Limiting Oxygen Index (LOI) Test for Plastics: Apparatus, Samples, and Step-by-Step Experimental Pro

The limiting oxygen index (LOI) test is the standard laboratory method for quantifying the flammability of plastic materials by determining the minimum oxygen concentration, in an oxygen/nitrogen mixture, that will just support candle-like combustion of a vertically mounted specimen. Performed in accordance with ISO 4589-2, ASTM D2863, and GB/T 2406.2, the oxygen index value is widely used for flame-retardant material development, quality control, and compliance with fire-safety specifications in the electronics, cable, railway, and construction industries. This article explains the testing principle, specimen requirements, apparatus, and the complete step-by-step experimental procedure for measuring the oxygen index of plastics.

Technical Overview

The limiting oxygen index (LOI), also known as the critical oxygen index, is defined as the minimum concentration of oxygen in a flowing oxygen/nitrogen mixture, expressed as a volume percentage, that will just support combustion of a vertically held specimen after ignition with a small external flame. The evaluation criterion is either a burning time exceeding 180 seconds or a flame front traveling a specified distance, typically 50 mm, down the specimen. The result is calculated from the gas flow rates:

• LOI = 100 x V(O2) / [V(O2) + V(N2)], expressed in percent
• Ambient air contains approximately 20.9 percent oxygen, so plastics with an LOI above 21 percent will not sustain steady burning in a normal atmosphere and are generally classified as self-extinguishing
• Burning proceeds downward from the ignited top of the specimen, in a candle-like mode, while a controlled gas mixture flows upward through a vertical glass chimney

The principal standards governing the procedure are ISO 4589-2 (plastics, ambient temperature), ASTM D2863, and the Chinese national standard GB/T 2406.2, which is technically aligned with ISO 4589-2. ISO 4589-3 extends the same method to elevated temperatures, because the oxygen index of most polymers decreases as temperature rises. Typical LOI reference values include polypropylene and polyethylene at approximately 17 to 18 percent, polystyrene and ABS at approximately 18 to 19 percent, polycarbonate at approximately 25 to 27 percent, rigid PVC at approximately 45 percent, and PTFE above 90 percent. Accredited third-party laboratories routinely perform this test as part of material qualification programs.

Testing Samples

The oxygen index result is sensitive to specimen geometry, particularly thickness, so the dimensions and type of specimen must always be stated in the test report. The main specimen categories are summarized below.

Rigid and Self-Supporting Plastics

Molded or sheet materials are tested as rectangular bars. ISO 4589-2 Type I specimens measure 80 to 150 mm in length, 10 mm in width, and 4 mm in thickness. ASTM D2863 specifies 70 to 150 mm in length, 6.5 mm in width, and 3 mm in thickness. Alternative thicknesses of 10 mm and 2 mm are covered by other ISO specimen types for comparison of thickness effects.

Flexible Films and Thin Sheets

Thin, limp materials that cannot stand vertically are prepared as Type V specimens, typically 140 mm x 52 mm, rolled along the long axis and mounted on a thin support rod so that a defined burning geometry is maintained inside the chimney.

Cellular Materials and Foams

Flexible and rigid foams are cut to the dimensions defined by the applicable standard and are supported so that they do not deform during the test; the skin and core orientation must be recorded because it influences the measured index.

Specimen Conditioning and Preparation

Specimens shall be free of flash, burrs, and visible defects. Unless otherwise specified, condition them for at least 40 hours at 23 +/- 2 degrees Celsius and 50 +/- 5 percent relative humidity before testing. Mark a reference line 50 mm below the top end of each specimen. Prepare a minimum of 15 specimens, and preferably 20, because the bracketing procedure consumes several specimens before the final series is obtained. For anisotropic materials such as laminates and fiber-reinforced plastics, prepare separate sets in the machine and transverse directions.

Testing Items

The primary deliverable of the experiment is the limiting oxygen index value, supported by qualitative observations of burning behavior. Typical reporting items include:

• Limiting oxygen index in percent, rounded to one decimal place
• Ignition method used: top-surface ignition (Method A) or propagation ignition (Method B)
• Specimen dimensions, thickness, and orientation
• Observations such as dripping of flaming particles, afterflame and afterglow behavior, charring, and swelling
• Assessment against specification limits, for example a minimum LOI of 26 or 28 percent required by certain product safety specifications

Testing Methods

Apparatus and Reagents

A standard oxygen index apparatus consists of the following components, whose calibration status must be verified before the test campaign begins:

• A vertical heat-resistant glass chimney, typically about 500 mm high with an internal diameter of at least 75 mm, fitted with a base gas distributor to ensure homogeneous flow
• A specimen holder that clamps the lower end of the specimen and holds it vertically on the chimney axis
• A gas mixing and delivery system with calibrated flowmeters or mass flow controllers for oxygen and nitrogen, and optionally an oxygen analyzer to verify the mixture concentration
• Supplies of oxygen and nitrogen of the purity specified by the standard
• A small igniting burner using propane or another suitable fuel, producing a low-luminosity flame
• A timer and a scale or marked gauge for judging the 50 mm burning extent

Test Environment and Pre-Test Checks

Carry out the test in a draught-free environment at standard laboratory atmosphere. Verify the calibration of the flow measuring devices, check the oxygen analyzer against atmospheric air (approximately 20.9 percent) or a certified calibration gas, and confirm that the chimney is clean and free of soot deposits from previous tests, as contamination alters the flow field and heat feedback to the specimen.

Step-by-Step Experimental Procedure

The complete sequence of operations is as follows:

• Step 1: Select the applicable standard and specimen type, and record the nominal dimensions and mean thickness of each conditioned specimen.
• Step 2: Clamp the specimen vertically in the holder so that its top end is at least 100 mm below the top rim of the chimney.
• Step 3: Set an initial oxygen concentration based on prior knowledge of the material, set the total gas flow to give a linear velocity through the chimney of 40 +/- 2 mm per second, and purge the chimney with the mixture for at least 30 seconds.
• Step 4: Ignite the specimen. For top-surface ignition, apply the burner flame to the top of the specimen for up to 15 seconds, applying and withdrawing it at about 5 second intervals so only the top surface is ignited. For propagation ignition, tilt the burner and apply the flame to the top surface and the upper portion of the vertical face for approximately 5 seconds.
• Step 5: Remove the igniting flame, start the timer, and observe the burning behavior without disturbing the gas flow.
• Step 6: Record the individual response as "O" if the burning time equals or exceeds 180 seconds, or if the flame front reaches or passes the 50 mm mark; otherwise record the response as "X". Note any dripping, afterglow, or charring.
• Step 7: Adjust the oxygen concentration for the next specimen. If the previous response was "O", decrease the oxygen concentration; if it was "X", increase it. Use coarse increments of about 1 percent until the response reverses, then reduce the increment, typically to 0.5 percent and finally to 0.2 percent.
• Step 8: Continue the bracketing sequence at the fine increment until a complete final series of alternating responses is obtained; the standards define the exact statistical treatment, which in practice means the oxygen index is derived from the last six responses measured at the fine increment.
• Step 9: Calculate the LOI as the arithmetic mean of the oxygen concentrations of the final series of responses, rounded to one decimal place.
• Step 10: Repeat the sequence with fresh specimens as required, and compile the full report including standard, specimen description, conditioning, ignition method, individual responses, and the final LOI value.

Calculation Example

If the limiting mixture is achieved with an oxygen flow of 2.80 L/min and a nitrogen flow of 7.70 L/min, the total flow is 10.50 L/min and LOI = 100 x 2.80 / 10.50 = 26.7 percent. This value indicates that the material requires an oxygen-enriched atmosphere to keep burning and will self-extinguish in ordinary air.

Common Interferences and Precautions

• Use gases of the specified purity; contaminated nitrogen can shift the measured index by a significant margin
• Never reuse a specimen that has been ignited, even if it did not meet the "O" criterion
• Clean soot from the chimney between campaigns, since deposits change the radiant environment
• Report the thickness explicitly, because LOI generally increases with thickness due to the greater heat sink effect
• Maintain constant total flow; changing the flow velocity changes the heat transfer around the specimen and biases results

Applications

Oxygen index data support flammability assessment across many sectors and are frequently combined with burning-rate tests such as the UL 94 vertical and horizontal burn tests for a complete material qualification package. Typical applications include:

• Development and optimization of halogen-free and mineral-filled flame-retardant polymer formulations
• Quality control of flame-retarded compounds, cable insulation, and cable sheathing materials
• Compliance evidence for railway rolling stock material requirements under EN 45545-2 and for aircraft interior material specifications
• Screening of engineering plastics for electrical and electronic enclosures and components
• Academic research into polymer combustion mechanisms and char-forming additives
• Comparative benchmarking of incoming materials by manufacturers and accredited third-party laboratories

FAQ

What oxygen index value is considered flame retardant?

As a practical guideline, materials with an LOI below 21 percent burn readily in air, values between 21 and 27 percent indicate slow-burning or self-extinguishing behavior, and values above 27 percent are generally regarded as flame retardant. Many product specifications set explicit minima, commonly 26, 28, or 30 percent, depending on the application and hazard scenario.

How is the LOI test different from the UL 94 test?

The LOI test measures a quantitative property, namely the oxygen concentration needed to sustain combustion, and gives a continuous numerical result. The UL 94 test rates the burning behavior of a specimen in ambient air, including afterflame time and flaming drips, and yields a classification such as V-0 or HB. The two methods are complementary and are often both specified.

How many specimens are needed for one determination?

A minimum of 15 specimens is normally required, and 20 is preferred, because several specimens are consumed during the coarse bracketing steps before the final fine-increment series begins. Highly variable or anisotropic materials may require additional specimens.

Why does the LOI depend on specimen thickness?

Thicker specimens act as a larger heat sink and lose combustion heat by conduction, so more oxygen is needed to keep them burning. For this reason, LOI values are only comparable when specimen dimensions, ignition method, and test standard are identical, and the thickness must always accompany the reported value.

Can the test be performed at elevated temperature?

Yes. ISO 4589-3 defines the oxygen index determination at elevated temperatures using a heated chimney, which is important for materials used in warm environments because the oxygen index decreases as temperature increases.

Are ISO 4589-2, ASTM D2863, and GB/T 2406.2 results interchangeable?

The test principle is the same, and results are usually close, but small differences in specimen dimensions, chimney geometry, and procedural details can cause deviations of one percent or more in the oxygen index. The report must always identify the exact standard used, and comparison against contractual limits should be based on the specified method.