The limiting oxygen index (LOI) test is one of the most widely used flammability evaluation methods for ABS (acrylonitrile butadiene styrene) thermoplastics, providing a quantitative measure of how easily the material sustains combustion. This article provides a comprehensive technical guide to ABS LOI determination, covering international standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, sample preparation requirements, step-by-step testing procedures, calculation methods, and practical applications in flame-retardant material development and quality control.
Technical Overview
ABS (acrylonitrile butadiene styrene) is a versatile amorphous thermoplastics widely used in electronic housings, automotive components, appliances, and consumer goods. Because standard ABS grades have a limiting oxygen index of approximately 18% to 21%, they burn readily in normal atmospheric air, which contains approximately 21% oxygen by volume. This inherent flammability makes LOI testing essential for evaluating both virgin ABS and flame-retardant ABS formulations.
The limiting oxygen index is defined as the minimum concentration of oxygen in a flowing mixture of oxygen and nitrogen, expressed as a volume percentage, that will just barely support candle-like downward combustion of a vertically mounted test specimen. A higher LOI value indicates better flame resistance. Materials with an LOI below 21% are generally considered flammable in air, while materials with an LOI above approximately 25% to 27% are regarded as flame-retardant or self-extinguishing in many regulatory contexts.
LOI determination offers several practical advantages: it produces a numerical result suitable for material comparison, requires relatively small specimens, uses inexpensive instrumentation, and correlates reasonably well with other flammability tests such as UL 94 vertical burn classification. For ABS, LOI values typically range from about 18% for unfilled general-purpose grades to 28% to 32% for brominated or phosphorus-based flame-retardant compounds.
Testing Samples
Rigid Self-Supporting Molding Materials
For standard ABS injection-molded or compression-molded specimens, ISO 4589-2 requires samples of 80 mm to 150 mm in length, 10 mm ± 0.5 mm in width, and 4 mm ± 0.25 mm in thickness. ASTM D2863 specifies similar geometry of 70 mm to 150 mm in length, 6.5 mm ± 0.5 mm in width, and 3 mm ± 0.5 mm in thickness. Specimens must be smooth, free from flash, bubbles, and surface defects, and the edges should be free from burrs that could influence flame propagation.
Flexible ABS Compounds
Flexible or soft ABS grades that cannot support their own weight are tested using a special sample holder with a U-shaped clamping frame that holds the specimen in a vertical position while allowing the upper 60 mm to remain exposed for ignition and burning evaluation.
ABS Film and Sheet Materials
Thin ABS films are rolled into a spiral and inserted into a holder so that the resulting rolled specimen approaches the standard geometry, ensuring a comparable surface-to-mass ratio and consistent burning behavior during the test.
Sample Conditioning
Prior to testing, specimens must be conditioned at 23°C ± 2°C and 50% ± 10% relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618. For ABS materials with volatile flame retardants or absorbed moisture sensitivity, the conditioning state must be strictly controlled, as both humidity and aging can measurably shift the LOI result.
Testing Items
The primary measured parameter is the LOI value itself, reported as the volume percentage of oxygen at the combustion boundary condition. Supporting observations recorded during testing include:
- Whether the specimen burns for more than 3 minutes after removal of the ignition source
- Whether the burned or charred length exceeds 50 mm from the top edge of the specimen
- Afterflame time, afterglow behavior, dripping of flaming particles, and whether cotton ignition occurs below the specimen
- The influence of specimen thickness, since LOI generally decreases as thickness increases for many thermoplastics
- Repeatability data from multiple specimens at each oxygen concentration level
Testing Methods
Applicable Standards
LOI determination for ABS is performed under one of the following equivalent standards: ISO 4589-1 (general guidance), ISO 4589-2 (ambient-temperature test), ASTM D2863 (Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics), and GB/T 2406.2 in the Chinese national framework. These standards share the same fundamental principle and differ mainly in specimen dimensions, pass criteria details, and reporting formats.
Test Apparatus
The LOI apparatus consists of a heat-resistant glass chimney (typically 450 mm to 500 mm high and 75 mm to 100 mm in internal diameter) mounted on a base with a non-flammable bead bed for gas mixing and distribution, a vertically adjustable specimen holder, a precision oxygen and nitrogen gas supply with calibrated rotameters or mass flow controllers, and an ignition burner fed by propane or butane producing a flame of 10 mm to 20 mm in length. A calibrated oxygen analyzer is strongly recommended to verify the actual oxygen concentration delivered to the chimney.
Test Procedure
- Mount the conditioned ABS specimen vertically in the holder so that at least 50 mm of the specimen top extends above the top of the chimney mouth is avoided; the specimen top should be at least 100 mm below the chimney top.
- Set an initial oxygen concentration based on prior knowledge of the material; for standard ABS begin near 19% to 21%, for flame-retardant ABS begin near 26% to 28%.
- Purge the chimney with the gas mixture for at least 30 seconds and verify flow stability before ignition.
- Apply the ignition flame to the top surface of the specimen, sweeping across the full width for up to 5 seconds; if ignition does not occur, repeat the application for another 5 seconds.
- Remove the burner and record the burning time and the extent of burned or charred length.
- Apply the pass or fail criteria: for ISO 4589-2, the specimen fails if burning exceeds 3 minutes or if the burning or charring front extends more than 50 mm down the specimen; it passes otherwise.
- Use the bracketing or direct-search approach: vary the oxygen concentration using large steps of about 2% initially, then progressively smaller intervals of 0.2% until the boundary between pass and fail is bracketed.
- Test at least three specimens at the final concentration to confirm repeatability of the boundary condition.
Calculation of LOI
The limiting oxygen index is calculated from the final oxygen concentration and the step interval using the formula LOI = cF + k·d, where cF is the oxygen volume percentage at the last data point of the series, d is the interval between concentration steps (typically 0.2%), and k is a correction factor determined from the observed pass and fail pattern of the final series of specimens as tabulated in ISO 4589-2. The result is reported to one decimal place, together with specimen dimensions, ignition mode, conditioning state, and whether testing was performed on the top or bottom edge.
Key Factors Affecting Accuracy
- Gas flow calibration errors: rotameters must be calibrated for oxygen and nitrogen separately, and total flow must typically remain between 10 and 13 L/min during testing.
- Specimen thickness and orientation: thicker ABS specimens exhibit lower apparent LOI values, so comparisons are valid only at identical geometry.
- Ignition technique: inconsistent flame application time or position significantly alters results, especially for thin flame-retardant ABS grades.
- Material homogeneity: poor dispersion of flame retardants, antimony trioxide synergists, or mineral fillers causes specimen-to-specimen scatter.
- Environmental conditions: ambient temperature, chimney drafts, and gas purity must be controlled within the tolerances of the standard.
Applications
LOI determination plays a central role across the entire ABS value chain:
- Flame-retardant formulation development: screening brominated, phosphorus-based, nitrogen-based, and intumescent flame retardant systems by comparing LOI increments per loading level.
- Quality control in compounding plants: routine LOI verification ensures batch-to-batch consistency of flame-retardant ABS compounds shipped to molders.
- Regulatory compliance support: LOI data supplement UL 94, IEC 60695, and RoHS/REACH documentation for electronic and electrical equipment housings, battery enclosures, chargers, and switchgear components.
- Material selection: engineers use LOI values to compare ABS against ABS/PC blends, HIPS, PVC, and polyamide alternatives when designing for fire safety in appliances, automotive interior parts, and building products.
- Academic and industrial research: studying combustion mechanisms, char formation, and synergistic flame-retardancy effects in modified ABS copolymers and nanocomposites.
- Aerospace and rail applications: elevated-temperature LOI testing per ISO 4589-3 supports qualification of ABS-based materials for specialized transport sectors.
FAQ
What LOI value qualifies ABS as flame retardant?
General-purpose ABS has an LOI of roughly 18% to 21%. Commercial flame-retardant ABS grades typically achieve LOI values of 26% to 32% through the addition of brominated flame retardants with antimony trioxide synergists or halogen-free phosphorus systems. Many specifications require a minimum LOI of 27% or 28% in combination with a UL 94 V-0 rating at the specified thickness.
Is LOI testing alone sufficient for fire safety certification?
No. LOI is a small-scale laboratory screening test. Regulatory approvals for end products generally require additional tests such as UL 94 vertical and horizontal burning, glow-wire ignition temperature (GWIT), glow-wire flammability index (GWFI), needle flame, and cone calorimetry depending on the application standard.
Why do my ABS LOI results differ between laboratories?
Common causes include differences in specimen thickness, conditioning humidity, gas flow calibration, ignition technique, and whether top or bottom ignition was used. Results are only comparable when the test standard, specimen geometry, conditioning, and ignition mode are identical.
How many specimens are required for one LOI determination?
A typical determination consumes 10 to 20 specimens: several for the initial coarse search and at least three to five for the final fine bracketing sequence at 0.2% oxygen intervals.
Does LOI correlate with UL 94 ratings?
There is a general correlation, but it is not exact. As a rule of thumb, thermoplastics with LOI above approximately 28% often achieve UL 94 V-0 at 1.6 mm thickness, while LOI below 25% rarely passes V-0. However, dripping behavior, char formation, and specimen thickness strongly influence UL 94 outcomes independently of LOI.