The Limiting Oxygen Index (LOI) test is one of the most widely used laboratory methods for evaluating the combustion characteristics of plastic materials, measuring the minimum oxygen concentration required to sustain flaming combustion. This article provides a comprehensive technical overview of oxygen index analysis, covering test principles, applicable standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, specimen requirements, testing procedures, and practical applications in flame-retardant material development and quality control.
Technical Overview
The Limiting Oxygen Index (LOI), also known as the Oxygen Index (OI), is a fundamental parameter used to characterize the burning behavior of polymeric materials. It 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 candle-like combustion of a vertically mounted test specimen for a specified time or over a specified length. The LOI is calculated using the following relationship:
- LOI (%) = 100 × V(O₂) / [V(O₂) + V(N₂)]
Because normal ambient air contains approximately 20.9% oxygen, materials with an LOI below approximately 21% will continue to burn freely in air after ignition, while materials with an LOI significantly above 21% tend to self-extinguish when the ignition source is removed. As a general classification guideline, plastics with an LOI below 21% are considered highly flammable, values between 21% and 27% indicate slow-burning or marginally self-extinguishing behavior, and values above 27% are generally regarded as flame-retardant or self-extinguishing. Typical LOI ranges include polyethylene and polypropylene at 17–18%, ABS at 18–20%, polycarbonate at 25–28%, rigid PVC at 45–49%, and PTFE above 95%. The oxygen index test is reproducible, relatively inexpensive, and quantitative, making it a cornerstone method for flame-retardancy research, material screening, specification compliance, and quality control in accredited third-party laboratories worldwide.
Testing Samples
Rigid Thermoplastics and Thermosets
Molded or machined bars of materials such as PE, PP, PS, ABS, PA, PC, PET, PBT, POM, PMMA, epoxy, and phenolic resins are tested as self-supporting rods or rectangular strips. Standard dimensions per ISO 4589-2 are 80 mm to 150 mm in length, 10 mm in width, and 4 mm in thickness, although thickness-dependent studies may use 1 mm to 10 mm specimens to evaluate geometry effects.
Flexible Films, Sheets, and Textiles
Thin flexible materials are tested using a special specimen holder with a frame that supports the film and defines the ignition zone, as thin specimens may otherwise deform, curl, or shrink during exposure to the ignition flame. Films are typically cut to dimensions of 140 mm × 52 mm and mounted on the frame defined in the relevant test standard.
Cellular and Foam Materials
Rigid and flexible foams, including polyurethane, polystyrene, and polyolefin foams, are tested with dimensions of 80 mm to 150 mm in length, 10 mm to 15 mm in width, and 10 mm to 12.5 mm in thickness. Specific size requirements depend on the material density and the applicable method clause.
Fiber-Reinforced Composites and Engineering Components
Glass-fiber or carbon-fiber reinforced laminates, prepreg materials, and sections cut from finished components can be evaluated, provided the specimen geometry can be prepared with clean, smooth edges and is representative of the end-use material. Specimens must be conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 88 hours prior to testing to ensure consistent moisture content.
Testing Items
The primary and supplementary measurands reported in an oxygen index analysis include:
- Limiting Oxygen Index (LOI) value, reported to the nearest 0.1% oxygen concentration
- Burning behavior observations, including afterflame time, afterglow, dripping, and whether combustion reaches the marking line
- Temperature-index testing at elevated temperatures (up to approximately 200 °C) per ISO 4589-3, which characterizes the decrease of LOI with temperature
- Thickness-dependence evaluation to establish LOI versus specimen thickness curves for design purposes
- Comparative LOI testing before and after flame-retardant formulation changes, aging, or environmental conditioning
Testing Methods
Oxygen index analysis is performed in accordance with internationally recognized standards, the most common of which are ISO 4589-2, ASTM D2863, and GB/T 2406.2. These methods are technically equivalent in principle and share the same fundamental procedure:
- The test specimen is clamped vertically in the center of a heat-resistant glass chimney, typically 100 mm high, through which a precisely controlled mixture of oxygen and nitrogen flows upward at a total velocity of approximately 40 mm/s.
- The gas concentration is set to an estimated starting value, and the top of the specimen is ignited with a small propane flame igniter for a defined application time.
- The specimen is observed to determine whether burning persists for more than 180 seconds or consumes more than 50 mm of the specimen length measured from the ignition point.
- The oxygen concentration is adjusted systematically, increasing it if the specimen fails to meet the burning criterion and decreasing it if the criterion is exceeded. The critical concentration is bracketed using the incremental step approach or the statistical up-and-down (Dixon) method until the limiting concentration is established.
- The final LOI is calculated from the volume flow rates of oxygen and nitrogen at the limiting condition, and at least fifteen to twenty specimens are typically consumed to complete one determination.
Calibration requirements include verification of gas flow meters, oxygen concentration analyzers, and timing devices. Test results are influenced by specimen thickness, orientation, ambient temperature, humidity conditioning, and the presence of fillers or plasticizers, so strict adherence to the specified conditioning and procedural parameters is essential for interlaboratory reproducibility.
Applications
Oxygen index analysis supports a wide range of industrial, regulatory, and research objectives:
- Flame-retardant formulation development, including optimization of halogenated, phosphorus-based, nitrogen-based, and inorganic filler systems
- Material selection and design verification for electrical and electronic equipment, where LOI data complement IEC and UL flammability classifications
- Quality control in plastics manufacturing, providing a rapid quantitative check on batch-to-batch consistency of flame-retardant compounds
- Compliance demonstration for construction products, transportation interiors, cables, and home appliances against EN, GB, and ISO-based regulatory requirements
- Academic and industrial research into polymer combustion mechanisms, char formation, intumescence, and synergistic additive effects
- Aged and weathered material evaluation, assessing how thermal oxidation, UV exposure, or moisture affect long-term fire performance
FAQ
What LOI value qualifies a plastic as flame retardant?
Although classifications vary by industry, a material with an LOI of 27% or greater is generally considered self-extinguishing in normal air. Materials intended for demanding applications such as electrical components or mass transport interiors are frequently specified with LOI values of 30% or higher.
How does the oxygen index test differ from the UL 94 test?
The LOI test measures the minimum oxygen concentration needed to sustain combustion of a vertical specimen, producing a continuous numerical value, whereas UL 94 evaluates flame spread, afterflame time, and dripping against pass/fail criteria such as V-0, V-1, V-2, and 5VA. The two methods are complementary: LOI is excellent for ranking and research, while UL 94 addresses end-use ignition scenarios. A high LOI generally, but not always, correlates with better UL 94 ratings.
Does a high LOI guarantee fire safety in real applications?
No. The LOI is a small-scale laboratory measurement under controlled gas flow and candle-like burning conditions. Real fire performance also depends on heat release rate, smoke production, toxic gas evolution, flame spread, and geometrical factors of the final product. LOI results should therefore be combined with complementary tests such as cone calorimetry, UL 94, and glow-wire testing for comprehensive fire hazard assessment.
What specimen dimensions and quantities are required for testing?
Standard self-supporting specimens are typically 80–150 mm long, 10 mm wide, and 4 mm thick. A minimum of fifteen specimens is recommended for a single determination, and additional specimens may be needed for verification runs or thickness-dependence studies.
How do temperature and thickness affect the measured LOI?
LOI decreases as ambient temperature rises, which is why ISO 4589-3 defines an elevated-temperature procedure for materials used near heat sources. Thickness effects vary by material: many thermoplastics show increasing LOI with thickness up to a plateau, so comparing results requires matching specimen geometry and test conditions exactly.