Oxygen Index Test for Combustion Performance of Thermal Insulation Materials

The oxygen index test, also known as the limiting oxygen index (LOI) test, is a fundamental laboratory method for evaluating the combustion performance of thermal insulation materials such as rigid polyurethane foam, polystyrene boards, phenolic foam, and mineral wool products. By measuring the minimum oxygen concentration required to sustain flaming combustion, this test provides a quantitative flammability index widely used for quality control, flame-retardant formulation development, and compliance with fire safety classification requirements under standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2. This article explains the technical principles, specimen requirements, test procedure, and typical applications of oxygen index testing for insulation materials.

Technical Overview

The limiting oxygen index (LOI) is defined as the minimum concentration of oxygen, expressed as a volume percentage, in a mixture of oxygen and nitrogen that will just support flaming combustion of a vertically mounted specimen under controlled laboratory conditions. Because ambient air contains approximately 20.9% oxygen, the LOI value provides an intuitive benchmark for flammability: materials with an LOI below 21% will generally continue to burn in normal air, while materials with higher LOI values are progressively more difficult to ignite and sustain combustion.

For thermal insulation materials, the oxygen index test is one of the most widely applied bench-scale fire performance methods. It is particularly relevant for combustible insulation products such as expanded polystyrene (EPS), extruded polystyrene (XPS), rigid polyurethane (PUR) and polyisocyanurate (PIR) foams, phenolic foam, polyethylene foam, and rubber-plastic insulation products. The test supports compliance demonstrations under building fire classification systems such as GB 8624 and EN 13501-1, as well as routine quality control of flame-retardant modifications.

The principal reference standards for this test include:

• ISO 4589-2 / EN ISO 4589-2 – Determination of burning behaviour by oxygen index, Part 2: Ambient-temperature test
• ASTM D2863 – Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics
• GB/T 2406.2 – Plastics – Determination of burning behaviour by oxygen index, Part 2 (Chinese national standard, technically aligned with ISO 4589-2)
• ISO 4589-3 – Elevated-temperature oxygen index determination, applicable when high-temperature service conditions must be simulated

LOI is expressed as: LOI = [O2] / ([O2] + [N2]) × 100%, where the gas volumes are measured at the equilibrium condition determined by the bracketing procedure defined in the standard.

Testing Samples

Rigid Foam Insulation Boards (EPS, XPS, PUR, PIR, Phenolic Foam)

Specimens are cut from representative board samples with clean, rectangular edges free of dust and skin discontinuities. For cellular materials, ISO 4589-2 typically specifies specimens of 125 mm to 150 mm in length, 10 mm in width, and 10 mm in thickness (Type V geometry), although alternative dimensions may apply depending on product form and standard edition. A minimum of 15 to 20 specimens is normally required so that the oxygen concentration can be bracketed through multiple adjustments.

Flexible and Elastomeric Insulation (Rubber-Plastic Foam, PE Foam)

Flexible foams are tested using the specimen geometry designated for non-self-supporting materials, often with a support frame or holder that maintains vertical orientation without restricting flame propagation along the exposed surface. Specimen orientation relative to the rise direction of the foam should be recorded, as anisotropic cell structure can influence the measured LOI.

Faced or Laminated Insulation Products

When insulation boards carry facings such as aluminum foil, glass tissue, or polymer films, the specimen configuration must be agreed in advance: the test may be performed on the core foam alone, on the complete composite, or on both, depending on the applicable product standard and certification scheme. The facing configuration must be documented in the test report.

Specimen Conditioning

Prior to testing, specimens shall be conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours (or shorter periods where justified by the standard), unless the governing specification states otherwise. Consistent conditioning is essential because moisture content significantly affects the ignition and burning behavior of hygroscopic insulation materials such as phenolic foam and mineral-based products with organic binders.

Testing Items

The core deliverable of the oxygen index test is the limiting oxygen index value of the material, reported to one decimal place together with the applicable standard, specimen geometry, and burning criterion. Depending on the client's purpose, the following test items are commonly performed:

• Limiting oxygen index (LOI) at ambient temperature in accordance with ISO 4589-2, ASTM D2863, or GB/T 2406.2
• Elevated-temperature oxygen index in accordance with ISO 4589-3 for materials used in high-temperature service environments
• Verification testing against declared LOI thresholds for flame-retardant grade qualification
• Comparative LOI evaluation of base resin versus flame-retardant modified formulations during research and development
• Aging or humidity-exposure followed by LOI re-testing to assess durability of flame-retardant performance
• Supporting data for fire classification under GB 8624 (e.g., Grade B1 ducting and board products) and related product certification requirements

The pass/fail burning criteria specified in ISO 4589-2 and GB/T 2406.2 require that a specimen either burns for a defined period (commonly 3 minutes, depending on specimen type) or consumes a defined burned length (commonly 50 mm), measured from the top edge. The LOI is the oxygen concentration at which the specimen neither satisfies nor fails the criterion in adjacent concentration steps, confirmed by the standard's verification procedure.

Testing Methods

The test is conducted using an oxygen index apparatus comprising a heat-resistant glass chimney, a specimen holder with a vertical rod, a precision gas mixing and flow control system, an oxygen analyzer or calibrated flow meters, and a small propane-fueled ignition burner. The general procedure is as follows:

• Mount the conditioned specimen vertically in the holder so that the top of the specimen is at a defined position below the chimney top, and install the assembly centrally within the chimney.
• Establish the initial oxygen concentration, typically 21% for exploratory testing or a value near the expected LOI when prior data are available.
• Purge the chimney with the oxygen–nitrogen mixture at the prescribed linear flow velocity (typically 40 mm/s) for at least 30 seconds to ensure a stable atmosphere before ignition.
• Apply the ignition flame to the top of the specimen in the manner specified by the standard, gradually moving the burner to cover the full top surface, then withdraw it and start timing.
• Observe whether the specimen meets or fails the burning criterion (burning time or burned length), record the result, and adjust the oxygen concentration using the bracketing (increment/decrement) method, typically in steps of 0.5% to 2% depending on proximity to the limiting value.
• Continue until the limiting concentration is bracketed within the required resolution, then perform the confirmation runs defined by the standard, commonly requiring a specified number of consecutive specimens to satisfy the "just burns / just extinguishes" pattern.
• Calculate and report the LOI value, along with specimen details, conditioning history, ignition observations (flaming, glowing, dripping, shrinkage), and any deviations from the standard.

Several technical points deserve attention when testing insulation materials. Foamed plastics may shrink, melt, or drip during the test; the operator must record such behavior because molten drips can extinguish or propagate the flame and affect the validity of the criterion. Anisotropic rigid foams should be tested with a documented orientation. In cases of borderline results, replicate testing across multiple specimens and boards improves confidence in the reported LOI. The test is a small-scale flammability screening method; it does not by itself represent behavior in a fully developed fire and should be complemented by reaction-to-fire tests such as the single burning item (SBI) test or the cone calorimeter where classification is required.

Applications

Oxygen index testing serves a broad range of engineering, regulatory, and commercial purposes across the insulation industry:

• Building code compliance: demonstrating that combustible insulation products meet the flammability requirements associated with GB 8624 fire performance grades, such as Grade B1 qualification for EPS, XPS, and PUR products used in wall, roof, and duct insulation systems in China.
• Product certification and factory production control: periodic LOI verification as part of voluntary product certification and internal quality assurance for insulation board manufacturers.
• Flame-retardant development: quantifying the effectiveness of halogenated, phosphorus-based, nitrogen-based, and mineral filler flame-retardant systems in polymer foam formulations.
• Incoming material inspection: acceptance testing of insulation materials by contractors, façade system integrators, and OEM purchasers against declared LOI specifications.
• Failure analysis: investigating fire incidents or non-conforming batches by comparing the LOI of field samples against reference material.
• Export compliance: supporting documentation for international markets where ISO 4589-2, EN ISO 4589-2, or ASTM D2863 data are requested by buyers, designers, and accredited third-party laboratories.

Typical LOI reference values in the industry include approximately 18–20% for unmodified EPS and PUR foams, values above 30% for well-flame-retarded grades used in B1-class applications, and 26% or higher for many rubber-plastic insulation products claiming reduced flammability. Exact acceptance thresholds must always be taken from the governing product standard or project specification rather than generic guidance.

FAQ

What is a good oxygen index value for insulation materials?

As a general guideline, an LOI of 21% or below indicates the material can burn in normal air, while values above 26% indicate improved flame resistance and values above 30% indicate strongly flame-retarded behavior. Many B1-grade insulation products in the Chinese market are specified with LOI ≥ 30%, but the actual acceptance criterion must follow the applicable product standard or project specification.

Which standards govern the oxygen index test?

The primary standards are ISO 4589-2 and EN ISO 4589-2 for ambient-temperature testing, ASTM D2863 for the equivalent candle-like combustion method, and GB/T 2406.2 as the Chinese national standard harmonized with ISO 4589-2. ISO 4589-3 covers the elevated-temperature variant.

How many specimens are required?

Typically 15 to 20 specimens of identical geometry and orientation are prepared, because the bracketing procedure consumes multiple specimens and the final confirmation stage requires several repeat runs. Additional specimens are recommended for materials that exhibit melting, dripping, or high variability.

Can mineral wool insulation be tested by the oxygen index method?

Mineral wool (rock wool and glass wool) is non-combustible inorganic fiber material and is normally classified at the highest fire class (e.g., A grade) without LOI testing. However, facings, coatings, and organic binders associated with such products may be evaluated by oxygen index or other reaction-to-fire methods when required.

Does LOI replace full-scale fire classification testing?

No. The oxygen index test is a small-scale, precisely controlled flammability comparison method. Formal fire classification under GB 8624 or EN 13501-1 depends on a suite of tests, including the single burning item (SBI) test and gross heat of combustion determination. LOI data are frequently used as supporting evidence, quality control metrics, and screening tools within those programs.

What factors affect the reproducibility of LOI results?

Key factors include specimen density and thickness, foam cell orientation, conditioning humidity, gas flow calibration, oxygen analyzer accuracy, ignition technique, and operator judgment of the burn-length criterion. Testing at an accredited third-party laboratory with calibrated apparatus and standardized procedures significantly improves comparability between results.

How long does a typical test take?

For a well-characterized material, a complete oxygen index determination including conditioning, bracketing, and confirmation typically takes one working day of laboratory time, with report issuance following internal review. R&D programs comparing multiple formulations may require several days depending on the number of sample sets.