Plastic Oxygen Index (LOI) Flammability Testing: Test Methods, Standards, and Classification

The Limiting Oxygen Index (LOI) test is a fundamental bench-scale method for evaluating the flammability of plastic materials, measuring the minimum oxygen concentration required to sustain candle-like combustion. Governed by internationally recognized standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, the oxygen index test is widely used for material screening, quality control, and flame-retardant formulation development. This article explains the testing principle, sample requirements, procedures, classification criteria, and typical applications of plastic oxygen index flammability rating testing.

Technical Overview

The Limiting Oxygen Index (LOI), also known as the oxygen index or critical oxygen index, quantifies how easily a plastic material will burn. It is defined as the minimum concentration of oxygen, expressed as a volume percentage in a flowing oxygen and nitrogen mixture, that will just support sustained candle-like combustion of a vertically mounted test specimen after ignition from the top. Because normal air contains approximately 20.9% oxygen, any material with an LOI below 21% will generally continue to burn freely in air, while materials with higher LOI values tend to self-extinguish once an external ignition source is removed.

The test is performed in a controlled laboratory environment using a transparent vertical glass chimney, a specimen holder, precision gas flow meters, an oxygen and nitrogen mixing system, and a small propane or butane ignition burner. The result is reported as a single numerical value to one decimal place, for example LOI = 28.5%. A higher value indicates better flame-retardant performance. The oxygen index method is valued for its excellent repeatability, small specimen requirements, and sensitivity to changes in flame-retardant loading, making it one of the most widely adopted fire-performance screening tools in the plastics industry.

• Governing standards: ISO 4589-2, ASTM D2863, GB/T 2406.2, and JIS K 7201-2
• Elevated-temperature testing: ISO 4589-3 (temperature index determination)
• Result expressed as volume percent of oxygen in an oxygen/nitrogen mixture
• Commonly paired with UL 94 (IEC 60695-11-10) for complete flammability characterization

Testing Samples

Molded and Machined Rigid Specimens

For rigid thermoplastics and thermosets, ISO 4589-2 typically specifies bar specimens measuring 80 to 150 mm in length, 10 mm in width, and 4 mm in thickness, while ASTM D2863 commonly uses specimens 70 to 150 mm long, 6.5 mm wide, and 3 mm thick. Specimens may be injection molded, compression molded, or machined from sheets, provided they are free from voids, flash, cracks, and surface contamination. Because LOI results can vary with thickness, the tested thickness must always be reported along with the result.

Flexible Plastics, Films, and Thin Sections

Thin films and flexible materials cannot stand freely in the specimen holder and require special support frames or rods as described in the annexes of ISO 4589-2 and ASTM D2863. These fixtures ensure that the specimen remains vertical during ignition without interfering significantly with gas flow around the burning zone.

Tubes, Pipes, and Cellular Materials

Tubing and pipe samples are tested using dedicated holders that grip the specimen vertically, and cellular plastics such as foamed polyurethane or polyolefin foams are cut to standardized dimensions appropriate to their form. In all cases, the specimen form and dimensions must conform to the applicable tables in the selected standard.

Specimen Conditioning and Quantity

Specimens are conditioned prior to testing in accordance with ISO 291 or ASTM D618, typically at 23 ± 2°C and 50 ± 5% relative humidity for a minimum of 88 hours, unless the material specification requires otherwise. A minimum of 15 specimens, and preferably 20 or more, is required for each test series because the bracketing procedure relies on multiple data points near the critical oxygen concentration. Materials with pronounced directionality, such as filled or reinforced compounds, may require testing in both machine and transverse directions.

Testing Items

Accredited third-party laboratories and in-house fire laboratories typically offer the following oxygen index test items for plastic materials:

• Ambient-temperature limiting oxygen index (LOI) determination per ISO 4589-2, ASTM D2863, or GB/T 2406.2
• Elevated-temperature oxygen index and temperature index (TI) determination per ISO 4589-3
• LOI comparison of flame-retardant formulations at different additive loadings
• LOI verification after heat aging, humidity exposure, or other preconditioning treatments
• Material screening and ranking for research and development programs
• Quality control verification of incoming resin, masterbatch, or finished compounds
• Combined flammability assessment together with UL 94 horizontal and vertical burn testing

Testing Methods

Apparatus and Test Setup

The test apparatus consists of a heat-resistant glass chimney (typically about 500 mm tall), a specimen holder that supports the specimen vertically in the center of the chimney, and a gas metering system that delivers a precisely controlled mixture of oxygen and nitrogen at a linear flow velocity of approximately 40 mm/s through the chimney. The oxygen concentration is adjustable between approximately 16% and 100%, and the mixing system must maintain concentration stability within ±0.1% during the test.

Ignition and Burning Criterion

The top of the vertically mounted specimen is ignited using a small flame from a gas burner applied for a defined period defined by the standard. After removal of the ignition source, the specimen is judged to have sustained combustion, commonly referred to as a "reaction," if it continues to burn for longer than a specified time or if the burned or damaged length exceeds the specified limit, typically on the order of 3 minutes or 50 mm depending on the standard version and specimen form. If the specimen extinguishes within these limits, it is recorded as having failed to sustain burning at that oxygen concentration.

Oxygen Concentration Bracketing and Calculation

The test begins at an estimated oxygen concentration and proceeds using a bracketing (ascending and descending) procedure: the concentration is increased after an extinguishing result and decreased after a sustained-burning result, with progressively smaller step sizes as the critical value is approached. The final LOI value is derived from the last several data points using the calculation method specified in the standard, often based on statistical estimation of the concentration at which burning is just supported. The reported LOI is rounded to one decimal place, and the full oxygen concentration series is documented in the test report.

Result Classification and Typical Values

Although the standards themselves do not assign pass or fail grades, the industry commonly interprets LOI values according to the following general guidelines:

• LOI below 21%: readily combustible in normal air; the material will typically continue burning after ignition
• LOI of 21% to 24%: slow-burning or marginally self-extinguishing in air
• LOI of 25% to 28%: generally classified as flame-retardant or self-extinguishing
• LOI above 28% to 30%: highly flame-retardant, frequently meeting demanding engineering requirements

Representative LOI values for common polymers illustrate the practical range of the method: polyethylene and polypropylene approximately 17% to 18%, polystyrene approximately 18%, ABS approximately 18% to 20%, polyamide approximately 22% to 27%, polycarbonate approximately 25% to 27%, rigid PVC approximately 45% to 49%, and PTFE above 90%. These values demonstrate why halogen-free and inherently flame-retardant engineering plastics such as PC, PPO blends, and PVC compounds perform well in the oxygen index test.

Applications

Polymer Formulation and R&D

Because LOI testing is fast, repeatable, and sensitive to small changes in flame-retardant content, it is the preferred screening tool during the development of halogenated, halogen-free, intumescent, and mineral-filled flame-retardant compounds. Formulators use LOI curves to optimize additive loading and cost before committing to full-scale UL 94 or IEC 60695 burning tests.

Wire and Cable Materials

Insulation and jacketing compounds for building wire, power cables, and data cables are routinely qualified and monitored using oxygen index testing. LOI values above 28% to 32% are commonly specified in procurement specifications for flame-retardant cable compounds, often alongside specific cable fire propagation standards.

Electrical and Electronic Equipment

Enclosure resins, connectors, sockets, and circuit board laminates are characterized with LOI data as supporting evidence of flame retardancy. While end-product compliance typically relies on UL 94 or IEC 60695-11-10 ratings, the oxygen index remains a core material data sheet parameter and a key input for material selection.

Transportation: Rail, Automotive, and Aerospace

Railway interior materials are frequently screened with LOI as part of the evaluation framework associated with EN 45545-2, and automotive and aircraft interior components similarly rely on oxygen index data as an element of broader flammability compliance packages. The method provides a consistent material-level metric that complements full-scale and component-level fire tests.

Quality Control and Incoming Inspection

Resin producers and compounders perform routine LOI testing to verify batch-to-batch consistency of flame-retardant formulations, and converters use it to confirm that incoming materials meet specification before processing. ISO/IEC 17025-accredited third-party laboratories issue formal test reports that are widely accepted in supply chain qualification and customer audits.

FAQ

What LOI value is considered flame retardant?

As a general industry convention, plastics with an LOI of 25% or higher are described as flame-retardant or self-extinguishing in air, and values above 30% indicate a high degree of flame retardancy. The exact threshold depends on the end-use specification, since each application standard defines its own acceptance criteria.

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

The oxygen index test measures the minimum oxygen concentration needed to sustain burning in a controlled gas mixture, while UL 94 evaluates afterflame time, dripping, and ignition of cotton when a specimen is exposed to a defined burner flame in normal air. LOI provides a quantitative material property that is ideal for ranking and QC, whereas UL 94 provides the flammability rating (such as V-0, V-2, or 5VA) commonly required for electrical product safety compliance. The two methods are complementary and frequently performed together.

How many specimens are required for one LOI determination?

A minimum of 15 specimens is generally required, and 20 or more is recommended, because the bracketing procedure must record several alternating burning and extinguishing results near the critical concentration to calculate a statistically valid LOI value.

Why might two laboratories report slightly different LOI values?

Small differences can arise from specimen thickness, conditioning history, ambient temperature and humidity, surface finish, material directionality, and minor apparatus differences. Well-controlled interlaboratory reproducibility is typically on the order of plus or minus 1 to 2 percentage points of oxygen, which is why test reports must state the standard used, specimen dimensions, and conditioning conditions.

Can the oxygen index be measured at elevated temperatures?

Yes. ISO 4589-3 extends the method to elevated temperatures, determining either the oxygen index at a specified temperature or the temperature index, which is the temperature at which the material's oxygen index drops to 21%. This is particularly relevant for materials used in warm environments where flame retardancy degrades with heat.

Is an oxygen index test report sufficient for product certification?

An LOI report from an ISO/IEC 17025-accredited third-party laboratory is widely accepted as material-level evidence of flame retardancy, but most end-product regulations, such as those for electrical equipment, railway components, or building products, require additional application-specific fire tests. LOI data is therefore typically submitted as part of a broader compliance documentation package rather than as a stand-alone certificate.