Determination of the Burning Oxygen Concentration of Foam Materials: Limiting Oxygen Index (LOI) Tes

The burning oxygen concentration of foam materials, more commonly known as the Limiting Oxygen Index (LOI), is a key quantitative indicator of the flammability and flame-retardant performance of cellular polymers. Determined in accordance with ISO 4589-2, ASTM D2863, and GB/T 2406.2, the LOI test measures the minimum oxygen concentration in an oxygen/nitrogen mixture required to sustain candle-like combustion of a vertically mounted specimen. This article explains the principle, specimens, test items, procedures, and applications of oxygen concentration determination for foam materials.

Technical Overview

The burning oxygen concentration of a foam material is defined as the minimum volumetric concentration of oxygen, in a flowing mixture of oxygen and nitrogen at ambient temperature, that will just support combustion of a vertically mounted specimen after ignition from its top edge. This value is universally reported as the Limiting Oxygen Index (LOI) and is expressed as a percentage of oxygen by volume.

Foam materials, including rigid and flexible polyurethane, expanded and extruded polystyrene, polyolefin foams, and rubber foams, are highly porous structures with an extremely large specific surface area and low density. Oxygen can readily diffuse into the open or closed cells, which makes most untreated polymer foams ignite easily and burn rapidly. The LOI test therefore provides a sensitive, repeatable, and quantitative measure of how easily a foam will continue to burn in a controlled oxygen atmosphere.

The governing test methods are internationally harmonized. ISO 4589-2 (also published as EN ISO 4589-2) and the Chinese national standard GB/T 2406.2 specify the ambient-temperature oxygen index procedure, while ASTM D2863 describes an essentially equivalent method. ISO 4589-3 extends the technique to elevated temperatures. Because normal air contains approximately 20.9% oxygen, a material with an LOI below 21% will generally continue to burn in air once ignited, whereas higher LOI values indicate progressively better self-extinguishing and flame-retardant behavior.

Testing Samples

Rigid Polyurethane and Polyisocyanurate (PIR) Foams

Rigid PU and PIR foams are widely used as thermal insulation in buildings, refrigerated equipment, and pipes. Specimens are typically cut to 100–150 mm in length with a 10 mm × 10 mm cross-section, preserving molded skins where they represent the delivered product.

Expanded and Extruded Polystyrene (EPS and XPS)

Polystyrene insulation boards, both EPS beads boards and XPS extruded sheets, are commonly tested to verify flame-retardant grades. Specimens must be cut cleanly with smooth, perpendicular edges because damaged cell walls can influence the ignition behavior.

Flexible Polyurethane Foams

Flexible slabstock and molded foams for furniture, mattresses, and automotive seating are low-density, easily deformable materials. Because they may bend during the test, ISO 4589-2 and ASTM D2863 permit the use of specimen supports to keep the sample vertical without restricting flame spread.

Polyolefin, EVA, and Rubber Foams

Cross-linked polyethylene (EPE/XLPE), EVA, EPDM, and PVC/NBR thermal insulation foams are tested in their supplied thickness or trimmed to the standard cross-section, depending on the specification agreed with the client or the product standard.

High-Performance Foams

Phenolic foam, melamine foam, polyimide foam, and silicone foam are inherently flame-resistant cellular materials with characteristically high LOI values, and oxygen index testing is frequently used to document their compliance.

Specimen Preparation and Conditioning

A minimum of 15 specimens is normally required for a single determination, and up to 30 may be needed when a new material is first evaluated. All specimens are conditioned, typically for at least 40 hours at 23 ± 2 °C and 50 ± 5% relative humidity, before testing in an atmosphere of similar conditions.

Testing Items

A standard oxygen concentration determination for foam materials delivers the following information:

• Limiting Oxygen Index (LOI) at 23 ± 2 °C, expressed in volume percent of oxygen, rounded to 0.1%
• The test standard applied (ISO 4589-2, GB/T 2406.2, or ASTM D2863) and the specimen type and dimensions
• Ignition behavior, including time to ignition and whether combustion is sustained after removal of the pilot flame
• Qualitative observations such as flaming drips, smoke release, char formation, molten collapse, or afterglow
• Optional elevated-temperature LOI determination in accordance with ISO 4589-3
• Comparison of the measured LOI against contractual or regulatory thresholds, for example a specified minimum of 30% oxygen for certain flame-retardant insulation products

Testing Methods

Test Apparatus

The oxygen index apparatus consists of a heat-resistant glass chimney, typically 450–500 mm high with an internal diameter of 75–100 mm, into which the specimen holder is centrally positioned. Metered oxygen and nitrogen supplies are blended in a mixing chamber, and the total flow is regulated to produce a superficial gas velocity of 40 ± 10 mm/s through the chimney. The oxygen concentration in the mixed gas is monitored with a calibrated analyzer, and a small propane or natural-gas pilot burner provides the top ignition source.

Test Procedure

The conditioned specimen is clamped vertically so that a free length of specimen is exposed well above the holder, and the chimney is purged with the gas mixture for at least 30 seconds to ensure a stable atmosphere. The pilot flame, typically 10–20 mm long, is applied to the top of the specimen in short intervals, generally repeated for up to 30 seconds, until ignition is achieved. After the ignition source is removed, the operator records whether the specimen burns for 180 seconds or more, or whether burning, charring, or damage extends downward to a mark 50 mm below the top of the specimen.

If either criterion is met, the response is recorded as "O" (oxygen-rich behavior); if the specimen self-extinguishes before either limit, it is recorded as "X". The oxygen concentration is then decreased following an "O" result and increased following an "X" result, using progressively smaller concentration increments to bracket the critical point.

Calculation and Expression of Results

The LOI is calculated from the bracketing sequence of O and X responses using the index formula specified in the standards, which applies a correction factor to the final concentration step. Well-controlled testing in accredited third-party laboratories achieves within-laboratory repeatability typically better than 0.5% oxygen. The report must state the gas velocity, specimen geometry, orientation, conditioning history, and any use of specimen supports so that results remain fully comparable between laboratories.

Quality Assurance

Oxygen analyzers are calibrated against certified calibration gases, gas flow controllers are verified regularly, and reference materials such as polymethyl methacethacrylate (PMMA) plates with established LOI values are tested periodically to confirm apparatus performance, as required by ISO 4589-2 and GB/T 2406.2.

Applications

Oxygen concentration determination is one of the most widely applied flammability screening tests for foam materials across multiple industries:

• Building and construction: verification of flame-retardant EPS, XPS, PU, and PIR insulation boards supporting fire performance classifications under GB 8624, EN 13501, and related product standards
• Furniture and bedding: qualification of combustion-modified flexible foams for mattresses, upholstered seating, and public-transport seating
• Transportation: material qualification for rail vehicles under EN 45545-2, automotive interior components, marine outfits, and aircraft cabin materials
• Electrical and electronics: assessment of gaskets, sealing strips, housings, and packaging foams used in thermal management and cable protection
• Research and development: comparative evaluation of halogen-free flame retardants, phosphorus and nitrogen additives, expandable graphite, and intumescent systems in foam formulations
• Factory quality control: batch-by-batch monitoring of flame-retardant dosing consistency in continuous foam production

Typical reported ranges illustrate the spread of performance: flexible polyurethane foam and untreated polyolefin foams often show LOI values of roughly 17–19%, flame-retarded polystyrene and polyurethane insulation foams commonly reach 25–30%, while PVC/NBR, phenolic, melamine, and polyimide foams frequently exceed 35%. Actual values depend strongly on formulation, density, and cell structure.

FAQ

What is a good LOI value for foam materials?

An LOI above 21% means the material will not sustain candle-like combustion in normal air, since air contains about 20.9% oxygen. In practice, many procurement specifications and flame-retardant product standards require LOI values of 26–30% or higher for foam insulation and seating materials, with values above 30% generally regarded as strongly flame retardant.

How does LOI testing differ from the UL 94 test?

The LOI test quantifies flammability as a continuous oxygen concentration value in a controlled atmosphere, making it ideal for ranking materials and tracking formulation changes. UL 94, in contrast, grades horizontal or vertical burn behavior in ambient air into classification levels such as V-0, V-1, or HF-1. The two methods are complementary, and many certification programs require both.

How many specimens and how much time are needed?

At least 15 specimens are required per determination, and up to 30 may be consumed for unfamiliar materials while bracketing the critical concentration. With standard conditioning, a complete determination is typically completed within a few working days.

Does a high LOI guarantee fire safety in real applications?

No. The LOI is a small-scale screening test. A complete fire hazard assessment of foam products should combine LOI data with heat release testing such as ISO 5660-1 cone calorimetry, smoke density and toxicity measurements, and reaction-to-fire classification tests such as the single burning item test under EN 13823 or GB/T 20284.

Can flexible or very thin foams be tested?

Yes. Limp specimens such as flexible polyurethane foam are tested using appropriate specimen supports so they remain vertical, and film-like or thin materials are tested with wider specimen types defined in ISO 4589-2 and ASTM D2863, ensuring the burner flame correctly contacts the specimen top.

Which standard should be specified, ISO, ASTM, or GB?

ISO 4589-2 and GB/T 2406.2 are technically aligned, and ASTM D2863 produces comparable oxygen index values, though specimen geometry and calculation details differ slightly. The choice should follow the requirements of the purchasing specification, regulatory scheme, or destination market, and accredited third-party laboratories can advise on the appropriate method before testing begins.