The limiting oxygen index (LOI) test is one of the most widely used methods for evaluating the combustion characteristics of plastic materials, and the reliability of its results depends heavily on properly prepared specimens. This article explains the specimen dimensions, conditioning, and preparation requirements specified in ISO 4589-2, ASTM D2863, and GB/T 2406.2, helping manufacturers and accredited third-party laboratories obtain repeatable and comparable oxygen index data.
Technical Overview
The limiting oxygen index (LOI), also referred to as the oxygen index (OI), is defined as the minimum concentration of oxygen in a flowing mixture of oxygen and nitrogen that will just support flaming combustion of a vertically mounted plastic specimen under controlled laboratory conditions. The result is expressed as a volume percentage, calculated as 100 multiplied by the oxygen volume divided by the total gas volume. Because normal ambient air contains approximately 20.9 percent oxygen, plastics with an LOI below 21 percent tend to continue burning in air once ignited, while materials with higher oxygen index values exhibit greater resistance to ignition and flame spread.
The test is governed internationally by ISO 4589-2 and ASTM D2863, with GB/T 2406.2 as the corresponding Chinese national standard and JIS K 7201-2 as the Japanese equivalent. General guidance is provided in ISO 4589-1, while ISO 4589-3 extends the method to elevated temperatures. Since LOI results are highly sensitive to specimen geometry, material preparation, and conditioning history, strict compliance with specimen requirements is essential for reproducible data.
Testing Samples
Rigid, Self-Supporting Plastics
For injection-molded, compression-molded, or machined rigid specimens, the standard dimensions are as follows:
- ISO 4589-2 and GB/T 2406.2: length 80 mm to 150 mm, width 10 mm ± 0.5 mm, thickness 4 mm ± 0.25 mm
- ASTM D2863: length 70 mm to 150 mm, width 6.5 mm ± 0.5 mm, thickness 3 mm ± 0.5 mm
- Edges shall be smooth and squared, free from burrs, cracks, flash, and surface defects
- A gauge mark shall be applied 50 mm from the ignition (top) end of each specimen
Flexible Films and Thin Sheets
Non-self-supporting films cannot be clamped directly in the standard holder. A rectangular strip, typically 140 mm long and 52 mm wide, is folded lengthwise and secured so that the effective specimen width is approximately 20 mm, maintaining a defined geometry throughout ignition and burning. Special holders or wire supports are used to keep the folded film stable inside the chimney.
Cellular (Foamed) Plastics
Expanded and foamed materials are tested with a larger cross section, generally 80 mm to 150 mm long, 10 mm ± 0.5 mm wide, and 10 mm ± 0.5 mm thick. Specimens must be cut cleanly without crushing, compressing, or heat-sealing the cells, because damaged cell structures significantly alter the burning behavior and the measured index.
Specimen Quantity, Conditioning, and Preparation
- At least 15 specimens are required per test, and laboratories typically prepare 15 to 20 plus spares for preliminary trials
- Condition at 23 °C ± 2 °C and 50 percent ± 5 percent relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618
- All specimens of one material shall be prepared identically, with molding or machining parameters documented
- Thickness shall be measured at several points and reported; material orientation and anisotropy shall be noted where relevant
- Surfaces shall be clean, dry, and free of oil and dust; solvents that swell or attack the polymer shall not be used for cleaning
- Specimens shall be representative of the production lot and taken from areas free of voids, sink marks, and color streaks
Testing Items
Typical oxygen index test items include:
- Limiting oxygen index at ambient temperature (23 °C), the primary measurement for most specifications
- Oxygen index at elevated temperatures in accordance with ISO 4589-3, commonly applied to heat-resistant engineering plastics and cable materials
- Temperature index, defined as the temperature at which the LOI of the material drops to 21 percent
- Supplementary observations such as dripping, charring, afterflame time, and afterglow behavior, which are recorded in the test report
Testing Methods
The test is performed in a vertical glass chimney, typically about 500 mm tall, using a specimen holder, an oxygen and nitrogen metering and mixing system, and a small gas flame igniter. The specimen is mounted vertically at the center of the chimney, and the general procedure is as follows:
- Purge the chimney with the gas mixture at the specified total flow rate, corresponding to a linear gas velocity of approximately 40 mm/s, for at least 30 seconds before ignition
- Apply the ignition flame to the top of the specimen for up to 5 seconds (top ignition), or use spread ignition for materials that melt, shrink, or draw away from the flame
- A specimen is judged to have burned if flaming persists for more than 180 seconds or if the flame front reaches or passes the 50 mm gauge mark; otherwise the result is recorded as no burning
- Adjust the oxygen concentration in progressively smaller steps, typically down to increments of 0.2 percent, following the pass/fail bracketing sequence defined in the standard until the critical concentration is bracketed
- Calculate the LOI as the volume percentage of oxygen at the critical concentration and round it according to the requirements of the applicable standard
The test report shall state the standard used, the specimen type and dimensions, material orientation, ignition mode, conditioning details, the oxygen index value, and any observed burning behavior such as dripping or afterglow. Because within-laboratory repeatability is typically better than inter-laboratory reproducibility, comparisons between datasets are valid only when specimen preparation and test conditions are identical.
Applications
Oxygen index data are used across many industries for material selection, product qualification, and regulatory compliance:
- Development and quality control of flame-retardant compounds, masterbatches, and filled polymer grades
- Railway rolling stock applications, where EN 45545-2 references ISO 4589-2 and sets minimum LOI limits for many requirement sets
- Wire and cable insulation and sheathing materials for electrical and construction installations
- Enclosures and components of electrical and electronic equipment, including engineering plastics for household appliances
- Interior and functional materials for automotive, shipbuilding, mining, and aerospace applications
- Academic research and comparative flammability screening of polymeric materials
FAQ
How many specimens are needed for one oxygen index test?
A minimum of 15 specimens is normally required to complete the bracketing sequence, and it is good practice to prepare additional spare specimens for preliminary trials and for materials with erratic burning behavior.
Can specimens be tested at their actual product thickness?
Yes, for informational purposes, but the LOI value depends on specimen thickness, so valid comparisons require identical dimensions. ASTM D2863 provides dedicated specimen types for testing thicker materials as received, and any nonstandard thickness must be clearly reported with the result.
What LOI value indicates a flame-retardant plastic?
Materials with an LOI above 21 percent will not burn easily in normal air. In industrial practice, an LOI of 27 percent or higher is commonly regarded as self-extinguishing or flame retardant, while many railway and cable specifications require values of 28 percent to 32 percent or more.
Are ISO 4589-2 and ASTM D2863 results interchangeable?
No. The two standards use different specimen dimensions and chimney geometries, which can produce systematically different values. The applicable standard shall always be stated when an LOI result is reported or quoted in a specification.
What should be done when specimens melt, drip, or shrink away from the flame?
The spread ignition mode and appropriate wire supports or special holders shall be used, and the observed behavior, such as dripping or flaming particles, shall be documented in the report so the result can be correctly interpreted.