The foam oxygen index combustion test, more formally known as the Limiting Oxygen Index (LOI) test, is a small-scale laboratory method that quantifies the minimum oxygen concentration required to sustain candle-like combustion of polymeric foam materials. Governed by internationally recognized standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, the test is one of the most widely used screening tools for evaluating and comparing the flame-retardant performance of flexible and rigid cellular plastics. This article explains the test principle, applicable standards, specimen requirements, test procedure, result interpretation, and typical industrial applications.
Technical Overview
The Limiting Oxygen Index (LOI) test measures 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 specimen. Because ambient air contains approximately 20.9% oxygen, the LOI value provides an immediate and intuitive benchmark: materials with an LOI below 21% will typically continue to burn in ordinary air, while materials with higher LOI values are progressively more difficult to ignite and sustain burning.
For foam materials, which possess high surface area, low density, and open or closed cellular structures that can promote flame spread, the LOI test is a particularly valuable quality-control and research tool. It enables manufacturers to verify flame-retardant formulations, compare candidate materials, and demonstrate compliance with product specifications and regulatory requirements in a reproducible, laboratory-controlled manner.
- Test principle: downward propagation of flame on a top-ignited vertical specimen in a controlled oxygen/nitrogen atmosphere.
- Result: LOI value in percent oxygen by volume, reported to the nearest 0.1%.
- Interpretation: higher LOI indicates greater flame resistance; LOI greater than 27% is generally regarded as self-extinguishing for many polymer applications.
- Key standards: ISO 4589-2, ASTM D2863, and GB/T 2406.2, with related elevated-temperature assessment in ISO 4589-3.
Testing Samples
Virtually all commercially significant polymer foams can be evaluated by the oxygen index method, provided that specimens can be cut with clean, uniform geometry and can be held vertically without significant deformation. Common sample categories include the following.
Flexible Polyurethane Foam
Flexible polyurethane foams used in upholstered furniture, mattresses, and automotive seating are frequently tested for LOI, often in combination with other flammability tests. Because low-density flexible foam is highly combustible in its unmodified form, flame-retardant grades are commonly formulated to achieve LOI values in the range of approximately 22% to 28%. Careful specimen preparation and support are required to prevent distortion during testing.
Rigid Polyurethane and Polyisocyanurate Foam
Rigid foams used as thermal insulation in construction panels, appliances, and pipe insulation are routinely evaluated by LOI. Polyisocyanurate-modified systems and flame-retarded formulations typically exhibit LOI values of 25% or higher. For building applications, LOI results usually supplement, rather than replace, reaction-to-fire classifications such as those determined under EN 13501-1 or GB 8624.
Expanded and Extruded Polystyrene Foam (EPS and XPS)
Polystyrene foams used in insulation boards and packaging are intrinsically combustible, with unmodified LOI values typically in the region of 18% to 20%. Flame-retarded grades incorporating halogenated or phosphorus-based additives are assessed by LOI to confirm formulation effectiveness and batch consistency.
Polyolefin, PVC, Phenolic, and Specialty Foams
Cross-linked polyethylene foam used in construction and expansion joints, PVC foam used in marine and signage applications, phenolic foam used in high fire-performance insulation, and elastomeric rubber foam used for HVAC insulation are all suitable candidates. PVC and phenolic foams characteristically achieve high LOI values, often exceeding 30% to 35%, reflecting their inherently flame-resistant chemistry.
Testing Items
The core deliverable of the test is the Limiting Oxygen Index itself, accompanied by observations that characterize the burning behavior of the specimen.
- Limiting Oxygen Index (LOI): the minimum oxygen volume fraction, rounded to 0.1%, at which the specimen just sustains combustion for the specified time or over the specified length.
- Ignition behavior: ease of ignition under the standardized pilot flame, including number of ignition attempts required.
- After-flame time: the duration of continued flaming after removal of the ignition source.
- Burning characteristics: melt dripping, flaming droplets, smoke emission, char formation, shrinkage, or collapse of the cellular structure.
- Conditioning verification: confirmation that specimens were conditioned under the specified temperature and humidity prior to testing.
Testing Methods
Applicable Standards
- ISO 4589-2: Plastics — Determination of burning behaviour by oxygen index — Part 2: Ambient-temperature test; the primary international reference method.
- ASTM D2863: Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index); the widely used North American equivalent.
- GB/T 2406.2: the Chinese national standard, technically aligned with ISO 4589-2, commonly specified in Chinese regulatory and procurement contexts.
- ISO 4589-3: extension of the method for determination of oxygen index at elevated temperatures, relevant to applications with higher operating temperatures.
Test Apparatus
The test is conducted in a dedicated oxygen index apparatus comprising a heat-resistant glass chimney, typically of approximately 75 to 100 mm diameter and 450 to 600 mm height, a specimen holder that supports the specimen vertically in the center of the chimney, a gas mixing and metering system with calibrated flow controls for oxygen and nitrogen, and a small propane or butane pilot flame igniter. Some apparatus configurations include a non-metallic downward-flow chimney per the ASTM variant; both configurations deliver a total gas flow on the order of the standardized rate so that the atmosphere within the chimney is stable and reproducible.
Specimen Preparation
Specimens are cut from representative foam samples with uniform, undamaged cells and precise dimensions. Typical rigid-plastic specimens measure 10 mm in width, 10 mm in thickness, and 80 to 150 mm in length; cellular materials may be tested with adjusted cross-sections as permitted by the governing standard to account for low density and cellular morphology. Before testing, specimens are conditioned under standard laboratory atmosphere, commonly 23°C and 50% relative humidity, for the period specified in the standard. A minimum of five to fifteen specimens is usually required because each individual specimen can only be tested once, and a bracketing sequence of several specimens is needed to converge on the final LOI value.
Test Procedure
The general procedure under ISO 4589-2 and its equivalents is as follows:
- Step 1: Mount the conditioned specimen vertically in the holder and position it centrally within the chimney.
- Step 2: Set the oxygen and nitrogen flow rates to achieve an initial estimated oxygen concentration, and allow the gas mixture to purge the chimney for at least 30 seconds to establish a stable atmosphere.
- Step 3: Apply the pilot flame to the top of the specimen for the prescribed duration, typically about 5 seconds, and then remove it.
- Step 4: Observe the specimen and determine whether combustion is supported. Depending on the specimen geometry and standard edition, support is judged by whether flaming persists for the specified period (commonly 180 seconds) or whether the flame front propagates over the specified burning distance (commonly 50 mm) after removal of the ignition source.
- Step 5: Record the outcome. If the specimen fails the criterion, increase the oxygen concentration for the next specimen; if it passes, decrease the concentration. Continue with progressively smaller concentration intervals until the difference between the highest non-supporting and lowest supporting concentrations falls within the specified tolerance.
- Step 6: Calculate the LOI as the volume percentage of oxygen in the final gas mixture: LOI = 100 × O2 / (O2 + N2). Report the result to the nearest 0.1% together with the standard used, specimen geometry, and observed burning behavior.
Interpretation of Results
While exact acceptance limits are always defined by the relevant product specification, the following general guideline is widely applied to polymeric materials:
- LOI below 21%: material generally continues to burn in ambient air; considered flammable.
- LOI of 21% to 24%: material burns slowly in air but with limited flame spread.
- LOI of 24% to 27%: material exhibits self-extinguishing tendencies in air.
- LOI above 27%: material is generally classified as flame retardant or self-extinguishing in air; high-demand applications such as railway components may specify LOI of 28% or greater.
It should be emphasized that the LOI test is a small-scale comparative method. Results are sensitive to specimen density, cell structure, thickness, and conditioning history, and LOI values alone do not fully predict behavior in large-scale or real-fire scenarios.
Applications
Oxygen index testing of foams supports a broad range of industrial, regulatory, and research objectives:
- Building and construction: qualification and incoming inspection of insulation foams for wall, roof, and floor systems, supporting reaction-to-fire assessment within classification frameworks such as EN 13501-1 and GB 8624.
- Railway: material qualification of foam components for interior applications under EN 45545-2, which incorporates ISO 4589-2 oxygen index testing within its requirement sets for certain components.
- Furniture and bedding: verification of flame-retardant flexible foam grades used in upholstered furniture and mattresses against manufacturer and procurement specifications.
- Automotive and aerospace: screening of seat foam, trim, and acoustic insulation materials during design and material selection.
- Electrical and electronics: evaluation of foam materials used in equipment housings, gaskets, and packaging where flame-retardant behavior is specified.
- Research and development: rapid, reproducible comparison of flame-retardant formulations, additive loading levels, and polymer modifications in a laboratory setting.
- Quality control: routine batch-to-batch verification of flame-retardant foam production by accredited third-party laboratories and in-house facilities.
FAQ
What is a good LOI value for foam materials?
As a general guideline, foam with an LOI above 27% is considered flame retardant or self-extingushing in ambient air. Specific requirements vary by application; for example, railway interior applications under EN 45545-2 typically demand LOI values of 28% or higher, while furniture foam specifications may accept lower values when combined with other protective measures.
Can flexible foams be tested by the oxygen index method?
Yes. Flexible foams are routinely tested using appropriately dimensioned and, where necessary, supported specimens. Because low-density flexible foams can deform or shrink during burning, careful specimen preparation and consistent test technique are essential for reproducible results.
Which standards govern the foam oxygen index test?
The principal standards are ISO 4589-2 and ASTM D2863 for ambient-temperature testing, GB/T 2406.2 as the aligned Chinese national method, and ISO 4589-3 for elevated-temperature oxygen index determination.
How is the LOI value calculated?
The LOI is calculated as 100 multiplied by the volume flow of oxygen divided by the sum of the volume flows of oxygen and nitrogen at the critical concentration determined by the bracketing procedure. The result is expressed as a percentage to the nearest 0.1%.
Does a high LOI guarantee fire safety in real applications?
No. The oxygen index test is a small-scale comparative screening tool. It provides an excellent measure of relative ignitability and combustion support, but comprehensive fire safety assessment normally requires supplementary tests, such as flame spread, heat release, smoke density, and full-scale or application-specific reaction-to-fire evaluations.
How many specimens are required for a test?
Because each specimen is consumed in a single determination and the method relies on iterative bracketing of the oxygen concentration, a typical test consumes five to fifteen specimens. Additional specimens should be prepared for materials that exhibit borderline or erratic burning behavior.
Do temperature and humidity affect LOI results?
Yes. LOI values generally decrease as ambient temperature increases, which is why ISO 4589-3 provides an elevated-temperature procedure. Moisture uptake by hygroscopic foams also influences results, which is why standardized conditioning before testing is mandatory.