a dimensionless percentage calculated from the volumetric flow rates of the two gases:
- LOI = [V(O2) / (V(O2) + V(N2))] × 100%
A material with an LOI below 21% will generally continue to burn freely in air once ignited, whereas a material with an LOI above 21% tends to self-extinguish under ambient conditions. In many industry specifications, particularly in rail and transport applications referencing EN 45545-2, an LOI of 28% or higher is a common threshold for classifying a polymer as flame retardant. Typical reference values include:
- Polyethylene (PE) and polypropylene (PP): approximately 17–18%
- Polystyrene (PS) and ABS: approximately 17–20%
- Polymethyl methacrylate (PMMA): approximately 17–18%
- Polyamide (PA): approximately 24–29%
- Polycarbonate (PC): approximately 25–35%
- Rigid polyvinyl chloride (PVC): approximately 45–49%
- Polytetrafluoroethylene (PTFE): greater than 95%
It should be emphasized that the oxygen index is a small-scale laboratory ranking tool. It does not by itself constitute a full fire hazard assessment, and results are influenced by specimen thickness, orientation, additives, and thermal aging.
Testing Samples
Rigid Plastic Specimens
For rigid plastics in sheet, compression-molded, or injection-molded form, the preferred specimen geometry under ISO 4589-2 and GB/T 2406.2 is a rectangular bar of 80 mm to 150 mm in length, 10 mm in width, and 4 mm in thickness. ASTM D2863 specifies a comparable bar of 70 mm to 150 mm in length, 6.5 mm in width, and 3 mm in thickness. Because LOI values depend on thickness, comparison between materials is only valid when identical geometries and test conditions are used.
Flexible Films and Thin Sheets
Thin films that cannot support themselves vertically are tested using a small U-shaped support frame in accordance with the annexes of ISO 4589-2 and ASTM D2863. A typical film specimen measures 140 mm × 52 mm and is clamped so that a defined portion remains exposed to the ignition source, ensuring repeatable ignition and flame propagation behavior.
Cellular Materials (Foams)
Rigid and flexible foams are tested with enlarged cross-sections that reflect their low density and high surface-to-mass ratio, with dimensions specified by the applicable product standard. Due to their porous structure, foams often exhibit lower LOI values than the corresponding solid polymers, and results should always be tied to the exact specimen thickness tested.
Tubes, Rods, and Molded Parts
Tubes and rods can be tested directly by mounting short lengths of typically 70 mm to 150 mm in an appropriate holder. Finished molded parts should be cut into standardized bars wherever possible; if geometry does not permit this, the actual configuration tested must be clearly documented in the test report.
Specimen Quantity and Conditioning
A minimum of 15 specimens is generally required to complete the concentration-bracketing sequence. Prior to testing, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 40 hours in accordance with ISO 291, unless the governing specification dictates otherwise.
Testing Items
The core deliverable of the oxygen index test is the room-temperature LOI value determined in general accordance with ISO 4589-2, ASTM D2863, or GB/T 2406.2. Depending on client requirements and product specifications, the test scope may also include:
- Room-temperature limiting oxygen index (23 °C) for quality control and material certification purposes
- Elevated-temperature oxygen index testing and determination of the Temperature Index (TI) in accordance with ISO 4589-3, which identifies the temperature at which the LOI falls to the oxygen concentration of air
- Assessment against specific classification criteria, such as LOI ≥ 28% commonly required for polymeric components in railway applications under EN 45545-2
- Comparative evaluation of flame-retardant formulations, filler loadings, or base resin grades
- Observation and recording of ignition behavior, afterflame time, dripping, and burning characteristics during the test
Testing Methods
Apparatus
The standard oxygen index apparatus consists of a heat-resistant glass chimney, typically at least 450 mm in height and 75 mm to 100 mm in internal diameter, into which a vertically adjustable specimen holder is inserted. Medical-grade oxygen and nitrogen are supplied from compressed gas cylinders, regulated through needle valves and calibrated flow meters, and mixed before entering the base of the chimney. Ignition is achieved with a small premixed propane flame, approximately 16 mm in height under ISO 4589-2, applied from the top of the chimney. An oxygen analyzer may be used to verify the concentration of the mixed gas stream.
Test Procedure
The test determines, by systematic adjustment of the oxygen concentration, the exact level at which the critical burning criterion is just met. The general procedure is as follows:
- Mount the conditioned specimen vertically in the holder and position it in the center of the chimney.
- Set the oxygen and nitrogen flow rates to achieve the target oxygen concentration, with a total gas flow corresponding to a linear velocity of approximately 40 ± 10 mm/s through the chimney.
- Purge the chimney with the gas mixture for at least 30 seconds to establish a stable atmosphere.
- Apply the ignition flame to the top surface of the specimen for up to 5 seconds and remove it.
- Observe whether flaming combustion spreads downward beyond the reference marks (a burned distance of 50 mm under ISO 4589-2, or 51 mm under ASTM D2863) or persists longer than 180 seconds after removal of the igniter.
- Increase the oxygen concentration if the specimen fails the criterion, or decrease it if the criterion is exceeded, using progressively finer increments down to 0.2% in the final bracketing stage.
- Repeat the sequence across the specimen set until the limiting concentration is confirmed by the required number of paired observations.
Calculation and Reporting
The LOI is reported as the volumetric oxygen concentration, rounded to 0.1%, at which the critical burning criterion is just satisfied. The test report includes the material identification, specimen dimensions and orientation, conditioning history, the standard applied, the individual oxygen concentrations tested, the calculated LOI value, and observations of any dripping, glowing combustion, or other anomalous behavior.
Relevant Standards
The oxygen index method is harmonized across several major standardization bodies, and the following documents are most frequently invoked by accredited third-party laboratories:
- ISO 4589-1 – Plastics – Determination of burning behaviour by oxygen index – Guidance
- ISO 4589-2 – Room-temperature test
- ISO 4589-3 – 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 – Chinese national standards corresponding to the ISO 4589 series
- JIS K 7201-2 – Japanese industrial standard aligned with ISO 4589-2
Applications
The oxygen index test is one of the most widely specified flammability screening methods in the plastics industry, and typical applications include:
- Research and development of flame-retardant compounds, including halogenated, phosphorus-based, mineral-filled, and intumescent systems
- Incoming material inspection and batch-to-batch quality control of resins, masterbatches, and compounded pellets
- Certification of polymeric components for railway rolling stock against EN 45545-2 requirement sets
- Qualification of wire and cable insulation and sheathing materials, where LOI complements tests under the IEC 60332 series
- Material selection for electrical and electronic equipment enclosures, often alongside UL 94 small-flame testing
- Supporting documentation for building products, aerospace interiors, and mass-transit vehicle components
- Academic research and competitive benchmarking of polymer flammability performance
Because the LOI value is sensitive to formulation changes, it serves as an efficient statistical process control parameter for manufacturers seeking to demonstrate consistent flame-retardant performance over time.
FAQ
What is a good LOI value for a flame-retardant plastic?
Since air contains about 21% oxygen, any material with an LOI above 21% shows a tendency to self-extinguish in normal air. For most regulated applications, an LOI of 28% or higher is regarded as indicative of a well-flame-retarded polymer, and this threshold is explicitly referenced in requirements such as EN 45545-2 for railway components.
How does the LOI test differ from the UL 94 test?
The LOI test measures the minimum oxygen concentration needed to sustain burning in a controlled oxygen-nitrogen atmosphere, producing a continuous numerical index. The UL 94 test evaluates ignition and flame spread of a specimen burning in ambient air, producing categorical ratings such as V-0, V-1, V-2, or HB. The two methods are complementary, and many specifications require both.
Why do LOI results vary with specimen thickness?
Thicker specimens act as a larger heat sink and tend to exhibit higher LOI values than thin specimens of the same material. For this reason, the tested thickness must always accompany the reported LOI value, and valid comparisons require identical geometries.
Does a high LOI mean the material cannot burn?
No. A high LOI indicates that the material requires an oxygen-enriched atmosphere to sustain candle-like combustion. In real fires, which involve high temperatures, radiant flux, and forced airflow, even high-LOI materials can ignite and contribute to fire growth. LOI is a ranking tool and must be combined with other fire test data for hazard assessment.
How many specimens are needed and how long does the test take?
At least 15 conditioned specimens are normally required to complete the full bracketing sequence for one test condition. A routine determination is typically completed within one working day when specimens are properly prepared and conditioned in advance.
Can the oxygen index test be performed at elevated temperatures?
Yes. ISO 4589-3 extends the method to elevated temperatures and defines the Temperature Index, which is the temperature at which the LOI of the material drops to the oxygen concentration of ambient air. This parameter is valuable for materials used in warm operating environments.
