The ABS plastic oxygen index test, also known as the Limiting Oxygen Index (LOI) test, is a standardized laboratory method used to evaluate the flammability characteristics of ABS (Acrylonitrile Butadiene Styrene) materials. Performed in accordance with ASTM D2863, ISO 4589-2, and GB/T 2406.2, this test determines the minimum oxygen concentration required to sustain candle-like combustion of a vertically mounted specimen. Because standard ABS grades have a relatively low LOI of approximately 18–19%, oxygen index testing is essential for verifying flame-retardant formulations used in electronics, appliances, and automotive applications.
Technical Overview
ABS (Acrylonitrile Butadiene Styrene) is a widely used engineering thermoplastic valued for its excellent impact resistance, dimensional stability, processability, and surface aesthetics. However, unmodified ABS is inherently flammable, with a limiting oxygen index of approximately 18–19%, meaning it continues to burn in normal ambient air, which contains about 21% oxygen. This characteristic makes oxygen index testing a critical quality control and material qualification step for manufacturers.
The oxygen index test quantifies 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 test specimen. A higher LOI value indicates better flame retardancy: materials with an LOI above approximately 26% are generally considered self-extinguishing in air. Flame-retardant ABS grades, typically modified with brominated or phosphorus-based additive systems, can achieve LOI values of 27% or higher.
The primary standards governing ABS oxygen index testing include:
- ASTM D2863 – Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics
- ISO 4589-2 – Plastics: Determination of burning behaviour by oxygen index, Part 2: Ambient-temperature test
- GB/T 2406.2 – Chinese national standard, technically aligned with ISO 4589-2
- NES 714 and IEC 61144 – Related methods used in specific industries and regions
Results from the oxygen index test are frequently used alongside vertical/horizontal burn ratings (such as UL 94) to provide a comprehensive flammability profile of ABS materials for regulatory compliance and product safety certification.
Testing Samples
Injection-Molded Specimen Strips
The most common sample form is injection-molded rectangular strips produced directly from the ABS resin or compound under evaluation. ISO 4589-2 specifies preferred dimensions of 80–150 mm in length, 10 mm in width, and 4 mm in thickness, while ASTM D2863 typically uses specimens measuring 70–150 mm long, 6.5 ± 0.5 mm wide, and 3.0 ± 0.5 mm thick. Molding conditions must be documented, as residual stresses and processing parameters can influence burning behavior.
Cut Specimens from Sheets or Plaques
Specimens may also be cut from compression-molded plaques, extruded ABS sheet, or calendered stock using a saw, blade, or milling cutter. Edges must be smooth and free of burrs, cracks, and flash, as irregular edges can affect flame propagation and ignition behavior. All cut surfaces should be cleaned and deburred before conditioning.
Specimens Machined from Finished Parts
For verification testing of production components such as equipment housings or appliance panels, strips may be machined from the actual molded part, provided the resulting dimensions meet the standard requirements. Test reports should note the part origin, location of cut, and any surface textures, coatings, or print layers present on the specimen.
Specimen Conditioning
Prior to testing, specimens must be conditioned to a consistent moisture and temperature equilibrium. Standard practice requires conditioning at 23 ± 2°C and 50 ± 5% relative humidity for a minimum of 88 hours in accordance with ISO 291 or ASTM D618, unless otherwise agreed. A minimum of 15 specimens is typically required to complete a full determination using the bracketing procedure.
Testing Items
The ABS oxygen index test program typically includes the following measurement and assessment items:
- Limiting Oxygen Index (LOI) value – the primary result, reported as the volume percentage of oxygen at the boundary between burning and self-extinguishing behavior
- Ignition and flame propagation observations – whether the specimen ignites readily, how far the flame front travels, and whether burning reaches the clamp or specimen top
- Afterflame and afterglow duration – the time the specimen continues to burn or glow after removal of the ignition source
- Burning behavior characterization – dripping of flaming particles, melting, charring, smoke generation, and curling of the specimen away from the flame
- Flame retardancy verification – confirmation that flame-retardant ABS grades meet specified LOI thresholds (for example, ≥ 27% or ≥ 32% depending on the application requirement)
- Comparison testing – evaluation of LOI differences between base resin, modified formulations, or competitive materials
Testing Methods
Test Apparatus
Testing is conducted on an oxygen index apparatus consisting of a heat-resistant glass chimney (typically 450–500 mm high and 75–100 mm in diameter), a specimen holder that supports the strip vertically in the center of the chimney, a controlled gas mixing and delivery system with calibrated flow meters or mass flow controllers for oxygen and nitrogen, and a small premixed gas flame igniter. The gas supply must use oxygen and nitrogen of at least 98% purity, and the flow system must be verified or calibrated at regular intervals.
Test Procedure
The specimen is clamped vertically at least 15 mm below the top of the chimney. A gas mixture of oxygen and nitrogen is introduced through the base of the chimney at a total flow rate specified by the standard (for example, 40 mm/s linear velocity in ISO 4589-2), and the system is purged for at least 30 seconds to ensure a stable atmosphere. The top of the specimen is then ignited using the small diffusion flame applied so that the specimen burns downward in a candle-like manner.
After ignition, the operator observes whether the specimen burns for a specified duration or over a specified length, or extinguishes on its own. The oxygen concentration is adjusted upward or downward, and a fresh specimen is tested at each new concentration. This bracketing procedure continues until the critical oxygen concentration—the narrow boundary between sustained burning and self-extinguishing—is established, typically using the step-by-step or binary search method defined in the standard.
Calculation and Expression of Results
The limiting oxygen index is calculated from the volume fractions of the gases at the critical concentration:
LOI = 100 × V(O₂) / [V(O₂) + V(N₂)]
The result is reported as a percentage, rounded to 0.1% or 0.2% depending on the standard, together with the specimen dimensions, ignition method used (Method A, B, or C where applicable), and observations of burning behavior. Replicate determinations and the calculated spread of results should be checked against the repeatability criteria of the standard.
Quality and Validity Checks
To ensure valid results, the laboratory must verify gas purity and flow calibration, confirm specimen conditioning, maintain ambient laboratory conditions within tolerance, and use only specimens free from defects. Reference materials with known LOI values are periodically tested to verify apparatus performance and operator technique.
Applications
Oxygen index testing of ABS materials supports a broad range of industrial and regulatory applications:
- Electrical and electronics – qualification of ABS and ABS alloy housings for computers, monitors, power adapters, chargers, and switchgear under IEC and UL safety requirements
- Home appliances – verification of flame-retardant grades used in television cabinets, washing machine control panels, vacuum cleaner bodies, and kitchen appliance enclosures
- Automotive – evaluation of interior trim components, dashboard elements, and electrical connector housings against FMVSS 302 and OEM flammability specifications
- Building and construction – assessment of ABS pipes, fittings, decorative panels, and profiles for flame performance classification
- Flame retardant R&D – screening and optimization of halogenated, phosphorus-based, and intumescent flame retardant packages in ABS and ABS/PC blends
- Incoming material control – routine quality control to confirm that supplied resin lots conform to the LOI specification agreed between resin producers, compounders, and molders
- Certification support – generation of test data for compliance dossiers supporting CE marking, UL recognition, and other market access programs
FAQ
What is the typical oxygen index of standard ABS?
Unmodified general-purpose ABS typically has a limiting oxygen index of approximately 18–19%. Because this value is below the 21% oxygen concentration of ambient air, standard ABS will continue to burn after ignition, which is why flame-retardant grades are required for most electrical applications.
What LOI value qualifies as flame-retardant ABS?
Flame-retardant ABS formulations generally achieve LOI values of 27% or higher, with high-performance grades reaching 30–35%. Many specifications pair the LOI requirement with a UL 94 V-0 rating at a defined thickness to fully define flame performance.
How does the oxygen index test differ from the UL 94 test?
The oxygen index test quantifies flammability as the minimum oxygen concentration needed to sustain burning, producing a numerical result that is highly reproducible and useful for material comparison. The UL 94 test evaluates the ability of a specimen to extinguish after ignition in ambient air and assigns a classification such as V-0, V-1, or V-2. The two methods are complementary and are commonly performed together.
Does specimen thickness affect the LOI result?
Yes. Thicker ABS specimens generally burn more slowly and can exhibit somewhat higher apparent LOI values than thin specimens. This is why the test report must state the specimen dimensions, and why comparisons should only be made between materials tested at the same thickness.
How many specimens are needed for one test?
A complete determination typically requires a minimum of 15 specimens to perform the bracketing procedure defined in ASTM D2863 and ISO 4589-2, with additional specimens available in case repeated checks are necessary.
Can filled, reinforced, or ABS alloy materials be tested?
Yes. The oxygen index method applies to ABS compounds containing glass fiber, mineral fillers, flame retardants, and to ABS/PC and ABS/PVC alloys, provided the specimens can be prepared to the specified dimensions and can be supported vertically without deformation during the test.