Determination of the Limiting Oxygen Index (LOI) of Plastics: Test Principles, Standards, and Data I

The limiting oxygen index (LOI) is a fundamental flammability parameter that quantifies the minimum concentration of oxygen in an oxygen/nitrogen mixture required to just sustain candle-like combustion of a plastic specimen. LOI data, determined in accordance with ISO 4589-2, ASTM D2863, and GB/T 2406.2, are widely used for flame-retardant material development, quality control, and compliance screening across the plastics industry. This article explains the measurement principle, sample requirements, test procedures, data calculation, and typical applications of LOI determination for plastic materials.

Technical Overview

The limiting oxygen index (LOI), also called the oxygen index (OI) or critical oxygen index, is defined as the minimum volumetric concentration of oxygen in a flowing oxygen/nitrogen mixture that will just support downward, candle-like flaming combustion of a vertically mounted plastic specimen under specified laboratory conditions. The result is expressed as a percentage of oxygen by volume, usually to one decimal place.

Because ambient air contains approximately 20.9% oxygen, the LOI value provides an intuitive benchmark: a material with an LOI below 21% will generally continue to burn in still air once ignited, while a material with an LOI above 21% tends to self-extinguish when the external flame is removed. Higher LOI values therefore indicate greater intrinsic resistance to sustained flaming combustion at room temperature.

• LOI is a small-scale laboratory screening test, not a direct assessment of full-scale fire hazard.
• The test compares the ease of extinction of different materials under identical, well-controlled conditions.
• LOI data are frequently reported on technical datasheets and referenced in material specifications for electrical, rail, cable, and construction applications.
• The method is applicable to rigid solids, flexible films, cellular plastics (foams), and fiber-reinforced composites in their end-use state.

Typical LOI ranges for common polymers illustrate the breadth of the scale: polyoxymethylene (POM) approximately 15%, polyethylene (PE) and polypropylene (PP) approximately 17–18%, polystyrene (PS) and ABS approximately 17–20%, polyamide 66 approximately 24–29%, polycarbonate (PC) approximately 26–28%, rigid PVC approximately 42–49%, and PTFE above 95%. Flame-retardant formulations are commonly engineered to reach LOI values of 28% or higher.

Testing Samples

Self-Supporting Rigid Plastics

Molded or machined bars that support their own weight are tested in vertical holders. ISO 4589-2 defines several specimen types, with the preferred Type I specimen measuring 80–150 mm in length, 10 mm in width, and 4 mm in thickness. Alternative types cover 10 mm thick and 2 mm thick bars, as well as 6.5 mm × 3 mm sheet specimens. Whenever possible, specimens should be taken from production parts or test plaques so that the measured LOI reflects the material in its actual processed state.

Flexible Films and Thin Sheets

Thin films that cannot stand upright are prepared as rolled specimens wound onto a mandrel or supported in special frames, allowing the top edge to be ignited in the same manner as rigid bars. Film testing is common for packaging laminates, battery separators, and insulation films where thickness strongly influences the burning behavior.

Cellular Materials (Foams)

Rigid and flexible foams are tested using support frames that hold the specimen vertically without restricting the flame front. Because foam density and cell structure affect the result, the sample report should always record the foam grade, density, and nominal thickness.

Composites and Laminates

Glass-fiber and carbon-fiber reinforced laminates, prepreg stacks, and multilayer boards can be tested as self-supporting bars. Specimen orientation relative to the reinforcement layers should be documented, since delamination and edge effects can influence the burning time and damaged length.

Specimen Conditioning and Quantity

Specimens are conditioned prior to testing, typically for at least 40 hours at 23 ± 2 °C and 50 ± 5% relative humidity in accordance with ISO 291 or the equivalent conditioning clause of the applicable standard. A full determination normally requires a minimum of 15 specimens, and laboratories are advised to supply 20 or more to allow the up-down (bracketing) concentration sequence to converge reliably.

Testing Items

The primary deliverable of the test is the ambient-temperature limiting oxygen index value of the plastic material, expressed in volume percent of oxygen. Supporting observations recorded during the determination include:

• Ambient-temperature LOI (vol%) measured at 23 °C per ISO 4589-2, GB/T 2406.2, or ASTM D2863.
• Elevated-temperature LOI and temperature index per ISO 4589-3 or GB/T 2406.3, where required.
• Burning time after removal of the ignition source and whether combustion reached the 50 mm reference mark.
• Observation of flaming versus glowing (afterglow) combustion and any dripping, melting, or char-forming behavior.
• Specimen geometry, conditioning history, and any deviations or special mounting arrangements.

Testing Methods

Applicable Standards

The principal methods are ISO 4589-2 (Plastics — Determination of burning behaviour by oxygen index — Ambient-temperature test) and ASTM D2863 (Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics). The Chinese national standard GB/T 2406.2 is technically aligned with ISO 4589-2, and JIS K 7201-2 provides the corresponding Japanese procedure. ISO 4589-3 and GB/T 2406.3 extend the method to elevated temperatures. While the techniques are conceptually identical, ISO and ASTM procedures differ in certain apparatus and calculation details, so results should be reported together with the method used.

Test Apparatus

A standard oxygen index apparatus consists of a heat-resistant glass chimney (typically 500 mm high and 75–100 mm in internal diameter), a specimen holder that clamps the lower portion of the bar, a precision gas mixing and metering system for oxygen and nitrogen, and a small propane flame igniter. Gas flow is regulated so that the total volumetric flow through the chimney is approximately 10 ± 2 cm³/s, corresponding to a linear gas velocity of roughly 40 mm/s. The oxygen concentration in the mixed stream is controlled through calibrated flow meters or mass flow controllers and can be verified with an oxygen analyzer.

Test Procedure

A reference mark is made 50 mm below the top edge of the conditioned specimen, which is then clamped vertically in the holder inside the chimney. The chimney is purged with the oxygen/nitrogen mixture at the selected concentration for at least 30 seconds before ignition. The igniter flame is applied to the top of the specimen for up to 30 seconds, with brief removals every few seconds to check whether burning is sustained.

After the ignition source is withdrawn, the specimen response is classified as follows:

• Failure (supports combustion): flaming or glowing combustion persists for 3 minutes or longer, or the damaged front reaches or crosses the 50 mm mark within that time.
• Pass (does not support combustion): combustion extinguishes before either criterion is met.

Starting from an estimated concentration, the oxygen level is adjusted in steps (typically 0.5–2% initially, narrowing near the critical point) in an up-down bracketing sequence: the concentration is raised after a failure result and lowered after a pass result. The sequence continues until the standard's calculation requirements are satisfied, usually based on the pattern of the final five results.

Calculation of the LOI Value

For each trial, the oxygen concentration of the gas mixture is calculated from the metered gas flows as the volume percentage of oxygen in the total oxygen/nitrogen stream. The final LOI is derived from the bracketing series using the calculation formula of the applicable standard, which weights the concentration steps according to the pass/fail sequence of the last several specimens. The result is reported to one decimal place together with the specimen type, dimensions, and conditioning state.

Precision and Factors Influencing Results

Within-laboratory repeatability of LOI determinations is typically better than ±1% oxygen absolute, while between-laboratory reproducibility can reach ±2–3%; differences of less than about 1% should therefore not be regarded as significant. Key influences on the measured value include:

• Specimen thickness: LOI generally increases with thickness, so the thickness must always accompany the reported value.
• Temperature: LOI decreases as ambient temperature rises, which is why ISO 4589-3 exists for high-temperature assessments.
• Moisture and conditioning: hygroscopic polymers such as polyamides show measurably different LOI values in dry versus conditioned states.
• Additives, fillers, and aging: flame retardants, plasticizers, reinforcements, and thermal or UV aging all shift the LOI, which is precisely why the method is valued for formulation studies.
• Gas purity and flow calibration: regular verification of flow metering and oxygen concentration is essential for accurate data.

Applications

LOI data determination serves a wide range of industrial and scientific purposes wherever the combustion resistance of plastics must be quantified, compared, or specified:

• Flame-retardant formulation development: optimizing halogenated and halogen-free systems based on phosphorus, nitrogen, intumescents, or mineral fillers such as aluminum hydroxide and magnesium hydroxide.
• Incoming material and production quality control: verifying that resin lots, compounds, and finished components conform to internal or contractual LOI specifications.
• Compliance and specification screening: supporting requirements in railway rolling stock (EN 45545-2), wire and cable materials, electrical equipment, and construction products where minimum LOI values are specified.
• Material selection: comparing candidate polymers on datasheets and during design reviews for electronics enclosures, connectors, insulation parts, and interior panels.
• Research and durability studies: evaluating aging, weathering, migration of flame retardants, and structure-property relationships in polymer science.
• Complementary fire performance assessment: pairing LOI with UL 94, IEC 60695-11-10, and heat release methods such as cone calorimetry to build a fuller flammability profile.

Accredited third-party laboratories routinely perform LOI determinations under ISO/IEC 17025 management systems, providing recognized test reports for suppliers, buyers, and certification bodies worldwide.

FAQ

Q: What is considered a good LOI value for a plastic?

A: As a rule of thumb, LOI below 21% means the material burns readily in air, 22–27% indicates a self-extinguishing to moderately flame-retardant material, and values of 28% or above are generally regarded as flame retardant. Many rail and cable specifications set minimum requirements around 28–32%; always verify against the specific product standard.

Q: Is the LOI test the same as a UL 94 flammability rating?

A: No. LOI measures the minimum oxygen concentration needed to sustain burning, whereas UL 94 evaluates flame spread, afterflame time, and ignition of a cotton indicator after a small-flame exposure. The two tests probe different failure modes and are complementary rather than interchangeable.

Q: Which standards govern LOI determination?

A: The primary methods are ISO 4589-2, ASTM D2863, GB/T 2406.2, and JIS K 7201-2 for ambient-temperature testing, and ISO 4589-3 with GB/T 2406.3 for elevated-temperature testing and temperature-index determination.

Q: How many specimens are required?

A: A minimum of 15 specimens is normally required for a complete determination, and 20 or more are recommended because several trials at each concentration step may be needed, particularly for materials near their critical oxygen concentration.

Q: Does specimen thickness affect the result?

A: Yes. Thicker specimens generally exhibit higher LOI values because more heat is absorbed by the material relative to the burning surface. Reported LOI values are only comparable when specimen geometry, especially thickness, is the same.

Q: Are ISO 4589-2 and ASTM D2863 results interchangeable?

A: The methods are closely related and usually produce similar values, but differences in apparatus details, specimen preparation, and calculation rules mean results should be quoted together with the method used and should not be assumed interchangeable for specification purposes.

Q: Can LOI data predict real-scale fire behavior?

A: No. LOI is a small-scale screening indicator of ease of extinction under controlled gas mixtures. Real fire performance depends on heat release, flame spread, smoke, and toxicity, which must be assessed with additional test methods.

Q: How long does an LOI determination take?

A: The instrumental portion of a determination typically takes a few hours once specimens are prepared and conditioned. Including conditioning time and reporting, accredited laboratories generally deliver results within one to three working days, subject to sample queue and project scope.