The oxygen index test, also known as the Limiting Oxygen Index (LOI) test, is a critical flammability assessment for ABS (Acrylonitrile Butadiene Styrene) plastics used in electronics, automotive, and appliance applications. This guide explains the major ABS oxygen index testing standards, including ISO 4589-2, ASTM D2863, and GB/T 2406, covering sample preparation, test procedures, acceptance criteria, and practical interpretation of results for manufacturers and quality engineers.
Technical Overview
The oxygen index test measures the minimum concentration of oxygen, expressed as a volume percentage in a flowing mixture of oxygen and nitrogen, that will just support candle-like combustion of a vertically mounted test specimen. For ABS, a widely used engineering thermoplastic, this value is a fundamental indicator of flammability performance and flame-retardant effectiveness.
Because ordinary air contains approximately 20.9% oxygen, materials with an LOI below 21% will continue burning in air after ignition, while materials with an LOI significantly above 21% tend to self-extinguish. Standard-grade ABS typically exhibits an oxygen index of 18% to 19%, classifying it as a flammable material. Flame-retardant ABS grades, modified with additives such as brominated compounds and antimony trioxide synergists, can achieve oxygen index values of 28% to 35% or higher, meeting demanding fire safety requirements in electronics and electrical equipment.
The oxygen index method offers several advantages: it provides a quantitative, reproducible numerical result rather than a simple pass/fail outcome; it requires relatively small specimens; and it is highly sensitive to small changes in material composition, making it valuable for both quality control and research and development of flame-retardant formulations.
Governing Standards
Several internationally recognized standards define the apparatus, specimens, and procedures for oxygen index testing. The most commonly applied to ABS are summarized below:
- ISO 4589-2: Plastics — Determination of burning behaviour by oxygen index — Part 2: Ambient-temperature test. The primary international standard specifying specimen dimensions, apparatus configuration, and the incremental test procedure.
- ISO 4589-3: Extension to elevated-temperature testing, used when materials are evaluated at temperatures above ambient conditions.
- ASTM D2863: Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index). The North American equivalent, broadly harmonized with ISO 4589-2 but with minor differences in specimen size and verification requirements.
- GB/T 2406.2: The Chinese national standard, technically aligned with ISO 4589-2, frequently referenced in Chinese regulatory and customer specifications for ABS materials.
- NES 714 / IEC 60695-2 series (related): Navy and electrical safety specifications that reference oxygen index limits as part of material qualification, often alongside complementary tests such as UL 94.
Testing Samples
Standard Specimen Dimensions
Under ISO 4589-2, the standard specimen for rigid, self-supporting ABS is 80 mm to 150 mm in length, 10 mm in width, and 4 mm in thickness. ASTM D2863 uses specimens of 70 mm to 150 mm in length, 6.5 mm in width, and 3 mm in thickness as the standard configuration. Specimens must be free from defects, burrs, and flash that could influence ignition behavior.
Specimen Conditioning
Prior to testing, ABS specimens must be conditioned to a controlled state, typically 23°C ± 2°C and 50% ± 5% relative humidity for a minimum of 88 hours in accordance with ISO 291 or ASTM D618. Because oxygen index results for some polymers are sensitive to moisture content, consistent conditioning is essential for reproducible data.
Sample Quantity and Preparation
A minimum of 15 specimens is generally required for a complete determination, since the procedure involves bracketing the critical oxygen concentration through a series of ignitions. Specimens may be injection molded, machined from sheet, or cut from finished parts; in all cases, the fabrication method must be documented, as surface condition and molecular orientation can influence results.
Testing Items
When an ABS material is submitted for oxygen index evaluation, the following parameters are typically determined and reported:
- Limiting Oxygen Index (LOI) value, expressed as the volume percentage of oxygen in the O2/N2 mixture
- Burning time after removal of the ignition source for each trial
- Burned or damaged length of the specimen for each trial
- Whether specimens meet the ignition criterion (three-minute burning time or 50 mm burned length under ISO 4589-2)
- Verification of apparatus performance using a reference material such as polymethyl methacrylate (PMMA)
- Optional observations: dripping, flaming drops, afterflame, afterglow, and smoke behavior
Testing Methods
Apparatus Description
The test apparatus consists of a heat-resistant glass chimney (typically 450 mm to 500 mm high and 75 mm to 100 mm in diameter), a specimen holder that clamps the sample vertically in the center of the chimney, a precise oxygen and nitrogen mixing and metering system with calibrated flow controllers, and a small hydrogen-free gas flame igniter. The total gas flow rate through the chimney is maintained at 40 mm/s ± 2 mm/s linear velocity, corresponding to approximately 10 L/min to 12 L/min depending on chimney diameter.
Test Procedure
The specimen is mounted vertically so that its top is at least 100 mm below the top of the chimney. The gas flow is set to the starting oxygen concentration, and the mixture is allowed to purge the column for at least 30 seconds. The operator then applies the igniter flame to the top of the specimen for up to 15 seconds, removing it periodically to check whether the specimen sustains ignition.
Following each ignition, the operator records whether the specimen burns for three minutes or more, or consumes 50 mm or more of its length (ISO 4589-2 criterion). If the specimen fails this criterion, the oxygen concentration is increased for the next specimen; if it passes, the oxygen concentration is decreased. This staircase approach, governed by the response of the previous specimen, continues until a critical bracket of concentrations is established.
Calculation of the Oxygen Index
The final LOI value is determined from the series of responses in the bracketing zone, either as the lowest oxygen concentration at which the material just sustains burning, or calculated using the statistical method specified in the standard (the NB/D-response method in ISO 4589-2). The result is reported as OI = (volume of oxygen / total volume of gas) × 100. Reproducibility within 0.2% to 0.5% oxygen is expected for a well-maintained apparatus operated by trained personnel.
Relationship to UL 94 Classification
Oxygen index testing is frequently performed alongside UL 94 horizontal and vertical burn testing. As a general correlation for ABS-based materials, an LOI of 28% or greater typically supports UL 94 V-0 performance at equivalent thicknesses, though the two tests measure different phenomena and one cannot formally substitute for the other in qualification programs.
Applications
Oxygen index data for ABS materials are used across many industries and regulatory contexts:
- Consumer electronics and IT equipment: Enclosures and internal components for monitors, printers, chargers, and power adapters, where flame-retardant ABS with elevated LOI satisfies IEC 60950-1 and IEC 62368-1 material requirements.
- Household appliances: Housings, control panels, and knobs for appliances requiring compliance with IEC 60335-1 and related household equipment safety standards.
- Automotive components: Interior trim parts, battery enclosures, and electrical connectors evaluated under FMVSS 302 and OEM material specifications.
- Building and construction: ABS pipe, conduit, and decorative elements where fire performance classifications reference LOI values.
- Material development and quality control: Incoming inspection of flame-retardant ABS compounds, batch-to-batch consistency verification, and optimization of halogenated, phosphorus-based, or intumescent flame-retardant packages.
- Railway and mass transit: Interior components subject to EN 45545-2, where oxygen index results contribute to fire safety assessment alongside smoke and toxicity testing.
FAQ
What is a good oxygen index value for ABS? Standard ABS has an LOI of approximately 18% to 19% and will burn readily in air. For flame-retardant applications, an LOI of 28% or higher is generally targeted, as this indicates self-extinguishing behavior under ambient conditions and typically correlates with UL 94 V-0 ratings.
Which standard should I specify for ABS: ISO 4589-2 or ASTM D2863? Both standards are technically equivalent in principle. ISO 4589-2 is preferred for international and European markets, while ASTM D2863 is customary in North America. GB/T 2406.2 is the appropriate reference for the Chinese market. Confirm the required standard with your customer or target regulation before testing.
How many specimens are needed for a valid test? A minimum of 15 specimens is recommended for a full determination, since the staircase procedure requires repeated trials to bracket the critical oxygen concentration. Additional specimens may be necessary if the initial concentration guess is far from the final LOI value.
Does specimen thickness affect the oxygen index result? Yes. Thicker ABS specimens generally exhibit slightly higher LOI values than thin ones because more heat is required to sustain burning of a larger cross-section. Results should always be reported together with the specimen dimensions used.
Can oxygen index testing alone prove a material is flame retardant? No. The LOI test measures behavior in a controlled oxygen-enriched or depleted atmosphere and does not capture all real-fire scenarios such as radiant heat exposure, flame spread, or dripping ignition. It is normally combined with UL 94, glow-wire (IEC 60695-2-10 to -13), or cone calorimetry testing for complete fire hazard assessment.
How often should the apparatus be verified? The gas analyzers and flow meters should be calibrated at regular intervals per the standard, and apparatus performance should be checked using a certified reference material such as PMMA, which has a documented oxygen index of approximately 17.3% to 18.2%, before beginning a new test series.