Limiting Oxygen Index (LOI) testing is the core laboratory method for quantifying the flame-retardant performance of modified ABS plastics used in electronics, appliances, automotive, and rail applications. Performed in accordance with ISO 4589-2, ASTM D2863, GB/T 2406.2, and JIS K 7201-2, the test determines the minimum oxygen concentration in an oxygen/nitrogen atmosphere that will just support combustion of a vertically mounted specimen. This article explains the testing principle, sample requirements, test procedure, data interpretation, and typical applications for flame-retardant and other modified ABS grades.
Technical Overview
Acrylonitrile butadiene styrene (ABS) is a widely used engineering thermoplastic valued for its toughness, dimensional stability, and good processability, but unmodified ABS burns readily in air, with a typical LOI of only 18 to 19 percent. Modified ABS grades, including flame-retardant ABS (FR-ABS), ABS/polycarbonate alloys, glass-fiber reinforced ABS, and mineral-filled compounds, are engineered to raise ignition resistance and self-extinguishing capability for regulated applications.
The Limiting Oxygen Index is defined as the minimum concentration of oxygen, expressed as a volume percent in a flowing oxygen/nitrogen mixture, that will just support candle-like combustion of a specimen for at least 3 minutes or over a distance of at least 50 mm after ignition with a small flame. Because normal ambient air contains approximately 21 percent oxygen, a material with an LOI above 21 percent will tend to self-extinguish in still air, and higher values indicate better flame retardancy. Commercial flame-retardant ABS grades typically achieve LOI values between 28 and 35 percent through brominated or phosphorus-based flame retardant systems, often synergized with antimony trioxide.
LOI testing is specified by internationally harmonized standards, including ISO 4589-2, ASTM D2863, GB/T 2406.2, and JIS K 7201-2. The result is used for quality control of compounds, incoming material verification, research and development of flame retardant formulations, and demonstration of compliance with product-level fire safety requirements, such as the oxygen index limits referenced in railway standard EN 45545-2. LOI data complement flammability classifications obtained from UL 94 (IEC 60695-11-10) burning tests and do not replace them.
Testing Samples
Molded Bar Specimens
The reference specimen geometry for ISO 4589-2 is a rectangular bar of 80 to 150 mm length, 10 mm width, and 4 mm thickness, produced by injection molding or compression molding under the material supplier's recommended processing conditions. ASTM D2863 and GB/T 2406.2 accept similar bar dimensions, typically 70 to 150 mm long, 6.5 mm wide, and 3 mm thick. Specimen dimensions must be reported with the result because thickness has a systematic influence on the measured LOI.
Sheet and Plate Materials
Modified ABS sheets, thermoformed plates, and laminates are tested by cutting strips to the standard bar dimensions while retaining the as-produced thickness. Results for non-standard thicknesses are valid but should only be compared with data at the same thickness.
Pipes, Tubes, and Profiles
For extruded ABS pipes and profiles, longitudinal strips are cut from the wall in accordance with the relevant product standard, mounted with the outer or inner surface oriented as specified, and tested in the same vertical configuration.
Films and Thin Sections
Thin modified ABS films that cannot stand freely are tested using a vertical support frame or rod arrangement, following the special procedures given in ISO 4589-2 and ASTM D2863 for flexible materials.
Finished Parts and Prototype Samples
Sections machined from finished components, 3D-printed coupons, or short-shot prototype parts can be evaluated, provided flat, uniform bars can be prepared. Flow direction, skin-core morphology, and filler orientation introduced by processing should be noted, as they can affect the result.
A minimum of 15 conditioned bars is recommended for a single LOI determination, because the bracketing procedure is iterative and consumes several specimens. Additional specimens should be prepared for repeat tests or when the expected oxygen concentration of the material is unknown.
Testing Items
Typical test programs for modified ABS materials include the following items:
- Ambient-temperature Limiting Oxygen Index determination per ISO 4589-2, ASTM D2863, GB/T 2406.2, or JIS K 7201-2, reported in volume percent to the precision defined by the standard.
- Elevated-temperature oxygen index testing per ISO 4589-3 for materials used in thermal environments.
- Thickness-series LOI measurements to generate design data for engineering documentation and material data sheets.
- LOI verification before and after heat aging, humidity aging, or UV exposure to assess the durability of the flame retardant package.
- Batch-to-batch quality control and incoming inspection of flame-retardant ABS compounds against a specified minimum LOI, commonly 27 or 28 percent.
- Comparative screening of halogen-free versus halogenated flame retardant formulations during research and development.
Testing Methods
Principle and Apparatus
The test is performed on a vertical glass chimney assembly comprising a specimen holder that clamps the bar at its lower portion, a controlled oxygen/nitrogen gas mixing and delivery system with calibrated flow measurement, a small premixed gas igniter, and a transparent column that shields the specimen from ambient drafts. Gas flows are regulated to the standardized total flow rate, corresponding to a nominal linear velocity of approximately 40 mm/s through the column in ISO 4589-2. Oxygen analyzers may be used to verify the delivered mixture.
Specimen Conditioning
Prior to testing, specimens are conditioned at 23 ± 2 °C and 50 ± 5 percent relative humidity for at least 88 hours according to ISO 291, or for 40 hours according to ASTM D618 Procedure A, unless the specification requires testing in the as-received or dry-as-molded state. Each specimen is measured, and a reference mark is applied 50 mm from the top edge to define the burning criterion.
Test Procedure
- Mount the conditioned specimen vertically in the center of the chimney and establish the oxygen/nitrogen mixture at an estimated starting concentration, allowing the system to purge for at least 30 seconds.
- Apply the ignition flame to the top of the specimen for the specified time until the full top face is ignited, then withdraw the igniter and start timing.
- Observe whether the specimen burns for 180 seconds or longer, or whether the flame front reaches or passes the 50 mm reference mark.
- If the burning criterion is met, reduce the oxygen concentration; if the specimen self-extinguishes before the criterion, increase the oxygen concentration. Initially coarse steps are used to bracket the critical concentration.
- Refine the oxygen concentration in small, uniform increments following the response-sequence calculation defined by the applicable standard until the limiting value is established.
- Repeat the confirmation runs with fresh specimens as required by the standard to finalize the reported index.
Calculation and Reporting
The oxygen index is calculated as the volume fraction of oxygen in the final gas mixture: LOI = 100 × V(O2) / [V(O2) + V(N2)], with the final value determined from the standardized response sequence of the last several tests rather than from a single observation. The test report must state the standard used, full specimen dimensions, conditioning history, ambient test conditions, the observed burning behavior, and the LOI value rounded as specified, typically to the nearest 0.1 or 0.2 percent.
Factors Influencing Results
Specimen thickness, moisture content, aging-induced migration or depletion of flame retardant additives, filler and glass-fiber orientation, processing-induced morphology, gas purity, and operator ignition technique can all shift the measured LOI. Strict adherence to specimen preparation and conditioning requirements is essential for reproducible, comparable results, and comparisons should always be made at identical thickness and test conditions.
Applications
LOI testing of modified ABS supports material qualification and fire safety compliance across many industries:
- Electrical and electronic products, including charger and adapter housings, television back covers, circuit breaker components, connectors, and switch assemblies, where LOI data accompany UL 94 V-0 classifications.
- Home appliances, such as washing machine control panels, microwave oven components, and fan housings.
- Automotive interior components and trim parts, where LOI screening complements horizontal burning requirements for interior materials.
- Railway and mass transit vehicles, where EN 45545-2 specifies minimum oxygen index values for listed interior and exterior components.
- Building and construction products, cable management accessories, and furniture components with flammability requirements.
- Compound manufacturing and supply chains, for batch quality control, incoming material verification, and supplier datasheet validation of flame-retardant ABS grades.
- Research and development programs optimizing halogen-free flame retardant packages, ABS/PC alloys, and reinforced grades.
Accredited third-party laboratories routinely perform these tests to generate the objective evidence required for regulatory submissions, customer specifications, and internal material approval.
FAQ
What LOI value qualifies modified ABS as flame retardant?
There is no universal threshold, but industry practice generally regards materials with an LOI of 27 percent or above as self-extinguishing, and many purchase specifications for flame-retardant ABS require a minimum of 28 percent. Since ambient air contains about 21 percent oxygen, any material with an LOI above 21 percent will not sustain candle-like burning in still air.
How does LOI relate to UL 94 ratings?
They are different and complementary tests. LOI quantifies the oxygen concentration needed to sustain combustion, while UL 94 (IEC 60695-11-10) evaluates afterflame time, afterglow, and flaming drips in a vertical or horizontal burn configuration. A high LOI generally correlates with better UL 94 performance, but no direct conversion exists, and product standards typically require both sets of data.
Does specimen thickness affect the LOI result?
Yes. Thicker specimens usually exhibit higher LOI values because more heat is conducted away from the burning zone relative to the fuel available. Results must always be reported with the specimen dimensions, and comparisons between materials are only valid at the same thickness.
Which standard should be specified: ISO 4589-2, ASTM D2863, or GB/T 2406.2?
These standards are technically harmonized and yield comparable data when specimen geometry and conditioning are matched. The choice depends on the target market, customer specification, or regulatory framework, with GB/T 2406.2 common in China, JIS K 7201-2 in Japan, and ISO or ASTM methods in international trade.
Why is it important to retest LOI after aging?
Flame retardant additives in modified ABS can migrate, volatilize, or degrade during prolonged thermal or humidity exposure, reducing the effective LOI over the product lifetime. Retesting after simulated service aging, for example by heat aging followed by ISO 4589-2 testing, verifies that the flame retardant performance is durable.
Can LOI testing be performed at elevated temperatures?
Yes. ISO 4589-3 defines the elevated-temperature oxygen index procedure, which is useful for modified ABS grades used near heat sources or in enclosed thermal environments where ignition resistance at operating temperature must be demonstrated.