The Limiting Oxygen Index (LOI) test is one of the most widely used bench-scale methods for evaluating the flammability of plastics and other polymeric materials. Performed in accordance with ISO 4589-2, ASTM D2863, and GB/T 2406.2, the test determines the minimum oxygen concentration in an oxygen/nitrogen mixture that will just sustain candle-like combustion of a vertically mounted specimen. This article explains the principle, apparatus, sample requirements, test procedure, calculation, and typical applications of the oxygen index combustion test for plastic materials.
Technical Overview
The Limiting Oxygen Index (LOI), also called the oxygen index or critical oxygen index, is defined as the minimum concentration of oxygen, expressed as a volume percentage in a mixture of oxygen and nitrogen, that will just support flaming combustion of a material. In the test, a specimen is held vertically in a glass chimney while a controlled mixture of oxygen and nitrogen flows upward around it. The top of the specimen is ignited with a small flame, and the oxygen concentration is adjusted step by step until the critical value that just sustains burning is found. The result is calculated as:
LOI = 100 × V(O2) / [V(O2) + V(N2)]
Because normal air contains approximately 20.9% oxygen, the LOI value gives an immediate indication of real-world burning behavior. Plastics with an LOI below 21% will generally continue to burn in ambient air after ignition, while materials with an LOI of 27% or higher are commonly regarded as self-extinguishing under this criterion. The higher the LOI, the greater the flame resistance of the material.
Because the oxygen concentration, gas flow rate, and specimen geometry are all tightly controlled, the LOI method offers excellent repeatability and comparability. It is widely used for material ranking, research and development of flame-retardant formulations, quality control, and demonstration of compliance with fire safety specifications. Accredited third-party laboratories routinely perform LOI testing under the following standards:
- ISO 4589-2 / EN ISO 4589-2 — determination of burning behaviour by oxygen index, ambient temperature
- ISO 4589-3 — oxygen index determination at elevated temperature
- ASTM D2863 — standard test method for measuring the minimum oxygen concentration to support candle-like combustion of plastics
- GB/T 2406.2 — Chinese national standard, technically aligned with ISO 4589-2
- JIS K 7201-2 — Japanese standard for oxygen index of plastics
Testing Samples
Rigid Plastics and Thermosets
Self-supporting rigid plastics, including thermoplastics and thermosets, are typically tested as rectangular bars of 80 mm to 150 mm in length, 10 mm in width, and 4 mm in thickness for ISO 4589-2, while ASTM D2863 commonly uses bars on the order of 70 mm to 150 mm long, 6.5 mm wide, and 3 mm thick. Specimens must be cut cleanly, free of flash, cracks, and sink marks, and the machine and transverse directions should be recorded for anisotropic materials such as filled or fiber-reinforced compounds.
Flexible Films and Thin Sheets
Thin films that cannot support themselves vertically are tested in a special frame or holder that supports the specimen without restricting flame propagation. Specimen dimensions and the supporting arrangement are defined in the standards, and the film thickness is always reported together with the LOI result because of its strong influence on the measured value.
Cellular Plastics and Foams
Low-density cellular materials are tested using larger cross-sections, commonly around 10 mm by 10 mm, to ensure representative burning behavior. The cellular structure, cell size, and density must be reported, as foams show markedly different oxygen index behavior than solid plastics of the same polymer.
Specimen Conditioning and Quantity
Prior to testing, specimens are conditioned in a standard atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity in accordance with ISO 291 or ASTM D618, typically for a minimum of 40 hours. A full determination normally requires at least 15 specimens, since the LOI value is derived from a statistical series of burns at incrementally adjusted oxygen concentrations. Additional specimens should be prepared when testing at multiple temperatures or when repeating measurements for verification.
Testing Items
Typical oxygen index test items offered for plastic materials include the following:
- Ambient-temperature limiting oxygen index (LOI) per ISO 4589-2, ASTM D2863, GB/T 2406.2, or JIS K 7201-2
- Elevated-temperature oxygen index and temperature index per ISO 4589-3
- LOI after defined conditioning states, such as dry, humidity-exposed, or heat-aged specimens
- Qualitative burning observations, including afterflame time, afterglow, flaming drips, and smoke release
- LOI verification as part of material qualification programs for railway, electrical, aerospace, and building applications
Many sector specifications set minimum LOI acceptance limits. For example, railway material requirements in EN 45545-2 call for LOI values of 28% or higher for certain interior and cable applications, and comparable thresholds appear in electrical equipment and rolling stock procurement specifications worldwide.
Testing Methods
Apparatus
The oxygen index apparatus consists of a heat-resistant glass chimney, typically 75 mm to 100 mm in internal diameter and at least 450 mm high, mounted on a base that contains the gas mixing and distribution system. A specimen holder, usually a thin rod with a clamp, supports the specimen vertically in the center of the chimney. Technical-grade oxygen and nitrogen of at least 98% purity are supplied through calibrated flow controllers and mixed before entering the base of the column. A small propane or butane diffusion flame igniter is used to ignite the top of the specimen, and timing devices plus a reference mark on the specimen are used to judge the burning criterion. Many modern instruments also integrate an oxygen analyzer to verify the actual oxygen concentration of the gas mixture.
Test Procedure
The general test sequence is as follows:
- Condition, measure, and mark the specimens, including a reference mark 50 mm below the top of the specimen where required by the standard.
- Clamp the specimen vertically in the holder and position it in the chimney with the top of the specimen at the specified distance below the chimney top.
- Set the total gas flow to achieve the standard linear velocity, approximately 40 mm/s in ISO 4589-2, and adjust the oxygen concentration to the initial estimated value.
- Allow the chimney to purge thoroughly so the atmosphere around the specimen is stable before ignition.
- Apply the ignition flame to the top of the specimen for a short time, typically no more than about 5 seconds, then withdraw it.
- Observe whether the specimen meets the burning criterion: continued burning for more than 3 minutes or flame front progression beyond the 50 mm reference mark is recorded as a burn response.
- If the specimen extinguishes before meeting the criterion, increase the oxygen concentration; if it burns past the criterion, decrease the oxygen concentration. Use coarse steps for initial bracketing, then fine increments of 0.2% oxygen for the final determination.
- Repeat the sequence across multiple specimens until the critical oxygen concentration is bracketed and the statistically defined LOI value is established, then verify the result with a confirmation run.
Calculation and Reporting
The LOI is reported as the volume percentage of oxygen in the oxygen/nitrogen mixture at the critical concentration, normally rounded to the nearest 0.1%. The test report should identify the standard used, specimen dimensions and thickness, conditioning, whether specimens were tested in the machine or transverse direction, the observed burning behavior such as dripping or afterglow, and any deviations or no-test situations, for example specimens that shrink or distort away from the flame.
Factors Influencing Results
Several factors can affect oxygen index results and should be controlled or documented. LOI decreases significantly as temperature rises, which is why elevated-temperature testing to ISO 4589-3 is valuable for materials used near heat sources. Specimen thickness, orientation, fillers, plasticizer migration, moisture content, and thermal aging all influence the measured value. Gas purity, flow calibration, chimney cleanliness, and consistent ignition technique are the main instrument-side variables. Under well-controlled conditions, repeatability within a single laboratory is typically on the order of 0.5% to 1.0% oxygen.
Applications
Oxygen index testing supports a broad range of industrial and regulatory needs:
- Development and screening of flame-retardant polymer formulations, including halogenated, phosphorus-based, mineral-filled, and intumescent systems
- Incoming material inspection and lot-release quality control for resin producers, compounders, and injection molders
- Demonstration of compliance with railway fire safety requirements, such as material requirement sets in EN 45545-2
- Qualification of cable insulation and sheathing compounds, electrical enclosures, connectors, and switchgear components
- Aerospace, automotive interior, mining, and construction material assessments where minimum oxygen index limits are specified
- Generation of material datasheet values; for reference, typical LOI values include approximately 17% to 18% for polypropylene and polyethylene, around 18% for polystyrene, 25% to 28% for polycarbonate, above 40% for rigid PVC, and greater than 95% for PTFE
- Complementary assessment alongside other flammability methods such as UL 94 horizontal and vertical burning tests and the IEC 60695-2 glow-wire test family
FAQ
What does the LOI value actually mean?
The LOI represents the minimum oxygen percentage needed to keep the material burning. Since air contains about 21% oxygen, a plastic with an LOI below 21% will burn in normal air, while values of 27% and above indicate a self-extinguishing tendency under the test criterion. LOI is best used for comparing materials under identical conditions rather than predicting full-scale fire performance on its own.
How is LOI testing different from UL 94 testing?
LOI quantifies flammability as a continuous oxygen concentration value under a controlled atmosphere, making it highly repeatable and ideal for ranking materials. UL 94 classifies burning behavior of specimens in ambient air, such as 5VA, V-0, V-1, V-2, and HB ratings. The two methods are complementary, and many specifications require both.
How many specimens are required and how long does testing take?
A full determination typically consumes 15 to 20 specimens. After conditioning is complete, the test itself is usually completed within a few hours, including the bracketing series, fine determinations, and verification runs.
Can the test be performed at temperatures above room temperature?
Yes. ISO 4589-3 defines the oxygen index determination at elevated temperatures, which is important for materials that operate near heat sources, because the LOI of most polymers decreases as temperature increases.
Do sample thickness and direction matter?
Yes. Thicker specimens generally burn differently from thin ones, and results should always be quoted with the tested thickness. For filled or reinforced plastics, results can differ between the machine direction and transverse direction, so the specimen orientation should be reported.
In summary, the plastic oxygen index combustion test is a precise, economical, and internationally standardized method for characterizing the flammability of polymeric materials. Accredited third-party laboratories equipped with calibrated oxygen index instruments can perform the test to ISO 4589-2, ASTM D2863, GB/T 2406.2, and related standards, providing manufacturers with reliable data for material development, quality control, and regulatory compliance.