Limiting oxygen index (LOI) testing determines the minimum oxygen concentration required to sustain candle-like combustion of cellular plastics, making it one of the most widely used bench-scale methods for evaluating the flame retardancy of foamed materials. This article explains the LOI test principle, applicable standards such as ISO 4589-2, ASTM D2863 and GB/T 2406.2, specimen requirements, test procedures, and typical applications for rigid and flexible foams.
Technical Overview
The limiting oxygen index (LOI), also known as the oxygen index (OI), is defined as the minimum concentration of oxygen, expressed as a percentage by volume in a flowing oxygen/nitrogen mixture, that will just support candle-like combustion of a vertically mounted specimen. Because normal air contains approximately 20.9% oxygen, the LOI gives an immediate and intuitive indication of how readily a material will ignite and propagate flame in real service conditions. The ambient-temperature test is standardized in ISO 4589-2, with technically equivalent methods in ASTM D2863, GB/T 2406.2 and JIS K 7201-2, and an elevated-temperature variant described in ISO 4589-3.
Cellular plastics such as expanded polystyrene (EPS), extruded polystyrene (XPS), flexible and rigid polyurethane (PUR), polyisocyanurate (PIR), phenolic, PVC and polyolefin foams combine high surface-to-mass ratios with low thermal inertia, so they tend to ignite easily unless flame retarded. LOI testing is therefore a standard tool for ranking the relative flammability of foams, validating flame-retardant formulations, and performing routine quality control on finished foam products.
As a general interpretation convention, the higher the LOI value, the greater the flame resistance:
- LOI below 21%: the material burns readily in ordinary air.
- LOI between 21% and 26%: the material burns with difficulty or slowly in air.
- LOI above 26%: the material is generally considered flame retardant or self-extinguishing in air.
Indicative LOI ranges for common foams include: untreated EPS/XPS and flexible polyurethane foam at roughly 17–19%; flame-retarded EPS and XPS at 28–32%; rigid PIR foam at 28–35%; phenolic foam at 35–45%. Actual values depend strongly on density, cell structure, thickness and test geometry, so comparisons should always be made between specimens of the same dimensions.
Testing Samples
Rigid Cellular Plastics
Rigid foams such as EPS, XPS, PUR/PIR and phenolic boards are typically cut into rectangular bars with a working length of 80–150 mm and a nominal cross-section of 10 mm × 10 mm, in accordance with ISO 4589-2. Specimens must be cut cleanly without crushing or thermally degrading the cells, and any molded skin or facing may be retained or removed depending on the product specification being evaluated. Because rigid foams are often anisotropic, the rise direction (vertical or horizontal) should be recorded, as it can measurably influence the result.
Flexible Cellular Materials
Flexible polyurethane foam, sponge and low-density cellular sheeting may deform in the specimen holder or shrink away from the ignition flame. For such materials, the standards define specific specimen types, including thinner bars and, per ASTM D2863, rolled or folded specimens for thin flexible sheet and film. Thickness effects are particularly significant for flexible foams, so the tested thickness must always be reported alongside the LOI value.
Specimen Conditioning and Quantity
A complete determination normally requires a minimum of 15 specimens, with additional specimens recommended for initial concentration scouting. Specimens are conditioned to equilibrium in a standard atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity (per ISO 291 or ASTM D618), commonly for at least 40 hours, since moisture content affects both ignition and burning behavior of many foams. A reference mark is placed 50 mm below the top edge of each specimen to judge flame spread, and cut edges must be smooth, square and free of dust.
Testing Items
Typical test items offered for foamed plastics include:
- Ambient-temperature limiting oxygen index determination in accordance with ISO 4589-2, ASTM D2863, GB/T 2406.2 or JIS K 7201-2.
- Elevated-temperature oxygen index testing in accordance with ISO 4589-3 for materials used in warm environments.
- Comparative LOI screening of flame-retardant formulations during research and development.
- Quality-control verification against product or purchasing specifications, for example a minimum LOI of 26% or 28% for certain flame-retardant insulation foams.
- Recording of qualitative burning behavior, including afterflame time, melting, dripping, char formation, shrinkage and smoke release.
The test report should state the LOI value to the nearest 0.1% oxygen, the governing standard, specimen type and dimensions, orientation, density, conditioning history, and all observations of burning behavior.
Testing Methods
Principle and Apparatus
A standard LOI apparatus consists of a vertical borosilicate glass chimney (typically 450–500 mm tall with an internal diameter of at least 75 mm), a specimen holder that supports the bar vertically inside the column, a gas mixing and metering system for oxygen and nitrogen, an oxygen analyzer or calibrated flow measurement accurate to about 0.1 vol%, and a small propane igniter. The combined gas flow is controlled to give the linear velocity through the column specified by the standard (40 mm/s in ISO 4589-2), ensuring a stable, repeatable atmosphere around the specimen. The test should be conducted under a ventilation hood with appropriate precautions for handling compressed oxygen and nitrogen.
Test Procedure
- Mount the conditioned specimen vertically in the holder and position it at the specified height within the chimney.
- Set an initial oxygen concentration based on the expected flammability of the foam (for example near 18–21% for untreated materials) and allow the gas mixture to purge and stabilize the column.
- Apply the igniter flame to the top edge of the specimen for up to 30 seconds, with intermittent contact, until the specimen ignites; then remove the igniter and start timing.
- Judge the result: combustion is classed as sustained if the specimen burns for 180 seconds or more after removal of the igniter, or if the flame front passes the 50 mm reference mark; otherwise the specimen is considered to have self-extinguished.
- Adjust the oxygen concentration upward or downward in progressively smaller increments (typically down to 0.2% oxygen near the critical value) and test fresh specimens until the pass/fail boundary is bracketed.
- Complete the statistical sequence, based on the up-and-down (bracketing) method defined in the standard, using at least the minimum number of specimens required.
Calculation and Reporting
The limiting oxygen index is calculated from the volumetric gas flows at the critical condition: LOI = 100 × V(O2) / [V(O2) + V(N2)]. The result is reported as a percentage of oxygen by volume to the nearest 0.1%. Within-laboratory repeatability is typically within about ±0.5% oxygen for a given specimen geometry, while reproducibility between laboratories may be wider due to differences in apparatus and specimen preparation, which is why the tested thickness and standard must always accompany the value.
Applications
LOI testing of cellular plastics supports material development, quality assurance and regulatory screening across many industries. Common applications include:
- Quality control and incoming inspection of flame-retardant EPS, XPS, PUR/PIR and phenolic insulation boards, where manufacturers and buyers specify minimum LOI limits as part of internal compliance programs that complement fire classification schemes such as GB 8624 and EN 13501-1.
- Research and development of flame-retardant systems, including phosphorus-based, halogen-free, nitrogen-phosphorus intumescent and mineral-filled packages, as well as reactive flame retardants for polyurethane foam.
- Screening of furniture, mattress and automotive seating foams, railway and marine interior materials, and acoustic/thermal foam components before submission to full-scale or regulatory fire tests.
- Comparison of competing materials, supplier qualification, and long-term production consistency monitoring performed by accredited third-party laboratories.
- Academic research on foam combustion mechanisms, char formation and the correlation between LOI, UL 94 ratings and cone calorimeter heat release data.
It should be emphasized that LOI is a small-scale screening and quality-control test. Regulatory fire classification of building products generally requires additional tests such as single-flame-source ignition (EN ISO 11925-2), the single burning item test (EN 13823), or heat release measurements, with LOI serving as a fast, economical tool for formulation control and pre-screening.
FAQ
What is considered a good LOI value for foam plastics?
Any material with an LOI above 21% will not burn easily in ordinary air. In practice, flame-retarded insulation foams are commonly specified at LOI ≥ 26–28%, and high-performance materials such as phenolic foam can exceed 35%. The requirement depends on the application and the applicable product specification.
Are ISO 4589-2, ASTM D2863 and GB/T 2406.2 interchangeable?
The three methods are technically aligned in principle, but they specify different preferred specimen dimensions and tolerances (for example, ISO 4589-2 uses a 10 mm × 10 mm bar while ASTM D2863 commonly uses a 6.5 mm × 3 mm bar). Because geometry influences the result, values obtained under different standards should not be compared directly without stating the governing method.
Why do thickness and orientation affect the LOI of a foam?
Thicker specimens act as a larger heat sink but also sustain larger flames, and the cellular structure of most foams is anisotropic, so burn behavior differs along and across the rise direction. Consistent specimen dimensions and documented orientation are essential for meaningful comparisons between lots or formulations.
Can an LOI result be used alone to certify fire safety?
No. LOI is a bench-scale ranking and quality-control test. Official fire classification of building or transport materials requires the full suite of tests defined by the relevant regulations, such as EN 13501-1, GB 8624 or EN 45545-2. LOI data support these programs but do not replace them.
How many specimens are needed and how long does a test take?
A determination requires at least 15 conditioned specimens plus spares for initial scouting. Including conditioning, a routine LOI test is typically completed within a few working days by accredited third-party laboratories.
Does temperature influence the oxygen index?
Yes. The LOI of most plastics decreases as ambient temperature rises. ISO 4589-3 provides the elevated-temperature oxygen index method for evaluating foams intended for warm service environments.