Oxygen index testing, also known as the Limiting Oxygen Index (LOI) test, is one of the most widely used small-scale methods for evaluating the flammability of plastic pellets and compounded resins. This article provides a technical analysis of oxygen index testing for plastic pellets in accordance with ISO 4589-2, ASTM D2863, and GB/T 2406.2, covering sample preparation, test procedures, result interpretation, and typical industry applications.
Oxygen index testing measures the minimum concentration of oxygen required to sustain flaming combustion of a plastic specimen, providing a quantitative indicator of flame retardancy. For plastic pellets, the test supports flame-retardant formulation development, quality control of compounds, and compliance verification across electrical, construction, and transportation industries.
Technical Overview
The Limiting Oxygen Index (LOI), also called the oxygen concentration index, is defined as the minimum volumetric concentration of oxygen in a flowing oxygen-nitrogen mixture that will just sustain flaming combustion of a vertically mounted specimen after ignition. Because ordinary air contains approximately 20.9% oxygen, an LOI value above roughly 21% indicates that a plastic will not continue to burn easily in normal air, while values below 21% indicate that the material, once ignited, will propagate flame under ambient conditions.
Oxygen index testing is one of the most widely adopted small-scale flammability tests for polymeric materials. For plastic pellets and compounded resins, it provides a reproducible, numeric measure of ease of extinction that supports retardant formulation development, incoming and outgoing quality control, material datasheet generation, and regulatory compliance verification. Testing is performed by accredited third-party laboratories and in-house facilities using instruments and procedures specified in ISO 4589-2, ASTM D2863, and GB/T 2406.2.
Testing Samples
Oxygen index methods require bar-shaped specimens of standardized geometry; therefore, plastic pellets are first converted into molded test bars before testing. Typical sample categories include:
Commodity Thermoplastics
Polypropylene (PP), polyethylene (LDPE, HDPE), polystyrene (PS), ABS, PMMA, EVA, and flexible or rigid PVC. Unmodified polyolefins typically show LOI values of 17-19%, whereas rigid PVC compounds may exceed 40% due to the intrinsic chlorine content of the polymer.
Engineering and High-Performance Resins
Polyamide 6 and 66 (PA6, PA66), polycarbonate (PC), PBT, PET, PPO/PPE blends, polysulfone, PPS, PEI, and PEEK. These materials generally exhibit LOI values between 21% and 35%, and their results are sensitive to moisture conditioning, filler loading, and glass-fiber content.
Flame-Retardant Compounds
Halogenated systems (brominated or chlorinated retardants synergized with antimony trioxide) and halogen-free systems (aluminum trihydroxide, magnesium hydroxide, phosphorus-based and intumescent nitrogen-phosphorus formulations). Oxygen index testing is a primary screening tool for optimizing retardant loading, with well-formulated halogen-free compounds frequently reaching LOI values of 28-40%.
Sample Preparation Requirements
- Pellets are converted into test bars, typically 80-150 mm long, 10 mm wide, and 4 mm thick, by injection molding (ISO 294-1) or compression molding (ISO 293, ASTM D4703).
- Specimens are marked with two reference lines 50 mm apart so that the flame-spread criterion can be assessed.
- A minimum of 15-20 specimens is generally required because the bracketing procedure consumes several bars; 0.5-1 kg of pellets is normally sufficient.
- Specimens are conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity (ISO 291) before testing, and hygroscopic resins such as polyamide are additionally dried or moisture-equilibrated as specified.
- Molding parameters must be documented, because thermal degradation, voids, or orientation introduced during pellet conversion can measurably shift the LOI result.
Testing Items
The core deliverable of oxygen index testing is the LOI value at room temperature; complementary flammability measurements are frequently requested within the same test program:
- Limiting Oxygen Index at 23 °C in accordance with ISO 4589-2, ASTM D2863, GB/T 2406.2, or JIS K 7201-2.
- Oxygen index at elevated temperature in accordance with ISO 4589-3, used to evaluate how flame retardancy changes with service temperature.
- Comparative burning characteristics under UL 94 (V-0, V-1, V-2, 5VA, 5VB, HB), often run in parallel with LOI for electrical products.
- Glow-wire ignition temperature (GWIT, IEC 60695-2-12) and glow-wire flammability index (GWFI, IEC 60695-2-13) for electrical and electronic components.
- Heat release rate and ignition time by cone calorimetry (ISO 5660-1) for advanced fire-performance engineering.
Testing Methods
Applicable Standards
- ISO 4589-1: general guidance for oxygen index testing.
- ISO 4589-2: determination of LOI at room temperature, equivalently adopted as GB/T 2406.2 and JIS K 7201-2.
- ISO 4589-3: LOI determination at elevated temperatures.
- ASTM D2863: minimum oxygen concentration to support candle-like combustion of plastics.
Apparatus
The test uses a vertical heat-resistant glass chimney, typically 450-500 mm high with a 75-100 mm internal diameter, in which a metered oxygen-nitrogen mixture flows upward at a linear velocity of 40 ± 10 mm/s. A specimen holder clamps the bar vertically at its base, and a small premixed burner flame of standardized length serves as the ignition source. Inline flow controllers and an oxygen analyzer (paramagnetic or zirconia type) establish and verify the oxygen concentration of the mixture, which is indicated to within 0.1% by volume.
Test Procedure
- The specimen is mounted vertically with its top edge a fixed distance below the chimney outlet, and the chimney is purged with the gas mixture for at least 30 seconds to reach equilibrium.
- The burner flame is applied to the top of the specimen using either the top-surface ignition or the propagating ignition procedure defined in the standard.
- Specimen behavior is judged as burning if the combustion front reaches the lower reference mark 50 mm below the upper mark, or if flaming or glowing persists beyond the criterion time specified for the specimen geometry.
- The oxygen concentration is adjusted stepwise: coarse steps are used initially, then fine steps of 0.2% oxygen bracket the transition between burning and non-burning behavior.
- The LOI is calculated as LOI = 100 × V(O2) / [V(O2) + V(N2)] and reported to one decimal place, together with the ignition procedure used and the specimen dimensions.
Result Interpretation
Although the LOI is a continuous value, the following classification is widely used for engineering interpretation:
- LOI below 21%: the material burns readily in ambient air once ignited, which is typical for unmodified PP, PE, PS, and ABS.
- LOI between 21% and 27%: the material is slow-burning or shows self-extinguishing tendencies; sustained ignition in air is difficult.
- LOI of 27-28% or higher: the material is generally considered self-extinguishing and is classified as flame retardant in many specifications.
Interpretation must also account for variables that influence the result, including specimen thickness (many materials show higher LOI at greater thickness), filler and glass-fiber content, plasticizer level, moisture content, conditioning history, ignition procedure, gas-flow stability, and any aging or migration of flame retardants after pellet molding.
Applications
- Flame-retardant formulation development: screening and optimizing mineral, brominated, phosphorus, and intumescent systems against target LOI values.
- Quality control of compounded pellets: verifying lot-to-lot consistency of flame-retardant masterbatches and engineering compounds.
- Wire and cable: qualification of insulation and sheathing compounds for building, appliance, and automotive wiring.
- Electrical and electronic: supporting material selection for connectors, housings, and structural parts alongside UL 94 and glow-wire requirements.
- Construction: fire-performance assessment of pipes, conduits, panels, and thermal-insulation foams.
- Transportation: demonstrating compliance of interior materials used in rail vehicles, buses, and aircraft with specified oxygen index limits.
FAQ
Can plastic pellets be tested directly without molding?
No. ISO 4589-2 and ASTM D2863 require bar specimens of defined geometry, so pellets must first be injection- or compression-molded into standard test bars. Testing loose granules is not representative of end-product behavior and is not accepted for compliance purposes.
How much sample is required for an oxygen index test?
A minimum of 15-20 test bars is typically consumed by the bracketing procedure. In practice, 0.5-1 kg of pellets is requested to allow mold trial runs and repeat measurements.
What is the difference between LOI and UL 94?
LOI quantifies the minimum oxygen concentration needed to sustain combustion and produces a numeric result, whereas UL 94 evaluates burning behavior in ambient air and assigns pass/fail classifications such as V-0 or HB. The two methods are complementary and are frequently specified together.
Does a higher LOI guarantee a UL 94 V-0 rating?
No. Although materials with LOI values above roughly 28% often achieve V-0, the correlation is not absolute because UL 94 depends on afterflame time, flaming drips, and ignition of the cotton indicator, which are governed by different mechanisms.
How long does oxygen index testing take?
Excluding specimen molding, conditioning and testing can typically be completed within 3-5 working days at an accredited third-party laboratory. Compression or injection molding of the pellets adds 1-3 days depending on tooling availability.
Why do polyamide results vary between laboratories?
PA6 and PA66 absorb moisture rapidly, and water content measurably affects LOI. Differences in drying, conditioning, and specimen age between laboratories are a common source of variance, which is why the conditioning state must always be reported together with the result.