Plastic Limiting Oxygen Index (LOI) Test Report: Standards, Test Methods, and Interpretation

The Limiting Oxygen Index (LOI) test is one of the most widely used laboratory methods for evaluating the flammability of plastic materials, quantifying the minimum oxygen concentration required to sustain candle-like combustion under controlled conditions. This technical article explains how plastic LOI testing is performed according to ISO 4589-2, ASTM D2863, GB/T 2406.2, and JIS K 7201-2, what a professional test report should contain, and how manufacturers, engineers, and researchers can interpret the results for quality control, flame-retardant development, and regulatory compliance.

Technical Overview

The Limiting Oxygen Index (LOI), also known as the Oxygen Index (OI), is defined as the minimum volumetric concentration of oxygen in a flowing mixture of oxygen and nitrogen that will just support flaming combustion of a vertically mounted specimen under specified test conditions. The concept was originally introduced by Fenimore and Martin in 1966 and has since become a fundamental screening tool for characterizing the relative flammability of polymers, compounds, and finished plastic components.

Because ambient air contains approximately 21% oxygen by volume, the LOI value provides an intuitive benchmark: a plastic with an LOI below 21% will generally continue to burn in normal air once ignited, whereas a material with an LOI significantly above 21% will tend to self-extinguish when the ignition source is removed. Although LOI is a comparative laboratory index rather than a direct fire-hazard assessment, it remains indispensable for material development, datasheet characterization, and specification compliance.

A typical engineering interpretation of LOI values is summarized below:

• LOI below 21%: readily flammable in air, ignition easily sustained.
• LOI between 21% and 27%: slow-burning, will typically not sustain combustion in still air but is not classified as highly flame retardant.
• LOI above 27%: generally considered self-extinguishing; many industry specifications for cable sheathing, electrical enclosures, and rail components require values of 28% to 35% or higher.

LOI testing is standardized internationally by several standards organizations, and a professional test report will clearly identify the exact standard edition applied. The most commonly referenced methods include:

• ISO 4589-2 (also adopted as EN 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: Chinese national standard, technically aligned with ISO 4589-2.
• JIS K 7201-2: Japanese industrial standard based on ISO 4589-2.
• ISO 4589-3: Extension of the method for determination of the temperature index of plastics at elevated temperatures.

Testing Samples

Specimen geometry has a direct influence on the measured LOI value, so the test report must state the specimen type, dimensions, orientation, and preparation method. Different sample categories are defined in the standards to accommodate the wide range of plastic product forms.

Rigid Molded Plastics

Self-supporting rigid plastics are typically tested as rectangular bars. ISO 4589-2 specifies specimens of 80 to 150 mm in length, 10 mm in width, and 4 mm in thickness for molded materials, while ASTM D2863 commonly uses bars of 70 to 150 mm in length, 6.5 mm in width, and 3 mm in thickness. Specimens are marked with a gauge line 50 mm from the top edge to define the burning criterion.

Flexible Films and Thin Sheets

Thin, flexible materials cannot stand freely and are therefore rolled along their long axis around a supporting rod and secured before testing. Standard film specimens are typically 140 mm in length and 52 mm in width, with the thickness reported as supplied. The rolled configuration provides a reproducible geometry for materials such as polyolefin films, protective coatings, and flexible laminates.

Cellular Materials, Tubes, and Special Geometries

Rigid and flexible foams are cut as bars with enlarged cross-sections, commonly on the order of 10 mm to 12.5 mm in width and thickness, to reflect the burning behavior of cellular structures. Tubes, pipes, and rods may be tested in lengths of approximately 70 to 150 mm, with the wall thickness and overall diameter recorded in the report.

Specimen Conditioning and Quantity

Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity in accordance with ISO 291 or ASTM D618 prior to testing. A full determination using the up-and-down (bracketing) method generally requires 15 to 30 specimens, depending on how sharply the material response changes near the limiting concentration. Additional specimens are retained for verification repeat tests.

Testing Items

A comprehensive plastic LOI test report typically covers the following items:

• Limiting Oxygen Index value: the principal quantitative result, reported as a percentage of oxygen by volume to one decimal place (for example, LOI = 29.5%).
• Ignition and burning behavior observations: ignition time, afterflame characteristics, dripping of flaming particles, and any glowing combustion or char formation.
• Specimen details: material identification, grade and color, supplier information, molding or processing method, dimensions, and orientation relative to processing direction.
• Conditioning regime: temperature, humidity, and duration prior to testing.
• Test atmosphere parameters: total gas flow rate, linear gas velocity, and oxygen/nitrogen metering accuracy.
• Optional temperature index determination: LOI measured at elevated temperatures (for example, 75 °C to 300 °C in 25 °C increments) per ISO 4589-3, extrapolated to determine the temperature at which the LOI falls to 20.9%.
• Statement of precision: repeatability limits within the same laboratory and, where applicable, reproducibility between laboratories.

Testing Methods

Apparatus and Calibration

The standard LOI apparatus consists of a heat-resistant glass chimney, typically at least 450 mm in height and 75 to 100 mm in internal diameter, a specimen holder that supports the bar vertically in the center of the column, a gas mixing and metering system supplied by oxygen and nitrogen cylinders, and a small pilot flame igniter fueled by propane or hydrogen. The mixed gas flows upward through the column at a linear velocity of 40 mm/s, which corresponds to a total volumetric flow of roughly 10 to 11 liters per minute depending on chimney diameter. Gas metering devices are calibrated against certified reference instruments, and the oxygen concentration is verified with an oxygen analyzer traceable to national standards.

Test Procedure

The determination follows the classic up-and-down (bracketing) approach:

• The specimen is mounted vertically with its top at a defined position below the chimney outlet, and the gas mixture is allowed to purge the column to establish a stable atmosphere.
• The pilot flame is applied to the top of the specimen intermittently, generally in intervals of a few seconds up to a total ignition contact period of about 30 seconds, until sustained flaming ignition is achieved.
• The specimen response is judged against a pass/fail criterion: the test is recorded as "burns" if flaming continues for more than 3 minutes or if the flame front passes the 50 mm gauge mark, and "extinguishes" otherwise.
• Based on the response, the oxygen concentration is adjusted in controlled increments, typically in the range of 0.2% to 1.0% depending on the stage of bracketing, and a fresh specimen is tested at the new concentration.
• Testing continues until the response sequence brackets the limiting concentration, after which the LOI is calculated from the series of responses using the statistical formula defined in the standard.
• The final concentration is verified by repeating the test at that setting to confirm the stability of the result.

Calculation and Expression of Results

The nominal oxygen concentration of each trial is calculated as the volumetric fraction of oxygen in the total gas stream, that is, the oxygen flow divided by the sum of oxygen and nitrogen flows, multiplied by 100. The final LOI is then derived from the up-and-down response sequence using the correction term specified in ISO 4589-2 or ASTM D2863. Results are reported to the nearest 0.1% oxygen, together with the standard designation, specimen geometry, and any observed deviations such as dripping or glowing combustion.

Essential Contents of a Professional Test Report

To ensure technical validity and acceptance by downstream users, a professional LOI test report issued by an accredited third-party laboratory should include, at minimum:

• Identification of the client, sample description, and unique laboratory sample number.
• The complete designation and edition year of the test standard applied.
• Specimen dimensions, quantity tested, and conditioning conditions.
• The measured LOI value with units and, where applicable, the step size used.
• Observations of burning behavior, including dripping, glowing, or char characteristics.
• Test dates, equipment identification, and the accreditation status of the laboratory.
• A clear statement that results relate only to the behavior of the test specimens under the specific laboratory conditions and do not by themselves constitute a fire-risk assessment of the end product.

Applications

LOI test reports serve a broad range of industrial and scientific purposes:

• Flame-retardant compound development: screening and optimizing halogen-free systems based on phosphorus, nitrogen, intumescent chemistry, and mineral fillers such as aluminum hydroxide and magnesium hydroxide, where LOI provides a rapid, quantitative comparison of formulations.
• Wire and cable industry: qualification of insulation and sheathing compounds; many purchaser and industry specifications require LOI values of 28% to 35% or higher for cable materials.
• Railway and mass transit: material performance programs in Europe, such as those referenced within EN 45545-2, use oxygen index as one of the parameters for specific material sets.
• Electrical and electronic equipment: LOI data is published in plastic datasheets alongside UL 94 ratings to support material selection for enclosures, connectors, and insulating parts.
• Construction and interior materials: supplementary characterization of pipes, panels, linings, and decorative components in combination with reaction-to-fire classifications under EN 13501.
• Quality control: incoming inspection of resin and compound lots and release testing of production batches against internal or contractual LOI limits.
• Research and academia: LOI is a near-universal comparative metric in polymer flammability research, thermal-degradation studies, and flame-retardancy mechanism publications.

FAQ

What LOI value is considered flame retardant?

As a common engineering convention, plastics with an LOI above 27% are regarded as self-extinguishing in air, while values below 21% indicate the material will continue to burn in normal atmospheres. Many application specifications set their own minimum values, for example 28% or higher for certain cable compounds, so the acceptance criterion depends on the end-use requirement rather than a universal threshold.

Can an LOI result be converted to a UL 94 rating?

No. The two methods measure different phenomena: LOI quantifies the minimum oxygen concentration needed to sustain burning, while UL 94 evaluates the burning behavior of a specimen subjected to a defined small-flame ignition, including afterflame time and flaming drips. The tests are complementary, and material datasheets commonly report both, but there is no direct conversion between them.

Does specimen thickness affect the measured LOI?

Yes. LOI is geometry-dependent, and for many polymers the measured value increases with specimen thickness because thicker sections lose heat to the bulk material more effectively. This is why the report must always state the exact dimensions tested, and why comparisons are only valid between specimens of matching geometry and conditioning.

How many specimens are needed for one determination?

A full up-and-down determination typically consumes 15 to 30 specimens, since a series of trials is required to bracket the limiting concentration and one or more verification repeats are performed. Laboratories therefore usually request 30 to 40 specimens per material to complete the test and allow for retests.

What is the temperature index, and when is it requested?

The temperature index, determined according to ISO 4589-3, is the temperature at which the oxygen index of the material falls to 20.9%, meaning it would just barely burn in ambient air at that temperature. It is frequently requested for electrical insulation materials, cable compounds, and components intended for elevated-temperature service environments.

How should LOI results be used in product safety assessment?

LOI is a sensitive and reproducible comparative index, but it does not by itself describe real-scale fire behavior. It should be used together with complementary tests such as UL 94 flame ratings, glow-wire tests under IEC 60695-2 series methods, cone calorimetry, and full-scale or application-specific fire performance standards when assessing the overall fire safety of an end product.

Is testing performed on raw material or on finished parts?

Both are possible. Raw resin or compound is typically molded or cut into standard bars, which gives the cleanest material characterization. Finished components can also be cut to standard geometry where wall thickness permits, and the report should then note the sampling location and orientation within the part.