A plastic oxygen index test report documents the Limiting Oxygen Index (LOI) of polymeric materials, providing a quantitative measure of flame retardancy that is essential for product compliance, quality control, and material selection. This guide explains the testing principle, sample requirements, methodology, and reporting standards based on ISO 4589-2, ASTM D2863, and GB/T 2406.2, helping engineers and procurement professionals correctly interpret laboratory results.
Technical Overview
The oxygen index test, also known as the Limiting Oxygen Index (LOI) test, is one of the most widely used bench-scale methods for evaluating the flammability characteristics of plastics. The LOI is defined as the minimum concentration of oxygen, expressed as a volume percentage, in a flowing mixture of oxygen and nitrogen that will just support candle-like combustion of a vertically mounted test specimen.
Since ambient air contains approximately 20.9% oxygen by volume, the LOI value provides an intuitive benchmark for flammability assessment. A material with an LOI below 21% will continue to burn in normal atmospheric air after ignition, while a material with an LOI above 21% will self-extinguish under ambient conditions. Higher LOI values generally indicate greater flame retardancy.
A professional oxygen index test report is typically issued in accordance with the following internationally recognized standards:
- ISO 4589-2 – Plastics – Determination of burning behaviour by oxygen index – Part 2: Ambient-temperature test
- 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 industrial standard for oxygen index determination
- IEC 61144 – Oxygen index test applicable to cable materials
- ISO 4589-3 – Elevated-temperature oxygen index test (temperature index)
As a general classification convention widely applied in industry, materials can be roughly grouped as follows: LOI below 21% is considered easily flammable, 21% to 28% is considered slowly burning or flame-retarded, and above 28% is considered self-extinguishing. Many procurement specifications for cables, rail components, and electronic enclosures impose minimum LOI thresholds of 28% or 30%.
Testing Samples
Rigid and Self-Supporting Plastics
For molded or sheet materials that retain their shape during testing, ISO 4589-2 specifies preferred specimen dimensions of 80 mm to 150 mm in length, 10 mm in width, and 10 mm in thickness. Alternative geometries, such as 140 mm × 6.5 mm × 3 mm, are permitted when the preferred size cannot be prepared, although results from different geometries may not be directly comparable. ASTM D2863 permits similar ranges, and the report should always state the exact dimensions used.
Flexible Films and Thin Sheets
Flexible materials that deform, curl, or shrink when mounted vertically are tested using a special specimen holder or support frame, typically a U-shaped clip that holds the top of the specimen in place. The report must identify whether a support was used, as this affects the validity of cross-comparisons between datasets.
Cellular Plastics and Foams
Rigid cellular materials are tested with larger cross-sections, commonly 150 mm × 50 mm × 13 mm, in accordance with foam-specific annexes of the relevant standards. The anisotropic cell structure of foams means that specimen orientation relative to the rise direction should be recorded in the report.
Sample Quantity and Conditioning
A complete LOI determination normally requires a minimum of 15 specimens to account for the bracketing procedure and confirmation burns. Specimens should be marked with reference lines (50 mm from the top for ISO, or gauge marks at specified intervals for ASTM) to assess flame spread distance. Unless otherwise specified, samples are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 88 hours prior to testing.
Testing Items
A standard plastic oxygen index test report covers the following core testing items:
- Ambient-temperature Limiting Oxygen Index (LOI) – the primary result, reported as a volume percentage of oxygen
- Oxygen concentration step interval (d) – the interval between oxygen concentrations used in the final determination, required for statistical validity
- Burning behavior observations – afterflame time, extent of burning, dripping, charring, and whether specimens meet pass/fail criteria
- Temperature index (optional) – LOI determined at elevated temperatures per ISO 4589-3, often required for cable and rail applications
- Material identification data – polymer type, grade, colorant content, flame-retardant system, density, and specimen orientation
Some accredited third-party laboratories also offer complementary flammability testing alongside the oxygen index, such as UL 94 horizontal and vertical burn tests, glow-wire tests per IEC 60695-2 series, and ignition temperature determinations, allowing a more complete flammability profile in a single test program.
Testing Methods
The oxygen index test is performed using a vertical glass chimney of approximately 450 mm to 500 mm in height, into which a precisely controlled mixture of oxygen and nitrogen flows at a total rate defined by the standard. The specimen is clamped vertically in the center of the column, and the top of the specimen is ignited using a small propane flame igniter applied in a defined manner.
The test procedure follows the general sequence below:
- Condition and mark the specimens, then mount the first specimen in the holder
- Establish the initial oxygen/nitrogen flow at a starting concentration estimated from experience or preliminary trials
- Purge the chimney with the gas mixture, then ignite the specimen top and observe the burning behavior
- Record whether the flame extinguishes before the criterion is reached (minimum 3 minutes of burning or 50 mm of specimen consumed for ISO 4589-2)
- Adjust the oxygen concentration upward if the specimen self-extinguishes prematurely, or downward if it burns beyond the criterion
- Use the bracketing approach to converge on the critical concentration, then verify the result with the specified nitrogen-balanced confirmation runs
The LOI is calculated from the final oxygen concentration, commonly expressed as LOI = 100 × V(O₂) / [V(O₂) + V(N₂)]. The reported value is rounded to one decimal place. The report should also include the estimated standard deviation derived from the oxygen concentration interval and the number of response observations, in accordance with the statistical annex of the standard.
A compliant test report includes the laboratory scope and accreditation information, the complete standard designation and edition used, full specimen descriptions and conditioning history, the test atmosphere, the calculated LOI value with its concentration step, and observations of individual specimen behavior. Any deviations from the standard, such as the use of specimen supports, must be explicitly documented.
Applications
Oxygen index test reports support a broad range of engineering and regulatory applications:
- Wire and cable industry – LOI requirements for insulation and sheathing compounds are specified in many national cable standards and utility procurement specifications
- Electrical and electronics – screening of enclosure, connector, and component materials for flammability compliance alongside UL 94 ratings
- Construction products – supporting evidence for reaction-to-fire evaluation of plastic pipes, panels, membranes, and insulation materials
- Rail transit and aerospace – material qualification programs where LOI thresholds and temperature index values are mandated by industry specifications
- Automotive engineering – evaluation of interior and under-hood polymer components
- Research and development – quantitative comparison of flame-retardant formulations, with LOI serving as a sensitive indicator of additive performance
- Quality control – incoming material inspection and batch-to-batch consistency verification for flame-retarded compounds
Because LOI is a small-scale laboratory result, it should not be interpreted in isolation as a predictor of full-scale fire hazard. Regulatory frameworks typically require it to be combined with other reaction-to-fire or fire-resistance data, and the test report itself often forms part of a larger technical file.
FAQ
What is considered a good LOI value for plastics?
Values above 21% indicate the material will not sustain combustion in normal air. For demanding applications such as cable sheathing, rail components, and aircraft interiors, specifications frequently require LOI values of 28% to 32% or higher. Highly flame-retarded compounds, including many halogen-free formulations, can achieve LOI values above 35%.
How is LOI different from the UL 94 test?
The LOI test measures the minimum oxygen concentration needed to sustain burning and yields a numerical result, while UL 94 evaluates ignition from a small flame and classifies materials into ratings such as V-0, V-2, 5VA, and HB. The two tests assess different aspects of flammability and are frequently requested together, as they are complementary rather than interchangeable.
How many samples are needed for a complete test?
At least 15 specimens are typically required for one full LOI determination using the bracketing method. If elevated-temperature testing is also required, additional specimen sets are necessary. Clients submitting thin or flexible materials should consult the laboratory in advance regarding support fixtures and minimum sample quantities.
How long does the oxygen index test take?
Once conditioning is complete, the combustion testing itself is usually completed within one working day, and reports are generally issued within three to five working days by accredited third-party laboratories, depending on the number of materials and whether complementary tests are included in the same program.
Does the report have an expiration date?
The test report itself documents the results for the specific samples submitted and does not formally expire. However, certification schemes and purchasing agreements often limit the age of acceptable test reports to two or three years, and any change to the material formulation or manufacturing process technically invalidates the previous result.
Which standard should be specified when requesting a test?
ISO 4589-2 and ASTM D2863 are broadly equivalent in principle but differ in detail; GB/T 2406.2 is aligned with the ISO method. The correct choice depends on the end market and the specification referenced by the buyer or regulator, so the applicable standard should be confirmed before samples are submitted to avoid retesting.