Plastic Oxygen Index (LOI) Testing: Principles, Standards, Methods and Applications

Plastic oxygen index testing, also known as the limiting oxygen index (LOI) test, measures the minimum oxygen concentration required to sustain the flaming combustion of plastic materials and is one of the most widely used bench-scale methods for evaluating flame retardancy. This technical article explains the test principle, the applicable standards such as ISO 4589-2, ASTM D2863 and GB/T 2406.2, specimen requirements, testing procedures, and typical applications across the wire and cable, electronics and polymer compounding industries.

Technical Overview

The plastic oxygen index test, more precisely the limiting oxygen index (LOI) test, quantifies the flammability of polymeric materials by determining the minimum concentration of oxygen, expressed as a percentage by volume, in a flowing oxygen–nitrogen mixture that will just sustain flaming combustion of a vertically mounted specimen under controlled laboratory conditions. For example, a plastic with an LOI of 28% will only continue to burn after ignition when the surrounding atmosphere contains at least 28% oxygen.

Because ambient air contains approximately 20.9% oxygen by volume, any material with an LOI above 21% tends to self-extinguish once the external ignition source is removed. This makes the oxygen index a convenient, repeatable, small-scale screening indicator of flame-retardant performance, widely used in material development, quality control and regulatory submissions to accredited third-party laboratories.

The test is governed by an internationally harmonized family of standards, including ISO 4589-1 (general guidance), ISO 4589-2 (ambient-temperature test), ISO 4589-3 (elevated-temperature test), ASTM D2863, GB/T 2406.2 and JIS K 7201-2. A common engineering classification based on LOI values is:

• LOI below 21%: readily flammable, continues burning in normal air.
• LOI between 21% and 27%: slow-burning, partial self-extinguishing behavior.
• LOI above 27%: generally considered flame-retardant or self-extinguishing.

It should be noted that LOI is a relative, small-scale flammability parameter. It correlates with, but does not replace, end-product fire performance tests such as UL 94 or the IEC 60332 cable fire test series, because real-scale fire behavior also depends on geometry, heat flux, airflow and specimen thickness.

Testing Samples

Rigid Moulded Specimens

Most thermoplastics and thermosets are tested as self-supporting rectangular bars produced by injection moulding, compression moulding or machining. Under ISO 4589-2 and GB/T 2406.2, standard bar specimens are typically 80 to 150 mm long and 10 mm wide, with thicknesses of 10 mm or 4 mm depending on the material class. ASTM D2863 commonly uses specimens 70 to 150 mm long, 6.5 mm wide and approximately 3 mm thick.

Films, Sheets and Flexible Materials

Thin films, flexible sheets and materials that distort on heating cannot stand freely and are tested using a support frame or U-shaped holder so that the specimen remains vertical during ignition. This allows plastics such as PE films, flexible PVC and elastomeric compounds to be evaluated with the same pass/fail criteria as rigid materials.

Cellular Plastics and Foam Materials

Rigid and flexible foams are tested using larger, thinner specimens with appropriate support, since cellular morphology, cell size and density strongly influence the burning rate and therefore the measured oxygen index.

Conditioning and Quantity

Specimens are conditioned to a standard atmosphere of 23 ± 2°C and 50 ± 5% relative humidity, typically for at least 40 hours, unless the material specification dictates otherwise. Because the result is obtained by statistical bracketing of several burning and non-burning responses, a minimum of 15 to 20 specimens per material is normally required for a valid determination.

Testing Items

Accredited third-party laboratories offering plastic oxygen index testing typically provide the following test items:

• Limiting oxygen index (LOI) at ambient temperature per ISO 4589-2, ASTM D2863, GB/T 2406.2 or JIS K 7201-2.
• Oxygen index at elevated temperatures and flammability temperature index per ISO 4589-3.
• Comparative LOI measurements before and after thermal aging, UV exposure or water immersion for material qualification programs.
• LOI as a quality-control parameter for flame-retardant masterbatches, halogen-free compounds and filled polymer systems.
• Verification testing supporting flame-retardant grade declarations requested by OEM specifications and purchasing agreements.

Testing Methods

Apparatus

The test is performed in a vertical heat-resistant glass chimney, typically at least 450 mm high with an internal diameter of 75 to 100 mm, mounted above a base containing glass beads that homogenize the gas mixture. Medical-grade oxygen and nitrogen are metered through calibrated rotameters or mass-flow controllers to achieve the target oxygen concentration, and a small diffusion-flame igniter, usually fueled by propane or hydrogen, is used to ignite the top edge of the specimen.

Ambient-Temperature Procedure

The conditioned specimen is clamped vertically in the holder with its upper end below the top of the chimney. The column is purged with the oxygen–nitrogen mixture at a defined linear velocity for at least 30 seconds. The igniter flame is then applied to the top of the specimen; once sustained ignition is achieved, the igniter is withdrawn and the operator observes whether the specimen burns for more than 180 seconds or consumes a downward burning length of more than 50 mm after removal of the ignition source.

Evaluation and Calculation

A response of "burns" requires lowering the oxygen concentration in the next test, while a response of "does not burn" requires raising it. Starting with coarse concentration steps, the operator refines the adjustment to increments of about 0.2% by volume near the limiting condition. The limiting oxygen index is then calculated from the final concentration series using the statistical k-factor method defined in ISO 4589-2 and ASTM D2863, and the result is reported as a volume percentage together with the specimen geometry and the burning criterion applied.

Elevated-Temperature Variant

For materials used near heat sources, ISO 4589-3 extends the method to elevated temperatures using a heated chimney assembly. The oxygen index is determined at successive temperatures, allowing derivation of a flammability temperature profile that is valuable for electrical insulation, appliance and transport applications.

Applications

Oxygen index testing is applied throughout the polymer value chain, from resin and compound development to end-product compliance verification. Typical application scenarios include:

• Wire and cable insulation and sheathing compounds, where flame-retardant formulations are frequently specified with minimum LOI values of 28% or higher.
• Development of halogen-free flame-retardant compounds based on metal hydroxide fillers, phosphorus additives and intumescent systems.
• Engineering plastics and printed-circuit-board laminates for electronics, where LOI supplements UL 94 ratings in material datasheets.
• Railway, aerospace and construction material selection, where LOI data support compliance programs aligned with EN, GB and UL fire-safety frameworks.
• Incoming inspection and batch-to-batch quality control of flame-retardant resins, masterbatches and recycled materials.

FAQ

What oxygen index value indicates a flame-retardant plastic?

As a rule of thumb, plastics with an LOI above 27% are regarded as flame-retardant or self-extinguishing, values between 21% and 27% indicate slow-burning behavior, and values below 21% mean the material will continue burning in normal air. Specific product standards and customer specifications may set different minimum values.

How is LOI different from the UL 94 burning test?

LOI measures the minimum oxygen concentration needed to sustain combustion in a flowing gas mixture, while UL 94 evaluates burning behavior, afterflame time and dripping in ambient air after ignition. The two methods are complementary, and flame-retardant grades are usually characterized by both parameters.

How many specimens are required for one LOI determination?

Because the result is obtained by bracketing burning and non-burning responses, at least 15 to 20 conditioned specimens of identical geometry are normally required. Additional specimens are recommended for materials with high variability or strong moisture sensitivity.

Which factors influence the repeatability of the result?

Specimen thickness, surface preparation, conditioning history, ambient humidity, gas purity, flow calibration and operator judgment during ignition all influence repeatability. Testing in an accredited third-party laboratory with calibrated flowmeters and standardized procedures typically achieves reproducibility within approximately ±0.5 to 1 volume percent.

Can LOI results be used directly for product certification?

LOI is primarily a material screening and quality-control parameter. For certification of finished goods, it is generally combined with standardized fire tests such as UL 94, the IEC 60332 series for cables, or the relevant EN and GB product standards, because end-use fire performance depends on the final product construction.