Plastic Oxygen Index Testing Standards Explained: A Technical Guide to LOI Determination

The Limiting Oxygen Index (LOI) test is one of the most widely used methods for evaluating the flammability of plastic materials, providing a quantitative measure of how much oxygen a material needs to sustain combustion. This article provides a detailed analysis of the major oxygen index testing standards, including ISO 4589-2, ASTM D2863, and GB/T 2406.2, covering sample requirements, test procedures, and result interpretation. Understanding these standards is essential for manufacturers, quality engineers, and product designers seeking to verify flame-retardant performance and achieve regulatory compliance.

Technical Overview

The oxygen index test, also known as the Limiting Oxygen Index (LOI) test, measures the minimum concentration of oxygen in a flowing mixture of oxygen and nitrogen that will just support candle-like combustion of a plastic specimen. Because ambient air contains approximately 20.9% oxygen by volume, the LOI value serves as a practical benchmark for assessing whether a material will continue burning under normal atmospheric conditions.

A material with an LOI value below 21% will generally burn readily in air, while a material with an LOI above 21% tends to self-extinguish once the ignition source is removed. In engineering practice, materials are often broadly classified as follows:

• LOI below 21%: Highly flammable materials that burn easily in air, such as polyethylene and polypropylene (typically 17–19%).
• LOI between 21% and 28%: Slow-burning or moderately flame-retardant materials that may still propagate flame under favorable conditions.
• LOI above 28%: Generally considered self-extinguishing or difficult to ignite, including many engineering thermoplastics and flame-retarded compounds.

The LOI method offers several advantages: it produces a numerical result suitable for quality control and material comparison, requires relatively small specimens, and delivers good repeatability when performed under standardized conditions. However, it should be noted that LOI results describe behavior in a controlled laboratory atmosphere and do not directly predict performance in real-scale fire scenarios. For this reason, LOI testing is frequently combined with other flammability assessments such as UL 94 vertical and horizontal burn tests.

Testing Samples

Oxygen index testing can be performed on a wide variety of plastic material forms. The specimen geometry depends on the material type and the specific standard applied, and sample preparation directly influences the accuracy of results.

Rigid Plastics

For self-supporting materials such as rigid sheets, rods, and molded or machined plaques, ISO 4589-2 and GB/T 2406.2 specify specimens typically 80 mm to 150 mm in length and 10 mm in width, with a preferred thickness of 4 mm (acceptable range 4 mm to 10 mm, or 10 mm to 12 mm for certain cases). Edges should be smooth and free from burrs, cracks, or stress concentrations that could affect burning behavior.

Flexible Films and Thin Sheets

Thin flexible materials require special specimen preparation, often using a frame or support to hold the film vertically in the specimen holder. Film specimens are frequently folded or rolled into specific geometries, as defined in the relevant standard annexes, to ensure reproducible ignition and flame spread.

Cellular Materials

For cellular plastics (foams), ISO 4589-3 extends the oxygen index method to include elevated-temperature testing. Specimen dimensions are adjusted to account for the lower density and faster burning characteristics of foamed materials, and dedicated holders are used to prevent specimen deformation.

Specimen Conditioning and Quantity

Prior to testing, specimens must be conditioned in a controlled atmosphere, typically 23 ± 2 °C and 50 ± 5% relative humidity for at least 40 hours (88 hours in some standards for certain hygroscopic materials). A full LOI determination generally consumes 15 or more specimens, because the oxygen concentration must be adjusted iteratively using the bracketing procedure to converge on the critical value. For materials with significantly different burning behavior in two directions (for example, extruded or fiber-reinforced plastics), testing in both orientations may be required.

Testing Items

The primary output of an oxygen index test program typically includes the following items:

• LOI value determination: The minimum oxygen volume fraction, expressed as a percentage, at which candle-like combustion is just sustained for the defined period or over the defined specimen length.
• Ignition method verification: Confirmation of behavior using either Method A (top surface ignition) or Method B (propagating ignition with a longer flame application), as applicable to the material under test.
• Burning behavior observation: Recording of afterflame time, afterglow, whether burning reaches the specimen clamp or a marked reference line, dripping of flaming particles, and other phenomena.
• Comparative assessment against specification limits: Many product standards and procurement specifications define minimum LOI thresholds (for example, LOI ≥ 28% or ≥ 32%) that the material must meet.
• Elevated-temperature LOI (optional): For materials used at higher service temperatures, the oxygen index can be determined at elevated temperatures in accordance with ISO 4589-3.

Testing Methods

Several international standards govern oxygen index testing, and they share the same fundamental principle while differing in certain procedural details.

ISO 4589-2

ISO 4589-2, Plastics — Determination of burning behaviour by oxygen index — Part 2: Ambient-temperature test, is the reference method used throughout Europe and much of Asia. GB/T 2406.2 is the Chinese national standard technically aligned with ISO 4589-2. The test apparatus consists of a heat-resistant glass chimney (minimum 450 mm high, 75 mm to 100 mm in diameter), a specimen holder, a gas mixing and flow control system with calibrated oxygen analyzers or flow meters, and a small propane-fueled igniter.

ASTM D2863

ASTM D2863, Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index), is the corresponding North American method. It is technically similar to ISO 4589-2, with differences in chimney dimensions, flow rates, and certain timing criteria. Because results from the two standards are usually comparable but not always identical, the applicable specification should always state which standard governs acceptance.

JIS K 7201-2

The Japanese Industrial Standard JIS K 7201-2 covers the oxygen index determination for plastics and is harmonized with the ISO methodology for ambient-temperature testing, making it widely accepted for Japanese market submissions.

Test Procedure Summary

The general procedure under these standards is as follows:

• Mount the conditioned specimen vertically in the holder so that at least 50 mm of the specimen extends above the top of the chimney.
• Establish a total gas flow of approximately 40 mm/s linear velocity through the chimney, with a starting oxygen concentration estimated from the expected flammability of the material.
• Apply the ignition flame to the top of the specimen for up to 30 seconds (Method A: surface ignition; Method B: propagating ignition), withdrawing and reapplying the igniter as specified.
• Observe whether burning is sustained for at least 3 minutes or reaches a reference line 50 mm below the top of the specimen; this defines "sustained combustion."
• Using the bracketing method, increase the oxygen concentration after a failure to ignite or decrease it after sustained burning, progressively narrowing the interval until the difference between pass and fail concentrations meets the prescribed tolerance.
• Calculate the LOI from the final bracketing results and report the value to one decimal place, together with specimen geometry, conditioning details, ignition method, and observed burning phenomena.

Result reporting should always state the standard used, specimen dimensions and orientation, and any deviations, because these factors influence reproducibility between laboratories.

Applications

Oxygen index data support material selection, product development, and regulatory compliance across many industries:

• Wire and cable: Insulation and sheathing compounds for building wires, power cables, and data cables are frequently specified with minimum LOI values, often alongside smoke density and vertical flame propagation tests.
• Building and construction: Flame-retardant pipes, panels, insulation foams, and decorative materials rely on LOI testing as part of fire performance classification and product certification.
• Electrical and electronic equipment: Enclosures, connectors, and circuit board laminates are characterized using LOI together with UL 94 flammability ratings to demonstrate compliance with equipment safety requirements.
• Rail transit and aerospace: Interior materials for rolling stock and aircraft are subject to stringent fire safety specifications in which LOI serves as a screening and quality control parameter.
• Automotive: Flame-retarded polymers used in engine compartments, battery systems, and electrical components are evaluated using LOI data during development and incoming material inspection.
• Research and quality control: Flame-retardant formulators use LOI as a rapid, sensitive indicator to compare additive effectiveness, optimize loadings, and monitor production consistency batch to batch.

Manufacturers exporting to regulated markets commonly engage accredited third-party laboratories to perform LOI testing under ISO/IEC 17025-accredited conditions, ensuring that results are accepted by certification bodies and downstream customers.

FAQ

Q1: What is considered a good oxygen index value for plastics?

Generally, an LOI above 21% indicates the material will not readily continue burning in normal air. For most flame-retardant applications, specifications call for LOI values of 28% or higher, and high-performance applications such as rail and aerospace interiors may require 32% or more. The required value always depends on the applicable product standard and end-use scenario.

Q2: How does LOI testing relate to the UL 94 test?

The two methods measure different things. LOI quantifies the oxygen concentration required to sustain combustion, while UL 94 evaluates the material's ability to extinguish after ignition in ambient air with defined ratings such as V-0, V-2, and 5VA. They are complementary and are frequently both required in material datasheets and specifications.

Q3: Are ISO 4589-2, ASTM D2863, and GB/T 2406.2 results interchangeable?

The methods are based on the same principle and typically produce similar values, but differences in apparatus geometry, flow conditions, and pass/fail timing mean results are not automatically interchangeable. Contracts and specifications should identify a single governing standard to avoid acceptance disputes.

Q4: How many samples are needed for one LOI test?

A complete determination usually requires 15 to 20 specimens because the bracketing procedure consumes several samples at different oxygen concentrations. Additional specimens should be prepared for repeat testing or for testing in multiple orientations when the material is anisotropic.

Q5: What factors can cause variability in LOI results?

Common sources of variability include improper specimen conditioning, incorrect gas flow calibration, inaccurate oxygen concentration control, variations in specimen thickness and edge quality, and inconsistent ignition technique. Using calibrated equipment and trained operators in accredited laboratories minimizes these effects.

Q6: Can oxygen index testing be performed at elevated temperatures?

Yes. ISO 4589-3 defines the elevated-temperature oxygen index procedure, which is important for materials that serve in warm environments, since LOI values typically decrease as temperature rises and a material flame-retardant at room temperature may burn at elevated temperature.