The oxygen index test, also known as the limiting oxygen index (LOI) or critical oxygen index test, is one of the most widely used laboratory methods for evaluating the flammability of plastic materials. By measuring the minimum oxygen concentration required to sustain candle-like combustion, the test delivers a quantitative and highly reproducible indicator of flame retardancy in accordance with ISO 4589-2, ASTM D2863, and GB/T 2406.2. This article explains the technical principles, sample requirements, test procedures, and practical applications of plastic oxygen index performance evaluation testing.
Technical Overview
The oxygen index (OI), commonly called the limiting oxygen index (LOI), is defined as 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 material under specified conditions. Because ambient air contains approximately 20.9% oxygen, the LOI provides an immediate indication of how a plastic behaves after ignition in a normal atmosphere: materials with an LOI below roughly 21% will generally continue to burn in air, while higher values indicate progressively greater resistance to sustained flaming combustion.
Oxygen index testing is governed by an internationally harmonized family of standards, the most widely applied of which include:
- ISO 4589-1 – Determination of burning behaviour by oxygen index: general guidance
- ISO 4589-2 – Ambient-temperature oxygen index test
- ISO 4589-3 – Elevated-temperature oxygen index test (temperature index method)
- ASTM D2863 – Standard test method for measuring the minimum oxygen concentration to support candle-like combustion of plastics
- GB/T 2406.1 and GB/T 2406.2 – Chinese national standards harmonized with ISO 4589
- JIS K 7201-2 – Japanese national method for oxygen index determination
The method is prized for its small specimen requirements, excellent repeatability, and fully quantitative output, making it ideal for quality control, material certification screening, and flame-retardant formulation research. It should be noted, however, that LOI is a small-scale ignition-resistance measurement; it does not by itself characterize full-scale fire hazard and is typically used alongside other flammability assessments such as UL 94 or IEC 60695 series tests conducted by accredited third-party laboratories.
Testing Samples
Specimen Types and Dimensions
ISO 4589-2 defines several specimen geometries to accommodate different material behaviors:
- Type I: 80–150 mm long, 10 mm wide, 4 mm thick — the standard bar for self-supporting rigid plastics
- Type II: 10 mm by 10 mm cross-section bars for certain moulded materials
- Type III: approximately 125 mm long, 6.5 mm wide, 3 mm thick — geometry aligned with ASTM practice
- Type IV: flexible specimens such as films and sheeting up to 10.5 mm thick, tested using a small-frame support
ASTM D2863 typically specifies specimens 70–150 mm long, 6.5 mm wide, and 3 mm thick, with provisions for other cross-sections when necessary. Specimen thickness must always be reported, since LOI values vary measurably with geometry.
Material Categories
Oxygen index testing is applicable to a broad range of polymeric materials, including:
- Thermoplastics: polyethylene (PE), polypropylene (PP), polystyrene (PS), ABS, polyamide (PA), polycarbonate (PC), PMMA, POM, PET/PBT, PVC, and fluoropolymers
- Thermosets: epoxy, phenolic, unsaturated polyester, and polyurethane systems
- Elastomers, rubber compounds, and thermoplastic elastomers
- Rigid and flexible foams used in insulation and cushioning
- Fiber-reinforced composites and laminates
Specimen Preparation and Conditioning
Specimens are normally injection or compression moulded in accordance with ISO 294 or ISO 293, or cleanly cut from sheets with smooth, blemish-free edges. Prior to testing, specimens are conditioned at 23 °C ± 2 °C and 50% ± 5% relative humidity for a minimum of 40 hours (or 88 hours in some regimes) in line with ISO 291. A minimum of 15 specimens is generally required per test series to complete the oxygen-concentration bracketing sequence.
Testing Items
Typical testing programs offered for plastic oxygen index performance evaluation include:
- Ambient-temperature limiting oxygen index (LOI) determination to ISO 4589-2, ASTM D2863, or GB/T 2406.2
- Elevated-temperature oxygen index (temperature index) determination to ISO 4589-3
- Flame retardancy classification and screening, for example verifying whether a compound meets a declared LOI threshold such as 27% or 32%
- Comparative evaluation of flame-retardant formulations, including halogenated, halogen-free, phosphorus-based, mineral-filled, and intumescent systems
- Incoming material inspection and batch-to-batch quality control for resins, compounds, and finished semi-finished products
- Verification of supplier datasheet values during material qualification
Testing Methods
Apparatus
The standard oxygen index apparatus consists of a heat-resistant glass chimney (typically 450–500 mm in height), a vertical specimen holder, supplies of oxygen and nitrogen with pressure regulation, precision flow measurement using calibrated rotameters or mass flow controllers, and a small diffusion-flame igniter fueled by propane or hydrogen. Many modern instruments also incorporate an oxygen analyzer to verify the actual concentration of the mixed gas stream delivered to the column.
Test Procedure
- Condition, measure, and mark the specimen with a 50 mm gauge graduation from the top edge
- Mount the specimen vertically in the holder so that its top is at least 100 mm below the top of the chimney
- Purge the column with the oxygen/nitrogen mixture at the specified total flow (corresponding to a linear velocity of 40 mm/s in ISO 4589-2) for at least 30 seconds
- Apply the igniter flame to the top of the specimen for up to 5 seconds (ISO) or per the ASTM ignition cycle, then remove the flame
- Record whether the specimen continues to burn for at least 3 minutes after ignition or whether the flame front consumes at least 50 mm of the specimen
- Adjust the oxygen concentration using the bracketing (bisection) method — coarse steps of 1–2% oxygen followed by fine steps of 0.2% — until the limiting concentration is bracketed
- Calculate the result: LOI = 100 × V(O2) / [V(O2) + V(N2)]
The final LOI is reported to the nearest 0.1% or 0.2%, together with specimen dimensions, ignition method, and conditioning history. Instrument performance is routinely verified using certified reference materials such as PMMA, whose oxygen index lies in the well-characterized range of approximately 17–19% depending on grade. Under well-controlled conditions, within-laboratory repeatability is typically within ±0.5% oxygen.
Applications
Oxygen index data support material selection, certification, and development across many industries:
- Wire and cable: insulation and sheathing compounds for building wiring, power cables, and data cables
- Construction: foam insulation boards, wall lining panels, piping, roofing membranes, and profiles
- Rail and mass transit: interior panels, seat materials, and cable systems requiring flame-spread screening
- Electrical and electronics: enclosures, connectors, and engineering components where ignition resistance is critical
- Automotive: interior trim, under-hood components, and battery-related polymer parts
- Research and development: optimizing loading levels of flame retardants and comparing halogen-free alternatives
- Regulatory support: preliminary screening before undertaking full-scale or end-product fire performance tests at accredited third-party laboratories
FAQ
What is considered a good oxygen index value?
Materials with an LOI of 21% or below generally burn readily in air. Values between 22% and 26% indicate improved ignition resistance, while an LOI of 27% or above is widely treated as the threshold for classifying a plastic as flame retardant, and values exceeding 32% characterize highly flame-retardant compounds. The exact acceptance criterion depends on the product standard and application.
How does the LOI test differ from UL 94 flame testing?
The oxygen index test determines the minimum oxygen concentration needed to sustain combustion of a vertically mounted specimen, producing a numerical value. UL 94 evaluates afterflame and afterglow times, dripping, and ignition of cotton after a defined flame application, producing ratings such as V-0, V-1, V-2, and 5VA. The two methods assess complementary aspects of flammability and are frequently specified together.
Does temperature affect the LOI result?
Yes. The oxygen index of most plastics decreases significantly as ambient temperature rises. ISO 4589-3 addresses this by determining the temperature index — the oxygen index measured at specified elevated temperatures — which is valuable for materials used in warm environments such as electrical enclosures and rail vehicles.
Can thin films and flexible materials be tested?
Yes. Films, textiles, and flexible sheeting are tested using small support frames or rollers specified in ISO 4589-2 (Type IV specimens) and equivalent provisions in ASTM D2863, ensuring the specimen remains in a defined vertical orientation during ignition.
How many specimens are needed and how long does testing take?
A typical determination requires 15 or more specimens to complete the concentration bracketing sequence. Including conditioning, most single-condition LOI determinations are completed within one to three working days at an accredited third-party laboratory.
What factors can influence the result?
Specimen thickness, molding quality, edge condition, conditioning history, gas-flow calibration, and ignition technique all influence the measured value. Strict adherence to the governing standard and regular calibration against reference materials are essential for obtaining comparable, defensible results.