Plastic Limiting Oxygen Index (LOI) Testing Process: Complete Procedure and Standards Guide

The Limiting Oxygen Index (LOI) test is one of the most widely used laboratory methods for evaluating the flammability characteristics of plastic materials. This article explains the complete plastic limiting oxygen index testing process, covering applicable standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, sample requirements, apparatus, step-by-step procedures, and result interpretation. Whether you are a materials engineer, quality control specialist, or product developer, this guide will help you understand how LOI data is generated and applied in flame-retardancy assessment.

Technical Overview

The Limiting Oxygen Index (LOI), also known as the Oxygen Index (OI) or Critical Oxygen Index (COI), is defined as the minimum concentration of oxygen, expressed as a volume percentage, in a flowing mixture of oxygen and nitrogen that will just support flaming combustion of a vertically mounted plastic specimen. The test is conducted under controlled laboratory conditions, typically at 23 ± 2 °C, in accordance with recognized standards such as ISO 4589-2, ASTM D2863, GB/T 2406.2, JIS K 7201-2, and NES 714.

Because normal ambient air contains approximately 20.9% oxygen, the LOI value provides an intuitive benchmark for relative flammability. Materials with an LOI below 21% will generally continue to burn easily in air under the test conditions, while materials with an LOI above approximately 27% are generally considered self-extinguishing. A higher LOI value indicates better flame resistance, making the test a fundamental screening tool for flame-retardant material development and qualification.

The LOI method is favored for its good repeatability, quantitative output, and relatively small specimen size. However, it measures candle-like upward combustion behavior and should be complemented by other fire performance tests, such as UL 94 horizontal and vertical burning tests, when full fire hazard assessment is required. Accredited third-party laboratories routinely perform LOI testing for compliance verification, research and development, and quality control purposes.

Testing Samples

Rigid Plastic Specimens

For self-supporting rigid plastics, the standard specimen geometry specified in ISO 4589-2 and GB/T 2406.2 is 80 mm to 150 mm in length, 10 mm in width, and 10 mm in height, or 10 mm in width and 4 mm in thickness. The preferred thickness is 4 mm, but alternative thicknesses of 3 mm, 6 mm, 10 mm, and 12 mm may be tested depending on the product form and the client's requirements. ASTM D2863 accepts similar dimensions, with typical specimens of 70 mm to 200 mm in length, 6.5 mm to 12.7 mm in width, and 3 mm to 6.4 mm in thickness.

Flexible Films, Sheets, and Thin Materials

Flexible materials that deform during burning cannot be tested as simple bars. They are prepared by rolling the film spirally onto a rod, folding it, or supporting it in a frame so that the specimen retains a defined geometry during the test. Specialized specimen preparation ensures the combustion behavior is representative of the material rather than distorted by sample collapse.

Specimen Marking, Conditioning, and Quantity

A reference mark is drawn 50 mm from the top edge of each specimen to provide the burning length criterion. Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 88 hours prior to testing, in line with ISO 291 or the equivalent conditioning requirements in the applicable standard. Because each specimen is consumed during testing, a sufficient quantity must be prepared; typically 15 to 20 specimens are required for a single LOI determination, as the concentration bracketing procedure consumes multiple specimens.

Material Scope

The LOI test is applicable to a broad range of polymer materials, including thermoplastics, thermosets, elastomers, films, sheets, laminates, and reinforced or filled compounds. ISO 4589-3 extends the method to elevated temperature index determination, and related oxygen index methods are also applied to rubber, textiles, and coatings.

Testing Items

The primary testing item is the determination of the Limiting Oxygen Index value of the plastic material, reported as a volume percentage of oxygen. Depending on client requirements and product specifications, the following items may also be included in a test program:

• LOI value at room temperature (23 ± 2 °C) per ISO 4589-2, ASTM D2863, GB/T 2406.2, or JIS K 7201-2
• Temperature index determination at elevated temperatures per ISO 4589-3
• Observation of burning behavior, including afterflame time, afterglow, dripping, and smoke release characteristics
• Comparison of LOI values across different specimen thicknesses or material formulations
• Verification of flame-retardant performance against procurement specifications or regulatory limits

Testing Methods

Test Apparatus

The LOI test is conducted on a dedicated oxygen index apparatus consisting of a heat-resistant glass chimney (typically 450 mm to 500 mm in height and 75 mm to 100 mm in diameter), a vertical specimen holder, a gas mixing chamber with a controlled outlet flow, precision flowmeters or mass flow controllers for oxygen and nitrogen, and an optional oxygen analyzer for verifying the mixed gas concentration. The ignition source is a small propane gas flame or an electric igniter applied from the top of the specimen. Modern instruments integrate digital flow control and software-based calculation to improve accuracy and repeatability.

Step-by-Step Test Procedure

The general testing procedure specified by ISO 4589-2 and equivalent standards is as follows:

• Condition the specimens as required, then verify dimensions and apply the 50 mm reference mark measured from the top of the specimen.
• Mount the specimen vertically in the holder so that the top of the specimen is at least 100 mm below the top of the chimney.
• Set an initial oxygen concentration in the nitrogen/oxygen mixture based on prior knowledge or an estimated value for the material class.
• Purge the chimney with the gas mixture for at least 30 seconds at a linear gas velocity of 40 ± 10 mm/s to ensure a stable atmosphere before ignition.
• Apply the ignition flame to the top surface of the specimen for up to 5 seconds, withdrawing and reapplying if necessary, until the specimen is well ignited.
• Start timing immediately after ignition and record whether the specimen burns for more than 180 seconds or the flame front reaches the 50 mm reference mark.
• Adjust the oxygen concentration downward if the specimen burns beyond either criterion, or upward if it extinguishes prematurely, following the specified step increments.
• Use progressively smaller concentration intervals, narrowing to approximately 0.2% oxygen near the critical value, to bracket the response of the material.
• Repeat the procedure with fresh specimens until the concentration difference between successive burning and non-burning results meets the criterion defined in the standard, then confirm the final value with additional repeat specimens as required.
• Calculate the LOI using the volumetric formula: LOI = 100 × V(O2) / [V(O2) + V(N2)], where V(O2) and V(N2) are the volumetric flow rates of oxygen and nitrogen at the critical concentration.

Result Interpretation

The reported LOI is typically the average of valid determinations, rounded to 0.1% or 0.2% depending on the standard. General interpretation guidelines are as follows:

• LOI below 21%: the material tends to burn readily in normal air and is generally classified as flammable
• LOI between 21% and 24%: the material burns slowly in air and requires flame-retardant improvement for most applications
• LOI between 25% and 28%: the material is difficult to ignite and shows self-extinguishing tendencies
• LOI above 28%: the material is generally regarded as self-extinguishing and suitable for many flame-retardancy requirements

It should be noted that LOI is a relative, small-scale laboratory index. Product qualification often requires additional fire performance tests, and results should always be correlated with the full fire safety requirements of the end application.

Applications

LOI testing is extensively used across industries where plastic flammability performance is critical:

• Wire and cable: evaluating insulation and sheathing compounds for flame-retardant grades and halogen-free formulations
• Electronics and electrical equipment: screening enclosure and component materials in combination with UL 94 and IEC 60695 based flammability tests
• Construction products: supporting material selection for panels, pipes, flooring, and thermal insulation systems under EN and GB framework requirements
• Rail transit and automotive: qualifying interior materials against strict fire safety specifications for rolling stock and vehicles
• Aerospace and marine: verifying flame-retardant performance of polymer components used in cabins and equipment
• Research and development: comparing flame-retardant formulations, filler loadings, and polymer modifications during material development
• Quality control: monitoring batch-to-batch consistency of flame-retardant compounds in routine production testing

Accredited third-party laboratories commonly issue LOI test reports for supplier declarations, regulatory submissions, and customer audits, ensuring that the data are traceable to recognized international standards.

FAQ

What is the Limiting Oxygen Index of a plastic?

The Limiting Oxygen Index is the minimum oxygen concentration, expressed as a volume percentage in an oxygen/nitrogen mixture, that will just sustain flaming combustion of a vertically mounted specimen for 180 seconds or over a burning length of 50 mm under specified test conditions.

Which standards govern plastic LOI testing?

The most commonly applied standards are ISO 4589-2, ASTM D2863, GB/T 2406.2, and JIS K 7201-2. These standards describe essentially the same principle with minor differences in specimen dimensions, gas flow parameters, and calculation details.

How many specimens are needed for one LOI test?

Because each specimen is consumed during testing and the concentration bracketing procedure requires multiple trials, at least 15 to 20 specimens are typically prepared for a single determination, with additional specimens recommended for verification runs.

Is a higher LOI always better?

A higher LOI indicates greater resistance to ignition and continued combustion under the test conditions. However, LOI is a relative small-scale index and does not alone predict real-scale fire hazard; it should be combined with other flammability tests, such as UL 94 or heat release measurements, for comprehensive assessment.

How does LOI relate to the UL 94 test?

LOI and UL 94 measure different aspects of flammability: LOI quantifies the oxygen concentration needed to sustain burning, while UL 94 evaluates the ability of a material to extinguish after ignition in a specific geometry. The two tests are complementary, and material specifications frequently require both.

What factors can influence LOI results?

Specimen geometry and thickness, material density, filler content, specimen conditioning, ambient temperature and humidity, gas flow stability, and ignition technique can all affect results. Strict adherence to the specified test conditions and use of calibrated equipment in accredited third-party laboratories are essential for reproducible data.