The limiting oxygen index (LOI) test is one of the most widely used small-scale methods for evaluating the flammability of cellular plastics such as polyurethane, polystyrene, polyolefin, and phenolic foams. Because foam materials readily absorb moisture and respond sensitively to ambient temperature, oxygen index testing is typically performed on specimens that have been conditioned in controlled temperature and humidity atmospheres in accordance with ISO 4589-2, ASTM D2863, GB/T 2406.2, and related conditioning standards. This article provides a technical overview of foam material oxygen index testing under temperature and humidity control, covering sample requirements, test items, methodology, applications, and frequently asked questions.
Technical Overview
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. Since ambient air contains approximately 20.9% oxygen, foam materials with an LOI below 21% burn readily in air, while higher LOI values indicate progressively greater difficulty of ignition and reduced flame propagation.
Foam materials are among the most combustion-sensitive polymer products due to their low density, high surface-to-mass ratio, and open or semi-open cell structures that allow oxygen to penetrate deeply into the material. Temperature and humidity exert a measurable influence on test results: moisture absorbed within the cell structure vaporizes during burning and acts as an inert diluent and heat sink, while elevated temperatures accelerate pyrolysis and typically reduce the measured index. For this reason, reproducible oxygen index measurement requires strict conditioning of specimens in standard atmospheres such as 23 °C ± 2 °C and 50% ± 5% relative humidity, as specified in ISO 291, ASTM D618, and GB/T 2918.
The principal method standards for this testing include ISO 4589-2, ASTM D2863, GB/T 2406.2, and JIS K 7201-2 for ambient-temperature oxygen index determination, and ISO 4589-3 for oxygen index determination at elevated temperatures. The test apparatus consists of a heat-resistant glass chimney, a vertical specimen holder, a gas mixing and metering system with calibrated flow controls, and a small premixed gas-flame igniter. Accredited third-party laboratories routinely perform this testing to support product certification, quality control, and research and development programs.
Testing Samples
Oxygen index testing under temperature and humidity conditioning is applicable to virtually all commercial cellular polymers. Typical sample categories submitted for testing include the following.
Polyurethane Foams (Flexible and Rigid)
Flexible polyurethane slabstock and molded foams used in furniture, mattresses, and automotive seating are tested to verify flame-retardant grades. Rigid polyurethane and polyisocyanurate foams used as building insulation boards are tested to demonstrate compliance with minimum index requirements in product specifications.
Polystyrene Foams (EPS and XPS)
Expanded polystyrene and extruded polystyrene insulation foams are evaluated in their finished, flame-retardant-modified form. Because these materials shrink and melt when heated, specimens are tested with appropriate support and special attention is given to specimen geometry and orientation.
Polyolefin Foams (PE, PP, EVA)
Cross-linked and non-cross-linked polyethylene foams, expanded polypropylene beads and molded parts, and EVA foams used in packaging, sports goods, footwear, and automotive components are commonly tested, particularly when fire-performance declarations are required.
Phenolic, NBR/PVC, Melamine, and Silicone Foams
Phenolic foam and elastomeric NBR/PVC foams for HVAC and pipe insulation, melamine foam for acoustic panels, and silicone foam for transit and industrial sealing applications are tested to characterize their inherently higher ignition resistance.
Specimen Requirements
Unless otherwise specified by a product standard, specimens are typically 80 mm to 150 mm in length, 10 mm in width, and 10 mm in thickness, cut cleanly without compressed or melted edges and free of surface skin where the core material is to be evaluated. A minimum of 15 specimens is generally required for each determination, with additional specimens reserved for preliminary concentration setting. Specimens are conditioned before testing in the specified atmosphere, commonly 23 °C ± 2 °C and 50% ± 5% relative humidity for at least 88 hours, or subjected to alternative humidity or drying regimes when the effect of moisture content is being investigated.
Testing Items
The core deliverable of this testing is the limiting oxygen index value, supported by conditioning parameters and burning-behavior observations. Typical test items include:
- Limiting oxygen index (LOI) of specimens conditioned at 23 °C / 50% relative humidity
- Limiting oxygen index after elevated-humidity conditioning or moisture-aging exposure, used to quantify moisture uptake effects
- Elevated-temperature oxygen index determination in accordance with ISO 4589-3
- Limiting oxygen index after drying, used to establish the dry-state baseline for hygroscopic foams
- Observation of afterflame and afterglow behavior, flaming drips, melting, shrinkage, curling, and charring
- Comparison of base resin and flame-retardant-modified foam formulations in research programs
Acceptance criteria are defined by the relevant product specification. Common specification thresholds for foam materials include LOI values of 26%, 28%, or 32%, with the higher values frequently required for building insulation products in certain regulatory classifications.
Testing Methods
Conditioning and Test Atmospheres
Specimens are conditioned in a climate-controlled chamber meeting the requirements of ISO 291 or an equivalent conditioning standard. The standard atmosphere 23/50 (23 °C ± 2 °C, 50% ± 5% relative humidity) is the default condition. When the influence of humidity is explicitly part of the test program, specimens may be exposed to more severe humidity conditions, such as elevated temperature and high relative humidity, followed by testing; in such cases the conditioning history is fully documented in the test report because absorbed moisture directly shifts the measured index.
Apparatus Verification
Before testing, the gas flows, oxygen and nitrogen purity, and mixing system are verified, and the apparatus is periodically checked using reference materials with certified oxygen index values, such as PMMA reference specimens. This verification ensures traceability and comparability of results between laboratories.
Test Procedure
A conditioned specimen is clamped vertically in the holder within the chimney, and a mixture of oxygen and nitrogen of known concentration is passed upward through the chimney at the specified linear velocity. After purging, the top of the specimen is ignited with the small diffusion flame of the igniter. The specimen response is judged against the response criterion defined by the standard, typically involving the flame front reaching a reference mark or the burning time exceeding a specified limit. Depending on the outcome, the oxygen concentration is adjusted upward or downward using a bracketing procedure with progressively smaller concentration steps, and fresh specimens are tested at each concentration until the critical concentration window is established.
Calculation and Reporting
The limiting oxygen index is calculated as the oxygen volume fraction in the gas mixture at the critical concentration and is reported to the nearest 0.1%. The test report includes the conditioning atmosphere and duration, specimen dimensions and orientation, ignition procedure, the measured index, and all relevant burning-behavior observations such as dripping, afterglow, and specimen distortion. For elevated-temperature testing under ISO 4589-3, the chimney and gas mixture are preheated to the specified test temperature, and the index is reported as a function of temperature.
Applications
Oxygen index testing of foam materials under controlled temperature and humidity conditions supports a wide range of industrial and regulatory purposes, including:
- Qualification of building insulation products, including EPS, XPS, and rigid polyurethane boards, as supporting evidence for classification under systems such as EN 13501-1 and GB 8624
- Verification of flame-retardant flexible foams for furniture, mattress, and seating applications against purchasing specifications
- Evaluation of foamed interior components and trim materials for automotive, rail, and mass-transit vehicles
- Quality control of incoming foam materials and flame-retardant masterbatch consistency in manufacturing
- Research and development of halogen-free and reactive flame-retardant foam formulations
- Comparison of material candidates during design selection, supported where necessary by complementary methods such as UL 94 vertical and horizontal burn testing
- Assessment of moisture-aging effects on the fire performance of hydrophilic and open-cell foams
FAQ
What oxygen index value is considered flame retardant for foam materials?
As a general guide, materials with an LOI above 21% will not continue burning in still air under test conditions. Industry specifications commonly regard foams with an LOI of 26% or higher as flame retardant, and certain building insulation specifications require values of 32% or greater. The applicable threshold is always defined by the relevant product standard or purchasing specification.
Why must temperature and humidity be controlled during oxygen index testing?
Foam materials can absorb significant amounts of water in their cell structure, and absorbed moisture increases the apparent oxygen index by cooling and diluting the combustion zone. Variations in conditioning temperature and humidity therefore lead to poor reproducibility. Standardized conditioning, typically 23 °C and 50% relative humidity, ensures that results from different laboratories and different production batches are directly comparable.
What is the difference between conditioning atmosphere and test atmosphere?
The conditioning atmosphere is the environment in which specimens are stored before testing, for example 88 hours at 23 °C and 50% relative humidity. The test atmosphere is the oxygen and nitrogen gas environment inside the chimney during measurement. For ambient testing, the room environment is maintained close to the conditioning condition; for elevated-temperature oxygen index testing per ISO 4589-3, the gas environment itself is heated, which usually lowers the measured index.
How do ISO 4589-2 and ASTM D2863 compare?
The two standards are technically harmonized in principle and apparatus, using the same candle-like burning concept and bracketing procedure. Minor differences exist in specimen support details, response criteria for certain material classes, and reporting requirements. Results are usually similar, but laboratories report against the specific standard cited in the product specification, and the standards should not be assumed to be interchangeable without verification.
How many specimens are required for one determination?
A minimum of 15 conditioned specimens is normally required for a single determination, with several additional specimens used for establishing the initial concentration bracket. Foams with high burning variability, strong melting behavior, or pronounced density gradients may require additional specimens to achieve a valid result.
Is a high oxygen index sufficient to prove fire safety?
No. The oxygen index test is a small-scale ignitability and combustion-sustainment screening method. It does not simulate real-scale fire scenarios involving radiant flux, flame spread, or heat release. For full compliance assessment, oxygen index data are combined with other fire test methods, such as UL 94 burn testing, flame spread and heat release measurements, and the classification scheme applicable to the end-use market.
