Quantitative Determination of the Limiting Oxygen Index (LOI) of Plastics

The limiting oxygen index (LOI) test is a fundamental laboratory method for quantifying the flammability of plastic materials by measuring the minimum oxygen concentration required to sustain candle-like combustion. Standardized in ISO 4589-2, ASTM D2863, and GB/T 2406.2, this small-scale test delivers a reproducible numerical indicator of flame retardancy that supports material development, quality control, and regulatory compliance. This article explains the technical principle, sample requirements, test procedure, calculation, and typical applications of oxygen index determination for plastics.

Technical Overview

The limiting oxygen index (LOI) is defined as the minimum concentration of oxygen, expressed as a percentage by volume in a flowing mixture of oxygen and nitrogen, that will just support flaming combustion of a vertically mounted test specimen under specified conditions. The higher the oxygen index, the greater the resistance of the material to sustained burning. Because ambient air contains approximately 20.9% oxygen by volume, a plastic with an LOI above roughly 21% will generally self-extinguish after removal of the ignition source, whereas a material with an LOI well below 21% will continue to burn in normal air.

Oxygen index determination is a small-scale laboratory screening method rather than a direct measure of real-scale fire hazard. Its principal value lies in providing a quantitative, reproducible basis for comparing the relative flammability of polymers, flame-retardant formulations, and finished compounds. The method is standardized internationally in ISO 4589-2 (Plastics — Determination of burning behaviour by oxygen index — Part 2: Ambient-temperature test) and ASTM D2863 (Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics). The corresponding Chinese national standard is GB/T 2406.2. Related methods include ISO 4589-3 for the determination of oxygen index at elevated temperatures and ISO 4589-1 for general guidance.

Representive LOI ranges for common polymers illustrate the practical significance of the parameter:

• Polyethylene (PE), polypropylene (PP), polystyrene (PS), and PMMA: approximately 17–19%, readily flammable in air.
• ABS and polycarbonate (PC): approximately 18–20% and 25–28% respectively, illustrating the effect of polymer structure and aromatic content.
• Rigid PVC: approximately 45% or higher, due to the chlorine content and char-forming tendency.
• PTFE and other highly fluorinated polymers: oxygen index above 90%, effectively non-combustible under test conditions.

Testing Samples

Rigid Thermoplastics and Thermosets

Specimens are machined or cut from compression- or injection-molded plaques, sheets, or rods. ISO 4589-2 specifies preferred dimensions of 80–150 mm in length, 10 mm in width, and 4 mm in thickness, with cylindrical rods of 3–10 mm diameter also acceptable. ASTM D2863 commonly uses specimens 70–150 mm long, approximately 6.5 mm wide, and approximately 3 mm thick. The surfaces must be clean, free of defects, and representative of the material as supplied.

Flexible Films and Thin Sheets

Thin films that cannot support themselves are prepared by winding the film spirally around a small mandrel or rod so that a self-supporting specimen of the required length is obtained. Shrinkage and curling during heating must be minimized, as specimen distortion can significantly influence the result.

Cellular Materials and Foams

Rigid and flexible foams are tested using specimens with larger cross-sections, typically on the order of 80–150 mm in length with width and thickness of approximately 10–15 mm, ensuring that the cell structure and density are representative of the end product.

Electrical Insulation and Finished Components

Insulation strips, cable sheathing, laminates, fiber-reinforced composites, and sections cut from finished parts may be tested provided that specimens of suitable geometry can be prepared. Results on such materials should always be reported together with the specimen geometry and the direction of any anisotropy.

Specimen Conditioning

Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 88 hours in accordance with ISO 291 or ASTM D618 prior to testing. A sufficient number of specimens is required for a valid determination; typically at least 15 specimens are consumed in a complete test series, since each concentration trial uses a fresh specimen.

Testing Items

The primary reported quantity is the ambient-temperature limiting oxygen index, expressed as the volume percentage of oxygen to the nearest 0.1%. Depending on client requirements, the test scope may be extended to include:

• Ambient-temperature limiting oxygen index (LOI) per ISO 4589-2, ASTM D2863, or GB/T 2406.2.
• Elevated-temperature oxygen index per ISO 4589-3, from which a temperature index — the temperature at which the oxygen index falls to approximately 21% — can be derived.
• Qualitative burning behavior observations: afterflame time, extent of charring, flaming drips, sample shrinkage or distortion, and smoke release during the test.
• Comparative assessment of flame-retardant grade classification against product or purchase specifications.

Testing Methods

Test Apparatus

The standard apparatus consists of a heat-resistant glass chimney, typically at least 450 mm high with an internal diameter of approximately 70–75 mm, within which the specimen is clamped vertically on a holder. Oxygen and nitrogen of high purity are metered through calibrated flow controllers into a mixing chamber and introduced at the base of the chimney at a total flow corresponding to a linear gas velocity of approximately 40 mm/s. The oxygen concentration of the mixture is verified with an oxygen analyzer. Ignition is achieved using a small, well-defined propane or butane diffusion flame applied to the top of the specimen.

Test Procedure

• Condition, dimension, and mark each specimen with a reference line 50 mm below the top edge.
• Mount the specimen vertically in the holder and purge the chimney with the gas mixture at the selected oxygen concentration before ignition.
• Apply the ignition flame to the top of the specimen for the prescribed duration, withdraw the igniter, and start timing.
• Observe whether the specimen burns beyond the specified criterion — in ISO 4589-2, burning for longer than 180 s or consuming more than 50 mm of the specimen constitutes continued combustion.
• If the specimen extinguishes before the criterion, increase the oxygen concentration; if it burns beyond the criterion, decrease the concentration. Fresh specimens are used for each trial.
• Refine the concentration in progressively smaller increments using a bracketing (up-and-down) sequence until the balance point is confirmed, typically to a resolution of 0.1–0.2% oxygen.

Calculation and Expression of Results

The oxygen index is calculated from the volumetric flow rates of the two gases: LOI (%) = 100 × V(O₂) / [V(O₂) + V(N₂)], where V denotes volume flow, with correction for gas purity where required. The final value is reported to the nearest 0.1%, together with the test standard used, specimen dimensions and orientation, conditioning regime, the combustion criterion applied, and all relevant burning behavior observations. Within-laboratory repeatability of the method is generally on the order of a few tenths of a percent oxygen when apparatus calibration and operator technique are well controlled.

Applications

Quantitative oxygen index data are used across the plastics value chain for formulation, qualification, and compliance purposes:

• Flame-retardant research and development: screening halogenated, phosphorus-based, nitrogen-based, inorganic, and intumescent flame-retardant packages, evaluating synergists, and optimizing loading levels.
• Quality control: verifying lot-to-lot consistency of resins, compounds, and masterbatches against internal or customer specifications.
• Regulatory and industry compliance: LOI is one of the recognized fire test methods within railway material requirement sets such as EN 45545-2, and it is widely referenced in electrical equipment, mining, and marine product specifications, as well as in IEC-related material assessments.
• Material selection and datasheet documentation: providing a comparable flammability figure for engineering polymers, insulation materials, laminates, and composites.
• Academic and industrial research: studying polymer degradation, char formation, and the influence of additives, fillers, and molecular structure on combustion behavior.

FAQ

What oxygen index value is considered flame retardant?

As a rule of thumb, materials with an LOI of 21% or below burn readily in ambient air. Values between 21% and 25% indicate a self-extinguishing tendency, while many product specifications classify materials with an LOI of 27–28% or higher as flame retardant. Classification thresholds ultimately depend on the applicable product standard or contractual requirement.

What is the difference between ISO 4589-2 and ASTM D2863?

The two methods are technically equivalent in principle but differ in details such as preferred specimen dimensions, chimney and flow configuration, ignition procedure, and the exact pass criteria. Results are usually comparable, but they should not automatically be treated as interchangeable; the report must always state which standard was applied.

How many specimens are needed for one determination?

A complete determination typically consumes at least 15 specimens, and often more than 20, because every oxygen concentration trial must be performed on a fresh specimen and the bracketing sequence requires multiple iterations around the balance point.

Can the oxygen index predict real-scale fire performance?

No. The LOI test is a small-scale screening method conducted under controlled, downward-burning candle-like conditions. It does not account for radiation, flame spread geometry, or ventilation effects present in real fires, and it should be complemented by other fire test methods — such as cone calorimetry, flame spread, and ignitability tests — whenever a full fire hazard assessment is required.

How does LOI compare with UL 94 ratings?

The two approaches are complementary. UL 94 is a pass/fail burning classification performed in ambient air, whereas the oxygen index provides a continuous numerical measure of the minimum oxygen needed for combustion. A high LOI generally correlates with better UL 94 performance, but there is no formal conversion between the two.

Why might results vary between laboratories?

Variations can arise from differences in specimen thickness and conditioning, thermoplastic shrinkage and flaming drips, gas purity and flow calibration, and operator judgment near the balance point. Testing in accredited third-party laboratories with calibrated apparatus and verified procedures minimizes inter-laboratory variation.