Oxygen Index Determination of Extruded Foam Boards: LOI Testing Principles, Standards and Applicatio

Oxygen index (LOI) testing measures the minimum oxygen concentration required to sustain candle-like combustion of extruded polystyrene (XPS) foam boards, making it one of the most widely used indicators of flame-retardant performance for extruded insulation products. Testing in accordance with ISO 4589-2, ASTM D2863 and GB/T 2406.2 supports quality control, product certification and fire safety assessment of extruded foam boards in building and engineering applications. This article explains the testing principle, sample requirements, test procedure, result interpretation and typical applications of oxygen index determination for extruded foam boards.

Technical Overview

Extruded foam board, most commonly extruded polystyrene (XPS) insulation board, is a rigid closed-cell foam produced by extruding polystyrene resin together with a blowing agent. Thanks to its low thermal conductivity, low water absorption and high compressive strength, it is widely used in walls, roofs, floors, cold storage facilities and civil engineering projects. Because the base polymer is combustible, the evaluation of burning behaviour is an essential part of product qualification and quality control.

The oxygen index, also known as the limiting oxygen index (LOI), 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 specimen. The value is calculated as LOI = 100 × V(O2) / [V(O2) + V(N2)]. Since ambient air contains approximately 20.9 % oxygen, a material with an LOI above this level will not sustain burning after removal of the ignition source under normal atmospheric conditions, and a higher LOI indicates better flame-retardant performance.

Oxygen index determination for extruded foam boards is performed in accordance with ISO 4589-2, ASTM D2863 or the technically equivalent Chinese standard GB/T 2406.2. For flame-retardant grades of XPS used in building applications, a minimum LOI of 30 % is commonly specified, supporting compliance with classification requirements such as the B1 class in GB 8624 for combustible building materials with improved burning behaviour.

Testing Samples

Standard Extruded Polystyrene Foam Board Specimens

Specimens are cut from representative production boards, typically 100 mm to 150 mm in length with a width of 10 mm to 15 mm and a thickness of 10 mm to 15 mm, consistent with the specimen configurations specified for cellular plastics in ISO 4589-2 and GB/T 2406.2. Edges must be cut cleanly and free of dust, collapse or melted residue. The extruded skin (surface crust) significantly influences the result, so it must be clearly defined in the test plan whether specimens are tested with the skin intact or with machined surfaces, and this configuration must be stated in the report.

Faced and Composite Extruded Foam Boards

For boards with facings such as laminated foil, polymer films or mortar coatings, the composite may be tested as installed, or the core foam may be tested separately. The chosen configuration must be documented, because facings can either inhibit or promote ignition and can change the apparent oxygen index considerably.

Specimen Conditioning and Quantity

Specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 40 hours in accordance with ISO 291 before testing. A minimum of 15 specimens is generally required for one determination because the bracketing procedure consumes several specimens, and additional specimens are needed when different orientations (machine direction versus transverse direction) or aged conditions are evaluated.

Testing Items

The primary deliverable of the test is the limiting oxygen index of the extruded foam board, accompanied by supporting observations that characterise its burning behaviour:

• Limiting oxygen index (LOI) value, expressed as a volume percentage of oxygen
• Verification of flame-retardant grade compliance, for example LOI ≥ 30 % for B1-grade XPS products
• Afterflame and afterglow behaviour, melting, shrinkage and flaming drips
• Comparison of burning behaviour in machine and transverse directions
• Oxygen index after aging treatments such as heat exposure or water immersion, where required by the product specification

Testing Methods

Test Apparatus

The oxygen index apparatus consists of a vertical heat-resistant glass chimney typically 450 mm or more in height with an internal diameter of 75 mm to 100 mm, a specimen holder that supports the specimen vertically in the centre of the chimney, a gas distribution and mixing section at the base, precision flow meters or mass flow controllers for oxygen and nitrogen, an oxygen analyzer for verifying the mixture concentration, and a small-flame igniter fuelled by propane or butane with a flame length of approximately 12 mm to 16 mm. The total gas flow is controlled to give a linear velocity of about 40 mm/s through the chimney, typically corresponding to 10 L/min to 15 L/min.

Test Procedure

Testing is carried out in an ambient environment of 23 ± 2 °C, following the sequence below:

• Mount the specimen vertically in the holder with its top edge at least 100 mm below the rim of the chimney, and mark a reference line 50 mm below the top of the specimen.
• Set an initial oxygen concentration based on experience or a trial determination, and purge the chimney with the gas mixture for at least 30 seconds before ignition.
• Apply the igniter flame to the top of the specimen for up to 30 seconds, sweeping across the whole top surface, until the specimen is well ignited, then remove the igniter and start timing.
• Record a failure if the specimen burns downward beyond the 50 mm mark or continues flaming for more than 180 seconds; otherwise record a pass.
• If the specimen fails, decrease the oxygen concentration; if it passes, increase the oxygen concentration, and repeat with narrowing concentration steps until the difference between the last passing and failing concentrations is no greater than 0.2 %.
• Calculate LOI = 100 × V(O2) / [V(O2) + V(N2)] and report the result to one decimal place, together with the standard used, specimen orientation and any atypical behaviour such as dripping or collapse.

Result Interpretation

For extruded foam boards, an LOI between 18 % and 22 % generally indicates a non flame-retarded or lightly retarded product, while properly formulated flame-retardant grades typically achieve 28 % to 36 %. It should be noted that the oxygen index is a small-scale comparative screening method: it does not by itself constitute a fire hazard classification, and classification under GB 8624 or EN 13501-1 requires additional reaction-to-fire tests such as the single burning item test and gross heat of combustion determination conducted by accredited third-party laboratories.

Applicable Standards

• ISO 4589-2: Plastics — Determination of burning behaviour by oxygen index — Ambient-temperature test
• ISO 4589-3: Determination of burning behaviour by oxygen index — Elevated-temperature test
• ASTM D2863: Measuring the minimum oxygen concentration to support candle-like combustion of plastics
• GB/T 2406.2: Chinese national standard technically aligned with ISO 4589-2
• JIS K 7201-2: Japanese equivalent method for oxygen index determination
• Classification and product frameworks including GB 8624, EN 13501-1, GB/T 10801.2 and EN 13164

Applications

Oxygen index determination is applied throughout the life cycle of extruded foam board products, from formulation development to conformity verification:

• Incoming and outgoing quality control of flame-retardant XPS production lines, including verification of flame-retardant dosage and dispersion
• Supporting evidence for fire performance declarations, such as B1 classification under GB 8624 or contributions to Euroclass assessment under EN 13501-1
• Exterior wall insulation, inverted roofing systems, ground slabs and underfloor heating layers where flame-retardant grades are specified
• Cold storage rooms, refrigerated logistics facilities and temperature-controlled buildings requiring durable insulation with controlled burning behaviour
• Civil engineering uses such as frost-heave protection for roads, railways and airport pavements
• Research and development of flame-retardant systems, including the transition to polymeric brominated flame retardants, and verification of performance retention after aging
• Independent verification and certification testing performed by accredited third-party laboratories for domestic acceptance and export markets

FAQ

What oxygen index value should extruded foam board achieve?

Ordinary, non flame-retarded XPS typically falls between 18 % and 22 %. For flame-retardant grades used in building insulation, an LOI of at least 30 % is commonly specified, and well-formulated products usually reach 30 % to 36 %. The exact acceptance limit depends on the applicable product standard and the declared fire classification.

Why is approximately 21 % oxygen a meaningful threshold?

Ambient air contains about 20.9 % oxygen by volume. A material whose LOI is higher than about 21 % cannot sustain candle-like combustion once the ignition source is removed in normal air. This makes the LOI a simple comparative measure of ease of extinguishment, although it is not a direct measure of real-scale fire hazard.

How many specimens are needed for one determination?

The bracketing procedure generally consumes a minimum of 15 specimens for a single LOI value. Additional specimens are required when results are needed in different orientations, at different thicknesses, or after aging treatments.

What factors can affect the oxygen index result?

Specimen dimensions and orientation, the presence of the extruded skin or facings, foam density and cell structure, the type and content of the flame retardant, aging of the flame-retardant system, and conditioning temperature and humidity all influence the measured value. Strict control of sampling, conditioning and gas calibration is therefore essential for reproducible results.

Is an oxygen index test sufficient for fire classification?

No. The LOI is a screening and quality-control method. Fire performance classes such as GB 8624 B1 or EN 13501-1 Euroclasses are assigned on the basis of a complete set of reaction-to-fire tests, including ignitability, the single burning item test and heat of combustion measurements, carried out by accredited third-party laboratories. The oxygen index serves mainly as production-control and supporting evidence.

Are GB/T 2406.2, ISO 4589-2 and ASTM D2863 results equivalent?

The three standards share the same principle and very similar apparatus, ignition procedures and pass/fail criteria, so results are generally comparable. Minor differences exist in specimen dimensions and data treatment, therefore the standard used should always be stated in the test report and comparisons should be made on a consistent basis.