Limiting Oxygen Index (LOI) Testing of Plastic Sheets and Boards

The limiting oxygen index (LOI) test is the fundamental laboratory method for evaluating the flammability of plastic sheets and boards, measuring the minimum oxygen concentration in an oxygen/nitrogen mixture that will just sustain combustion of a vertically mounted specimen. Performed in accordance with ISO 4589-2, ASTM D2863, GB/T 2406.2 and JIS K 7201-2, it is widely used to verify whether plastic sheet materials meet flame-retardant requirements in construction, rail transit, electronics and other regulated industries. This article explains the test principle, specimen requirements, procedure, calculation, acceptance criteria and typical LOI values for common plastics.

Technical Overview

The limiting oxygen index (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 flaming combustion of a vertically held test specimen after it has been ignited from its top edge. Because ambient air contains approximately 20.9% oxygen, the LOI value directly indicates how a plastic sheet will behave in air: materials with an LOI of 21 or lower will continue to burn once ignited, while materials with higher LOI values tend to self-extinguish. The higher the LOI, the better the flame-retardant performance of the material.

LOI testing of plastic sheets and boards is primarily governed by the following standards, which are technically aligned in principle and apparatus:

• ISO 4589-2 — Plastics, determination of burning behaviour by oxygen index, ambient-temperature test
• ASTM D2863 — Standard test method for measuring the minimum oxygen concentration to support candle-like combustion of plastics (oxygen index)
• GB/T 2406.2 — Chinese national standard, equivalent to ISO 4589-2
• JIS K 7201-2 — Japanese industrial standard for the oxygen index method
• ISO 4589-3 — Elevated-temperature oxygen index determination

As a quantitative and reproducible measure, the LOI test is used by materials engineers, certification bodies and accredited third-party laboratories for quality control, flame-retardant formulation development, incoming material inspection and compliance demonstration against product specifications and fire-safety regulations.

Testing Samples

Rigid Self-Supporting Plastic Sheets

For rigid thermoplastic and thermoset sheets such as PP, PE, ABS, PVC, PC, PMMA, PA and FRP panels, bar specimens are cut from the delivered sheet. The preferred specimen under ISO 4589-2 is 80–150 mm long, 10 mm wide and 4 mm thick (Type I), while ASTM D2863 typically uses 70–150 mm long, 6.5 mm wide and 3 mm thick bars. Sheets are normally tested at the as-received thickness within the permitted range (approximately 3–10 mm), and because the LOI value depends on thickness, results are only comparable between specimens of identical dimensions.

Thin and Flexible Sheet Materials

Thin films, membranes and flexible sheets that cannot stand upright on their own are tested using a support frame or a special holder specified in the standards, so that the specimen remains vertical in the combustion chimney without artificial reinforcement that would alter the burning behaviour.

Cellular and Foamed Plastic Sheets

For foam boards and cellular plastic sheets such as XPS, PU and PVC foam panels, specimens with a larger cross-section (typically an 80–150 mm length with a 10–15 mm square section) are used. The effect of surface skins versus cut cores should be considered, as the presence or absence of a skin layer can measurably change the oxygen index result.

Specimen Quantity and Conditioning

A minimum of 15 specimens is generally required, because the oxygen concentration must be bracketed through several ignitions. Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 40 hours before testing, since absorbed moisture can significantly influence the combustion behaviour of many plastics.

Testing Items

The core deliverable of the test is the ambient-temperature limiting oxygen index of the plastic sheet, reported as a percentage to one decimal place. Depending on the specification, the test scope may also include:

• Ambient-temperature LOI at 23 °C per ISO 4589-2, ASTM D2863, GB/T 2406.2 or JIS K 7201-2
• Elevated-temperature LOI per ISO 4589-3, relevant for materials used in hot environments
• Temperature index determination, i.e. the temperature at which the LOI falls to 21, which approximates the upper service temperature for self-extinguishing behaviour
• Compliance screening against defined thresholds, such as LOI ≥ 27, LOI ≥ 28 or LOI ≥ 32 required by particular product or industry specifications
• Comparison of different grades, thicknesses or flame-retardant formulations for material selection and R&D purposes

Testing Methods

The test is conducted in an oxygen index apparatus consisting of a heat-resistant glass combustion chimney (typically at least 450 mm high and 75 mm in diameter), a vertical specimen holder, a gas mixing and metering system for oxygen and nitrogen, calibrated flow controllers or an oxygen analyzer, and a small propane or butane ignition burner.

The general test procedure is as follows:

• Condition and measure the specimens, then mark reference positions on each bar to allow assessment of the burned length.
• Clamp the specimen vertically in the holder so that the top of the specimen is at a defined position below the chimney outlet.
• Set an initial oxygen concentration based on the expected flammability of the material and purge the chimney with the gas mixture for at least 30 seconds.
• Apply the igniter flame to the top of the specimen to ignite the upper portion, following the ignition method specified in the standard.
• Remove the igniter and observe the combustion: the response is recorded as "burns" if flaming continues for more than 180 seconds or if the flame front passes a mark 50 mm from the top, otherwise it is recorded as "extinguishes".
• Using the bracketing (up-and-down) procedure, increase the oxygen concentration after an "extinguishes" response and decrease it after a "burns" response, repeating with fresh specimens and progressively smaller concentration steps.
• Calculate the limiting oxygen index from the final series of measurements as LOI = 100 × V(O2) / [V(O2) + V(N2)], where V denotes the volumetric flow rates of oxygen and nitrogen.
• Report the LOI value together with the standard used, specimen dimensions and orientation, conditioning, ignition method and any observed burning phenomena such as dripping, glowing or curling.

The LOI result is a property of the material-geometry combination rather than an intrinsic constant, so consistent specimen preparation and laboratory control are essential for reproducibility. Typical approximate LOI ranges for common sheet materials include: PP and PE at 17–18%, PS at 17–19%, ABS at 18–19%, PMMA at 17–18%, PA at 24–28%, PC at 25–28%, flame-retardant modified grades at 28–40%, rigid PVC at 40% or above, and PTFE above 90%. As a general interpretation, values below 21 indicate the material burns readily in air, values of 21–24 indicate slow candle-like burning, values of 25–27 indicate self-extinguishing tendencies, and values of 28 and above are commonly accepted as flame retardant.

Applications

LOI testing of plastic sheets and boards supports fire-safety compliance and material qualification across many industries:

• Building and construction: decorative wall panels, ceiling boards, partitions and cladding sheets, supporting fire-performance classification of building products under regulations such as GB 8624 and EN 13501 contexts
• Rail and mass transit: interior panels, seat components and trim sheets for trains, metros and buses, where specifications frequently require LOI values of 32 or higher
• Electrical and electronics: enclosures, switchgear linings, insulating boards and printed-circuit substrates evaluated alongside IEC and UL based flammability requirements
• Shipbuilding and mining: low-flame-spread sheet materials for cabins, ducting and cable management systems
• Furniture and appliances: engineering panels and laminates requiring reduced ignitability
• Research, development and quality control: screening flame-retardant masterbatches, optimizing filler and additive levels, incoming inspection of resin lots and supplier qualification by manufacturers and accredited third-party laboratories

FAQ

What is considered a good LOI value for a flame-retardant plastic sheet?

A value of 28% or above is generally regarded as flame retardant, since normal air contains only about 21% oxygen. Many rail-transit and high-specification applications demand an LOI of 32% or higher, while general engineering specifications commonly cite thresholds of 27–28%. The exact requirement depends on the applicable product standard or contract specification.

Which standards should be specified when ordering an LOI test?

ISO 4589-2, ASTM D2863, GB/T 2406.2 and JIS K 7201-2 are the principal ambient-temperature methods and are closely harmonized; ISO 4589-3 is used for elevated-temperature testing. Because results between standards and specimen thicknesses are not directly interchangeable, the required standard and the sheet thickness should always be stated explicitly on the test request.

How many specimens are needed and how are they prepared?

At least 15 specimens are normally required because the bracketing procedure consumes several bars before the final oxygen concentration is confirmed. Specimens are cleanly cut from the sheet with square edges, free of burrs and cracks, and conditioned at 23 °C and 50% relative humidity for a minimum of 40 hours before testing.

Does specimen thickness affect the LOI result?

Yes. Thicker specimens usually produce higher LOI values because more heat is conducted away from the flame zone relative to the burning mass. LOI data are only valid for the geometry tested, which is why reports must always state the specimen dimensions and thickness.

How does LOI testing differ from the UL 94 test?

The LOI test measures the minimum oxygen concentration needed to sustain burning and yields a continuous numerical result, whereas UL 94 assesses burning behavior, afterflame time and dripping in normal air and assigns a classification such as V-0, V-1 or V-2. The two methods evaluate different aspects of flammability and are frequently specified together to fully characterize a plastic sheet.

Can the test be performed at elevated temperatures?

Yes. ISO 4589-3 extends the oxygen index method to elevated temperatures, which is important for sheets used near engines, electrical equipment or hot process environments. The temperature index derived from such testing indicates the temperature at which the material's LOI drops to 21, the practical limit of self-extinguishing behavior in air.

What information appears in a typical LOI test report?

A compliant report includes the test standard, material identification and grade, specimen dimensions and orientation, conditioning history, ignition method, individual oxygen concentrations and specimen responses, the final LOI value to one decimal place, observations such as dripping or glowing combustion, and the temperature and humidity of the laboratory.