The oxygen index test, also known as the limiting oxygen index (LOI) test, is the primary method for evaluating the flame-retardant performance of extruded polystyrene (XPS) insulation boards. Performed in accordance with GB/T 2406.2, ISO 4589-2, and ASTM D2863, it measures the minimum oxygen concentration required to sustain combustion of the material and provides essential data for fire safety classification and quality control. This article explains the test principle, specimen requirements, testing procedure, result interpretation, and typical applications of the XPS board oxygen index test.
Technical Overview
Extruded polystyrene (XPS) board is a closed-cell, rigid foam insulation material manufactured by continuously extruding polystyrene resin combined with a blowing agent. Because base polystyrene is an inherently flammable polymer with a natural limiting oxygen index of only about 18–19%, flame retardants are incorporated during production to improve fire performance. The oxygen index test is the standard laboratory method used to quantify how effectively this flame retardancy has been achieved.
The limiting oxygen index (LOI) is defined as the minimum volume fraction of oxygen, in a mixture of oxygen and nitrogen flowing upward through a vertical glass chimney, that will just support candle-like combustion of a vertically mounted specimen. Because ambient air contains approximately 21% oxygen, a material with an LOI below 21% will continue to burn easily in air, while a material with a higher LOI tends to self-extinguish once an ignition source is removed. A higher LOI value therefore indicates better flame-retardant performance.
- GB/T 2406.2-2009 — Determination of burning behaviour by oxygen index (Chinese national standard)
- ISO 4589-2 — Plastics 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
For building applications, XPS boards intended for flame-retardant grades are typically expected to reach an LOI of 30% or higher, while ordinary grades are commonly specified at a minimum of 26%. Unmodified polystyrene foam generally measures between 18% and 22%. The test is widely carried out by accredited third-party laboratories, manufacturers' quality control departments, and research institutions.
Testing Samples
Specimen Dimensions and Preparation
Test specimens are cut directly from finished XPS boards. In accordance with GB/T 2406.2 and ISO 4589-2, specimens are typically 80–150 mm in length and 10 ± 0.5 mm in width. For rigid foams, the preferred thickness is 10–15 mm; where the product thickness is less than 10 mm, the specimen is tested at its full thickness and the actual thickness is reported. Specimens must be cut cleanly with smooth edges, free of dust, cells crushed during cutting, visible defects, and surface contamination. Depending on the specification, specimens may be tested with or without the extruded surface skin, and this condition should be recorded in the report.
Conditioning Requirements
Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours, following GB/T 2918 or ISO 291. Proper conditioning is critical because moisture content can measurably influence the burning behavior of foam materials.
Sample Quantity
A minimum of 15 specimens is generally recommended to complete the full incremental test sequence, and additional spare specimens should be prepared in case of invalid observations. Samples should be taken from representative positions of the board or production batch to ensure the results reflect actual product quality.
Testing Items
The oxygen index test evaluates the following items:
- Limiting oxygen index (LOI) value, expressed as the volume percentage of oxygen in the oxygen/nitrogen mixture
- Burning behavior observations, including after-flame time, whether the flame front passes the 50 mm reference mark, flaming drips, and char or melt formation
- Compliance verification against specified acceptance thresholds, such as a minimum LOI of 30% for flame-retardant grade products
Testing Methods
Apparatus
The test is performed using an oxygen index tester consisting of a vertical heat-resistant glass chimney (minimum 450 mm in height, typically 75–100 mm internal diameter), a gas mixing chamber, precision flow meters or mass flow controllers for oxygen and nitrogen, and optionally an oxygen analyzer to verify the mixed gas concentration. A specimen holder clamps the specimen vertically on the centerline of the chimney, and a small propane or butane gas igniter with a defined flame length is used as the ignition source. The total gas flow is controlled to provide a linear velocity of approximately 40 mm/s through the chimney.
Test Procedure
Method A (top surface ignition) is the most commonly applied procedure for XPS boards:
- Set the initial oxygen concentration based on experience or a preliminary trial, and purge the chimney with the gas mixture for at least 30 seconds
- Mount the specimen vertically with a reference mark positioned 50 mm below the top edge, and pre-purge the chimney
- Apply the igniter flame to the entire top surface of the specimen in 5-second intervals, alternating with 5-second removal periods, for a maximum application time of 15 seconds
- Record the response: the specimen is judged to have burned if any individual flaming period exceeds 10 seconds after removal of the igniter, or if the flame front reaches the 50 mm mark; otherwise the specimen is judged not to have burned
- Adjust the oxygen concentration upward or downward in steps, using larger steps initially and fine steps of approximately 0.2% near the balance point, repeating the observation at each concentration
- Continue the bracketing sequence until the series of observations required by the standard is complete
Method B (propagation ignition), in which the igniter flame is applied to ignite the entire top of the specimen to assess flame propagation, may be used as an alternative when specified by the product standard or customer requirement.
Calculation and Reporting of Results
The LOI is calculated as the volume fraction of oxygen in the oxygen/nitrogen mixture at the balance point, derived from the final series of concentration observations using the calculation formula specified in GB/T 2406.2 and ISO 4589-2. The result is reported as a percentage, and the test report should include the standard and method used, specimen dimensions and conditioning, the LOI value, and any relevant observations of burning behavior such as dripping, smoking, or glow persistence. Because the LOI of foam materials is thickness-dependent, comparing results between different products requires specimens of comparable thickness.
Applications
Oxygen index testing of XPS boards serves a wide range of practical purposes:
- Building envelope applications: exterior wall insulation, external thermal insulation composite systems (ETICS), roof and inverted-roof insulation, and below-grade or foundation insulation
- Cold-chain and industrial applications: cold storage facilities, refrigerated rooms, and process insulation where fire performance must be documented
- Floor and civil engineering applications: underfloor heating panels, floor leveling boards, road and railway subgrade insulation, and frost-heave protection layers
- Product certification and regulatory compliance: supporting fire performance grading of building materials and demonstrating conformity with procurement specifications
- Factory production control: routine batch release testing and incoming material inspection by manufacturers, distributors, and contractors
- Research and development: evaluating flame-retardant formulations, comparing halogen-free alternatives, and studying the retention of flame retardancy after heat aging or weathering
FAQ
What LOI value should flame-retardant XPS board achieve?
Flame-retardant XPS boards used in building applications are generally required to reach an LOI of 30% or higher, while ordinary grades are commonly accepted at a minimum of 26%. Specific thresholds should always be confirmed against the applicable product standard and project specification.
Do density, thickness, and surface skin affect the result?
Yes. The LOI of foam materials is influenced by specimen thickness, foam density, cell structure, and the presence of the extruded surface skin. Thicker specimens often behave differently from thin ones, which is why specimen dimensions must always be reported alongside the LOI value.
Is the oxygen index test sufficient to prove fire safety?
No. The LOI measures the ease of ignition and the ability to sustain combustion under controlled laboratory conditions. Full fire performance classification of building products additionally requires tests such as single-flame-source testing, the single burning item test, or heat release measurement using cone calorimetry, depending on the classification system applied.
How is the oxygen index test different from UL 94 testing?
The oxygen index test determines the minimum oxygen concentration needed to support combustion in a flowing gas mixture, while UL 94 evaluates the specimens' response to a small flame ignition source under fixed atmospheric conditions. The two methods are complementary, and both are widely used to rank the flammability of plastic and foam materials.
How many specimens are needed and how long does testing take?
At least 15 conditioned specimens are normally required for a complete determination. The laboratory test itself, including apparatus stabilization and the incremental concentration sequence, typically takes a few hours, and lead time also depends on the mandatory conditioning period before testing.