Determination of Oxygen Index Values for Thermal Insulation Materials: Test Methods, Standards, and

Oxygen index (Limiting Oxygen Index, LOI) testing is one of the most widely used small-scale fire performance methods for evaluating the flammability of thermal insulation materials such as rigid polyurethane foam, polystyrene foam, phenolic foam, and elastomeric insulation. This article explains how oxygen index values for insulation materials are determined in accordance with ISO 4589-2, ASTM D2863, and GB/T 2406.2, covering sample requirements, test procedures, calculation methods, typical result ranges, and practical applications in quality control and fire safety compliance.

Technical Overview

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 test specimen for a specified time or over a specified length. Because ambient air contains approximately 20.9% oxygen by volume, materials with an LOI above approximately 21% will tend to self-extinguish after removal of an ignition source, while materials with lower values will continue to burn in normal atmospheric conditions.

For thermal insulation products, which are often lightweight, cellular, and highly combustible in their unmodified form, the oxygen index is a key screening indicator of flame-retardant performance. It is used in research and development of flame-retardant formulations, routine factory quality control, incoming material inspection, and as supporting data for fire classification systems such as GB 8624 and EN 13501-1. The determination is typically performed in accordance with ISO 4589-2, ASTM D2863, or the technically equivalent Chinese national standard GB/T 2406.2, with elevated-temperature testing available under ISO 4589-3.

The oxygen index is calculated from the volumetric flow rates of the two gases at the critical concentration:

• LOI (%) = 100 × V(O2) / [V(O2) + V(N2)]

where V(O2) and V(N2) are the volume flow rates of oxygen and nitrogen, respectively, at the equilibrium burning condition determined by the bracketing procedure.

Testing Samples

Oxygen index determination is applicable to a broad range of thermal insulation materials that are rigid, flexible, cellular, or fibrous. Because the test geometry influences the result, specimens must be cut so that the dimensions conform to the requirements of the selected test standard.

Foam Plastic Insulation

Rigid and semi-rigid cellular plastics represent the largest category of insulation materials tested for oxygen index, including:

• Expanded polystyrene (EPS) boards and molded parts
• Extruded polystyrene (XPS) boards
• Rigid polyurethane (PU) and polyisocyanurate (PIR) foams
• Phenolic foam boards and panels
• Polyethylene (PE) and cross-linked polyethylene foams
• Melamine foam products

Standard bar specimens are typically cut to approximately 80–150 mm in length, 10 mm in width, and 4 mm in thickness for self-supporting materials. For materials that are not self-supporting at this geometry, a support frame or alternative specimen holder is used as permitted by the standard.

Elastomeric and Rubber-Based Insulation

Flexible elastomeric foam insulation, such as nitrile-butadiene rubber (NBR) foam tubes and sheets used on HVAC piping and refrigeration lines, is commonly tested for oxygen index because many procurement specifications require minimum values. Specimens are cut from sheet material or slit from tubing to the standard geometry.

Fibrous and Inorganic Insulation

Mineral wool products, including glass wool and rock (stone) wool, are fundamentally non-combustible and usually fall outside the intended scope of the oxygen index method. However, faced or composite products containing combustible facings, binders, or polymeric layers may be tested to characterize the combustible component or to verify compliance with product specifications.

Coatings, Films, and Facings

Thin materials such as reflective foil facings, fire-retardant coatings, membranes, and reinforced skins associated with insulation systems can be evaluated using the film and sheet specimen methods described in ISO 4589-2, which specify narrower strips and rolled-edge mounting on a small frame.

Testing Items

Accredited third-party laboratories typically offer the following determinations related to the oxygen index of insulation materials:

• Ambient-temperature oxygen index (LOI) determination to ISO 4589-2, ASTM D2863, or GB/T 2406.2
• Elevated-temperature oxygen index (temperature index) determination to ISO 4589-3
• Oxygen index testing as supporting evidence for fire performance classifications under GB 8624 and EN 13501-1
• Comparative oxygen index testing of flame-retardant formulations during product development
• Verification of lot-to-lot consistency against internal or contractual LOI specifications

Testing Methods

The determination follows a standardized procedure designed to identify, by systematic adjustment of the oxygen concentration, the exact atmosphere in which burning is marginally sustained.

Specimen Preparation and Conditioning

Specimens are cut cleanly from representative material, free of compressed zones, skin irregularities, or damage introduced during cutting. Edges are smoothed where necessary to avoid artificial flame propagation paths. Unless otherwise specified, specimens are conditioned prior to testing at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum period defined by the standard or product specification, commonly at least 88 hours, in accordance with ISO 291. The number of specimens is normally at least 15, and in many cases a full determination consumes 20 or more specimens as the bracketing sequence converges.

Apparatus

The test is performed on an oxygen index apparatus consisting of:

• A heat-resistant glass vertical chimney of specified internal diameter, typically approximately 70–100 mm depending on the standard
• A specimen clamp and holder that supports the specimen vertically in the center of the chimney
• Precision gas meters, mass flow controllers, or calibrated rotameters for oxygen and nitrogen supply
• A gas mixing chamber and distributor system to deliver a smooth, uniform upward flow through the chimney, commonly corresponding to a linear velocity of approximately 40 mm/s
• A small, adjustable hydrogen or hydrocarbon gas flame igniter used to ignite the top of the specimen
• A timer and ruler or marked scale for judging the burning time and damaged length

The oxygen and nitrogen concentrations must be traceable, and flow calibration is verified regularly to ensure the accuracy of the reported LOI value.

Test Procedure

The determination proceeds through the following stages:

• The conditioned specimen is clamped vertically in the chimney and the total gas flow is set to the standard value.
• An initial oxygen concentration is selected based on experience or a preliminary trial; the gas mixture is allowed to purge and stabilize the chimney atmosphere.
• The igniter flame is applied to the top surface of the specimen for a defined period, typically up to 30 seconds, with brief reapplications if necessary, until the specimen is well ignited.
• The response is recorded as either a burning event, meaning combustion persists for more than the specified period or propagates beyond the specified damaged length, or a non-burning event.
• The oxygen concentration is adjusted stepwise in the appropriate direction, and further specimens are tested using the bracketing (critical concentration) approach until the equilibrium condition is established.
• Verification specimens are tested at the critical concentration to confirm the final result.

Under ISO 4589-2 and GB/T 2406.2, a burning event is generally one in which the specimen burns for more than 180 seconds after removal of the ignition source or the damaged length exceeds 50 mm from the top of the specimen, with the precise criteria depending on the ignition method and material category applied.

Calculation and Expression of Results

The final oxygen index is calculated from the volumetric flow rates of oxygen and nitrogen at the critical concentration and is reported to one decimal place as a volume percentage. The test report includes the material identification, specimen dimensions and orientation, ignition method used, conditioning history, the individual oxygen concentration series, and the calculated LOI value, together with any observed burning behavior such as flaming drips, afterglow, or shrinkage.

Precision and Influencing Factors

When performed by competent operators on well-conditioned specimens, the repeatability of the oxygen index determination within a single laboratory is typically on the order of 0.5% oxygen or better, while reproducibility between laboratories is generally within approximately 1% to 1.5% oxygen. Results can be influenced by:

• Specimen density, cell structure, and thickness, particularly for low-density foams
• Moisture content and conditioning history
• Cutting quality and the presence of skins or facings on the tested surface
• Accuracy of gas flow measurement and mixture uniformity
• Operator judgment during ignition and response assessment

For these reasons, small differences between reported values should only be interpreted with reference to the precision of the method.

Applications

Oxygen index determination for insulation materials supports a wide range of engineering and regulatory objectives:

• Verification of flame-retardant grade claims for polymeric insulation products in procurement and acceptance inspection
• Supporting evidence in the fire performance evaluation of building products under classification frameworks such as GB 8624 and EN 13501-1, alongside reaction-to-fire tests such as the single burning item test and ignition source tests
• Research and development of halogen-free and intumescent flame-retardant systems for polyurethane, polystyrene, and elastomeric foams
• Factory production control, where periodic LOI checks confirm the consistent dosing of flame retardants
• Comparative screening of candidate materials for rail transit, marine, appliance, and industrial thermal insulation applications
• Failure analysis when insulation materials involved in fire incidents are examined for flame-retardant content

Typical indicative ranges reported in the literature include approximately 18–20% for untreated polystyrene foams, approximately 22–30% for flame-retardant grades of extruded polystyrene and polyurethane, approximately 30–40% for polyisocyanurate systems, and in excess of 35% for phenolic and melamine foams, with elastomeric NBR foams commonly specified at 32% or above. These values are formulation-dependent and must always be confirmed by testing.

FAQ

What oxygen index value qualifies an insulation material as flame retardant?

There is no universal threshold, but as a practical guideline, materials with an LOI below approximately 21% burn readily in air, while values of 26% or above are commonly required in product specifications for flame-retardant insulation, and values of 32% or higher are frequently specified for elastomeric foam and high-grade products. The applicable limit must always be taken from the relevant product standard, certification scheme, or contract.

Which standards govern the determination?

The primary methods are ISO 4589-2 and its American counterpart ASTM D2863, together with the Chinese national standard GB/T 2406.2, which is technically aligned with ISO 4589-2. Elevated-temperature testing is covered by ISO 4589-3.

How many samples are needed for one determination?

A complete determination typically requires 15 to 20 specimens of the standard bar geometry, and laboratories usually request additional spare material to allow repeat determinations or verification testing.

Does a high oxygen index guarantee fire safety in a real building fire?

No. The oxygen index is a small-scale laboratory ranking tool. Full-scale and intermediate-scale reaction-to-fire behavior, including heat release, smoke production, and flame spread, must be assessed with the appropriate fire test methods and classification systems for the intended end use.

Can mineral wool insulation be tested by this method?

Non-combustible mineral wool generally does not sustain candle-like combustion and therefore falls outside the practical scope of the method; oxygen index testing is mainly relevant for combustible polymeric insulation and for combustible components of composite products.

How long does a typical determination take?

For a well-characterized material, one determination including conditioning, calibration checks, and testing of the specimen series is usually completed within a single working day in the laboratory, with reporting following internal quality procedures.