Plastic Oxygen Index (LOI) Determination Test Report: Standards, Procedure, and Interpretation

The limiting oxygen index (LOI) test, performed in accordance with ISO 4589-2, ASTM D2863, and GB/T 2406.2, quantifies the minimum oxygen concentration in an oxygen/nitrogen mixture required to sustain candle-like combustion of a plastic specimen. This technical article provides a complete guide to oxygen index determination test reporting, covering sample preparation, apparatus, test procedure, calculation, and how the resulting LOI value is interpreted for flame-retardancy assessment in quality control and material development.

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 volume percent, in a flowing mixture of oxygen and nitrogen that will just support flaming combustion of a vertically mounted specimen under controlled laboratory conditions. Because ambient air contains approximately 20.95% oxygen by volume, the LOI value provides an immediate qualitative benchmark: plastics with an LOI of 21 or lower will generally continue to burn in ordinary air after ignition, while materials with LOI values of 27 or above are commonly regarded as self-extinguishing or flame retardant in normal atmospheres.

Oxygen index determination is one of the most widely used screening methods for evaluating the flammability of polymers. The test is highly reproducible, sensitive to small formulation changes such as flame-retardant loading or filler content, and relatively economical to perform, which makes it a standard tool in both research laboratories and accredited third-party laboratories. The LOI result is quantitative and is often reported on technical data sheets alongside complementary fire-test ratings such as UL 94 and IEC 60695-11-10 classifications.

A complete oxygen index determination test report documents the material identification, specimen geometry and preparation, conditioning history, the applicable standard and ignition method, the calculated LOI value, and all relevant burning observations such as dripping, afterglow, and char formation. This documentation is essential for traceability, comparison between material grades, and demonstration of compliance with customer or regulatory flammability requirements.

Testing Samples

Rigid Thermoplastics and Thermosets

Self-supporting rigid plastics are tested as rectangular bars. ISO 4589-2 and GB/T 2406.2 specify specimens of 80 mm to 150 mm in length, 10 mm in width, and 4 mm in thickness, while ASTM D2863 specifies bars of 70 mm to 150 mm long, 6.5 mm wide, and 3.0 mm thick. Specimens must have smooth, defect-free edges, since flash, burrs, or stress cracks can influence ignition behavior and distort the result.

Flexible and Non-Self-Supporting Materials

Elastomers, soft thermoplastics, and thin films that deform, curl, or cannot stand vertically on their own are tested using a support rod or a small frame so that the specimen remains upright in the gas flow. The support arrangement must be described in the test report because it can affect the observed burning behavior.

Cellular Materials and Foams

Rigid and flexible foams are typically tested with a larger cross section of approximately 10 mm by 10 mm to reflect the lower density and different heat-transfer behavior of cellular structures. The skin or molded surface is usually retained unless the standard or specification requires otherwise.

Fiber-Reinforced Composites and Laminates

Composite specimens are prepared at the thickness representative of the end use. Fiber orientation, resin content, and laminate construction strongly influence the LOI, so these parameters should be recorded in detail in the report to allow meaningful comparison between samples.

Specimen Quantity and Conditioning

A minimum of 15 test specimens is recommended for a determination using the normal technique, with additional spares reserved for materials that show high variability. Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity in accordance with ISO 291 for at least 88 hours before testing. Each specimen is marked with a reference line 50 mm below the top edge to support the pass/fail judgment during testing.

Testing Items

The core deliverable of an oxygen index test report is the ambient-temperature LOI value, but a professionally structured report typically covers the following items:

• Ambient-temperature limiting oxygen index at 23 °C, expressed as volume percent oxygen in nitrogen, reported to one decimal place.
• Confirmation of the ignition method used, normally top-surface ignition or propagation ignition as defined in the standard.
• Burning observations for each oxygen concentration step, including afterflame and afterglow behavior, flaming or non-flaming drips, char formation, curling, and shrinkage.
• Specimen geometry verification, including length, width, and thickness, together with the method of preparation such as injection molding, cutting, or machining.
• Conditioning history and any pre-treatment such as drying, annealing, thermal aging, or immersion, where required by the product specification.
• Optional elevated-temperature oxygen index testing to ISO 4589-3 or related methods, in which the LOI is determined at temperatures such as 70 °C, 125 °C, or higher, reflecting the fact that LOI decreases as ambient temperature rises.
• Instrument verification data, including gas-flow calibration and cross-check of the oxygen concentration using a calibrated oxygen analyzer or certified gas mixtures.

Testing Methods

Oxygen index determination is governed by several internationally recognized standards that are technically harmonized in principle but differ in specimen dimensions and certain procedural details:

• 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 (Oxygen Index).
• GB/T 2406.2, the Chinese national standard, which is technically aligned with ISO 4589-2.
• ISO 4589-3, covering oxygen index determination at elevated temperatures for materials where service temperature affects flammability.

Apparatus

The test is conducted in a dedicated oxygen index apparatus consisting of a vertical borosilicate glass chimney mounted on a U-shaped base containing a mixing chamber filled with glass beads to homogenize the gas mixture. Precision needle valves, calibrated rotameters or mass-flow controllers regulate the oxygen and nitrogen feed, and the mixed gas passes upward through the chimney at a linear velocity of approximately 40 mm/s. A specimen holder clamps the lower end of the bar so that it stands vertically on the chimney axis, and a small propane or butane pilot igniter is used to ignite the top of the specimen. A timer and a calibrated oxygen analyzer complete the setup.

Test Procedure

The determination follows a bracketing sequence to locate the transition oxygen concentration:

• Mount the conditioned specimen vertically in the holder and verify that the 50 mm reference mark is visible.
• Set an initial oxygen concentration, purge the chimney with the flowing mixture for at least 30 seconds, and allow the gas flow to stabilize.
• Apply the pilot flame to the top of the specimen using the specified ignition method for a maximum of 30 seconds, withdrawing the igniter as soon as sustained ignition is achieved.
• Record the result of that concentration: the response is a failure if flaming persists for 3 minutes or longer, or if the flame front reaches the 50 mm mark within 3 minutes; otherwise the specimen passes at that concentration.
• Adjust the oxygen concentration in progressively smaller steps, typically down to 0.2% by volume, alternately bracketing pass and fail responses until the transition point is bracketed and stabilized across the required number of specimens.
• Calculate the LOI from the final oxygen concentration and the step size using the normal technique (NT) formula defined in the standard, and report the result as a single oxygen index value.
• Repeat verification tests as needed and record all auxiliary observations, including dripping, afterglow, and specimen distortion, for inclusion in the report.

Report Content Requirements

A compliant test report identifies the material and its source, states the standard and ignition method used, describes specimen dimensions and preparation, records the conditioning atmosphere and duration, presents the final LOI value together with the step size, and notes any abnormal burning behavior. The testing laboratory, test date, and instrument identification are included to ensure full traceability. Because LOI is sensitive to thickness, the tested thickness must always accompany the numeric result to prevent misinterpretation.

Applications

Oxygen index test reports support a wide range of industrial and regulatory activities:

• Flame-retardant formulation development, including optimization of halogen-free systems based on phosphorus, nitrogen, intumescent chemistry, aluminum hydroxide, and magnesium hydroxide fillers, where LOI provides a fast, quantitative feedback signal.
• Incoming inspection and lot-release quality control of engineering plastics, wire and cable compounds, and insulation materials.
• Supporting evidence for compliance programs in electrical and electronic equipment, where LOI complements UL 94, glow-wire, and needle-flame tests under IEC 60695 series methodologies.
• Material selection and datasheet publication for construction products, rail and mass-transit components, automotive interiors, mining equipment, and aerospace applications, where specifications frequently define minimum LOI thresholds.
• Academic and industrial research into polymer combustion behavior, degradation mechanisms, and the temperature dependence of flammability.
• Comparative benchmarking of competing resin grades, recycled-content compounds, and upgraded flame-retardant variants during material qualification.

FAQ

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

As a practical guide, materials with an LOI below 21 burn readily in normal air, values between 21 and 27 indicate slow-burning or marginally self-extinguishing behavior, and values above 27 are generally classified as flame retardant. LOI values above 32 are typically required for demanding applications such as certain cable and public-transport components.

How does the oxygen index test differ from the UL 94 test?

The LOI test quantifies the minimum oxygen concentration needed to sustain combustion in a controlled atmosphere, producing a numerical value, while UL 94 rates burning behavior in ambient air using defined specimen orientations and afterflame timing. The two methods are complementary, and materials are frequently characterized with both.

Why is the 21% oxygen level significant?

Normal air contains approximately 20.95% oxygen. A material whose LOI is at or below this level can sustain flaming combustion after ignition in ordinary air, whereas a material with an LOI above 21 will self-extinguish when the ignition source is removed in an ambient atmosphere.

Does specimen thickness affect the LOI result?

Yes. Thicker specimens generally exhibit higher LOI values because more heat is conducted away from the burning zone relative to the mass of material involved. For this reason, the tested thickness must always be stated alongside the LOI value, and comparisons are only valid between specimens of the same geometry.

How many specimens are required for one determination?

A minimum of 15 specimens is recommended for the normal technique, which requires bracketing the pass/fail transition at a final step size of 0.2% oxygen. Additional specimens are advisable for anisotropic composites or materials with high lot-to-lot variability.

Can the oxygen index be measured at elevated temperatures?

Yes. ISO 4589-3 defines procedures for determining the LOI at elevated temperatures. Because LOI decreases as temperature rises, high-temperature testing is valuable for applications where components operate above ambient conditions.

Is LOI testing a pass/fail regulatory test?

LOI is primarily a quantitative screening and characterization method rather than a standalone regulatory pass/fail test. However, many product standards, industry specifications, and customer agreements define minimum LOI acceptance thresholds, in which case the reported value directly supports compliance decisions.

How should dripping behavior be handled in the report?

Dripping, whether flaming or non-flaming, does not change the numerical LOI calculation but is an important qualitative observation. Standards require that any dripping, afterglow, charring, or unusual specimen behavior be documented in the test report because it can be relevant to end-use fire risk assessment.