Oxygen Index Testing of Rigid Plastic Foams: Standards, Methods and Applications

The limiting oxygen index (LOI) test is one of the most widely used small-scale flammability methods for evaluating the fire performance of rigid plastic foams used in thermal insulation, construction, and transportation. By determining the minimum oxygen concentration required to sustain candle-like combustion, laboratories can assess and compare the flame-retardant characteristics of materials in accordance with 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 rigid cellular plastics.

Technical Overview

The oxygen index test, also known as the limiting oxygen index (LOI) test, measures the minimum volumetric concentration of oxygen in a flowing oxygen/nitrogen mixture that will just support combustion of a vertically mounted specimen. Combustion is considered sustained when the specimen burns for at least 3 minutes after removal of the ignition source, or when the damaged length reaches 50 mm, whichever occurs first. The result is expressed as a percentage, with higher values indicating greater flame resistance.

Since ambient air contains approximately 20.9% oxygen, a rigid foam with an LOI above roughly 21% will tend to self-extinguish after ignition in normal atmospheric conditions, while a material with an LOI below this threshold will continue to burn. This makes the oxygen index a practical and repeatable screening indicator for flame retardancy. It should be noted that the LOI is a small-scale laboratory measurement and does not by itself represent the fire hazard of a material under real-scale fire conditions; full-scale or reaction-to-fire tests are required for definitive classification.

The principal standards governing this test include 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), and the Chinese national standard GB/T 2406.2, which is technically aligned with ISO 4589-2. ISO 4589-3 extends the method to elevated-temperature testing, which is useful for materials exposed to higher service temperatures.

Testing Samples

Rigid cellular plastics submitted for oxygen index testing should be representative of the finished product, with uniform density, cell structure, and thickness. Specimens must be cut cleanly without crushing, melting, or tearing the cells, because damaged edges can significantly influence ignition and burning behavior.

Specimen Dimensions

Under ISO 4589-2 / GB/T 2406.2, rigid cellular materials are typically tested using Type IV specimens measuring 80–150 mm in length, 10 mm in width, and 10 mm in thickness. Under ASTM D2863, the preferred specimen is 70–150 mm long, 6.5 mm wide, and 3 mm thick, although a 6.5 mm × 12.5 mm cross section is commonly applied to foamed materials to reflect their actual geometry. Materials that are thinner or thicker than the standard dimensions may be tested using thickness-adjusted criteria agreed between the client and the laboratory.

Conditioning and Quantity

Prior to testing, specimens are conditioned at 23 ± 2°C and 50 ± 5% relative humidity for a minimum of 88 hours in accordance with ISO 291 or ASTM D618, unless otherwise specified. A minimum of 15 specimens is generally recommended, because the bracketing procedure used to determine the LOI requires multiple ignition trials across a range of oxygen concentrations. Flame-retarded, faced, or coated foams should be tested in their delivered state, and any skins, facings, or composite layers must be documented in the test report.

Typical Foam Materials Tested

• Rigid polyurethane (PUR) and polyisocyanurate (PIR) foams for building panels and pipe insulation
• Expanded polystyrene (EPS) and extruded polystyrene (XPS) insulation boards
• Phenolic foams used in fire-sensitive insulation applications
• Rigid PVC and chlorinated polymer foams
• Polyolefin and specialty structural foams for aerospace, rail, and marine uses

Testing Items

The core deliverable of the test is the limiting oxygen index value, reported as a percentage of oxygen in the O2/N2 mixture by volume, rounded to the nearest 0.1%. Additional reported or derived items typically include:

• LOI value at ambient temperature (23°C) according to ISO 4589-2, GB/T 2406.2, or ASTM D2863
• Burning time and damaged (char) length observations at each oxygen concentration step
• Confirmation of whether specimens ignite and sustain flaming combustion in the tested atmosphere
• Evaluation against customer or regulatory acceptance thresholds, such as an LOI of 26%, 28%, or 30% specified in purchase specifications
• Optional elevated-temperature oxygen index testing per ISO 4589-3 for thermal performance assessment
• Comparative LOI studies for flame-retardant formulation development, including halogen-free, phosphorus-based, nitrogen-based, and intumescent systems

Testing Methods

Apparatus

The test apparatus consists of a heat-resistant borosilicate glass chimney (typically 70–100 mm in diameter and 450–600 mm high) through which a precisely controlled oxygen/nitrogen mixture flows upward; a specimen holder that clamps the specimen vertically at the base of the chimney; a small propane or butane diffusion flame igniter; and calibrated mass flow controllers or rotameters for regulating gas flow. The total gas flow rate is maintained within the range specified by the standard, generally around 40 mm/s linear velocity through the chimney.

Test Procedure

The standard testing sequence proceeds as follows:

• Condition and measure the specimens, then mount one vertically in the holder so that the top of the specimen extends an appropriate distance above the chimney base.
• Set the initial oxygen concentration based on experience or prior data, allow the gas mixture to purge the chimney for at least 30 seconds, and verify flow stability.
• Apply the ignition flame to the top of the specimen for up to 30 seconds, withdrawing and reapplying the flame as specified, until the specimen is well ignited.
• Record whether the specimen burns for 180 seconds or more, or reaches a damaged length of 50 mm; these outcomes define "sustained combustion."
• Adjust the oxygen concentration upward or downward in a bracketing sequence, testing fresh specimens at each concentration until the critical oxygen concentration is bracketed within the required increment.
• Calculate the LOI as 100 × VO2 / (VO2 + VN2), where VO2 and VN2 are the volumetric flow rates of oxygen and nitrogen at the critical balance point.

Factors Affecting Results

Several material and procedural variables influence oxygen index results for rigid foams and must be controlled for reproducibility:

• Density and cell structure: lower-density foams with open cells generally burn more readily than higher-density closed-cell foams.
• Specimen thickness: thicker specimens tend to show higher measured LOI values than thin ones; results are only comparable at identical geometry.
• Flame-retardant loading and distribution, including migration, blooming, or aging of additives.
• Surface condition: skins, facings, coatings, and cut versus molded surfaces alter ignition behavior.
• Conditioning history: moisture content and thermal aging can shift the LOI, particularly for hygroscopic or humidity-sensitive formulations.

Well-managed laboratories using calibrated equipment typically achieve repeatability within approximately 0.5% oxygen and reproducibility within about 1–1.5% oxygen for a given material, consistent with the precision statements of ISO 4589-2 and ASTM D2863.

Applications

Oxygen index testing of rigid plastic foams supports a wide range of engineering, regulatory, and commercial objectives:

• Building insulation: Providing LOI evidence that supports reaction-to-fire classification of PUR/PIR, EPS, XPS, and phenolic insulation boards under systems such as GB 8624 and EN 13501, where LOI data supplement larger-scale single-burning-item and cone calorimeter tests.
• Flame-retardant R&D: Ranking candidate formulations, comparing halogenated versus halogen-free systems, and optimizing additive loading levels during product development of PIR trimerization-modified foams, phosphorus-expanded systems, and graphite-enhanced EPS.
• Quality control: Routine batch verification that incoming resin systems, foams, and fabricated components meet specified minimum LOI thresholds in purchase agreements.
• Transportation: Qualifying rigid foam cores, buoyancy materials, and interior insulation for rail, marine, and aerospace applications where self-extinguishing behavior in air is a baseline requirement.
• Cold chain and pipe insulation: Confirming that rigid foam lagging and panel materials installed around piping, vessels, and refrigerated enclosures meet contractual flame-retardance levels.
• Regulatory submissions: Supplying standardized LOI data within technical dossiers for product certification, customs verification, and buyer declarations.

FAQ

What LOI value is considered acceptable for rigid insulation foams?

There is no universal pass/fail threshold, but in practice many specifications require an LOI of at least 26% for general-purpose flame-retarded foams, while stringent building, transportation, and marine applications frequently specify 28% or higher. Any value above approximately 21% indicates the material will not sustain candle-like combustion in ordinary air.

How does the oxygen index test differ from UL 94?

The LOI test quantifies the minimum oxygen concentration needed to support combustion, producing a numerical result, whereas UL 94 evaluates ignition from a small flame under fixed atmospheric conditions and assigns ratings such as 94V-0, V-2, or HF-1. The two methods are complementary, and flame-retarded foams are often evaluated under both.

Does foam density affect the oxygen index result?

Yes. Density, cell size, and cell openness all influence burning behavior. Results for rigid foams should therefore always be compared at the same specimen dimensions and similar densities; accredited third-party laboratories document these parameters in the test report.

How many specimens are needed, and what is the typical turnaround?

At least 15 conditioned specimens of the standard size are normally required because the bracketing procedure consumes several pieces per concentration step. Testing itself is usually completed within one working day once specimens are conditioned, so overall turnaround is typically three to five working days including conditioning.

Can the oxygen index test alone prove a material is fire-safe?

No. The LOI is a small-scale, ambient-temperature screening method that ranks relative ease of extinction. It does not measure heat release, smoke production, or flame spread, and regulatory fire classification of construction products must rely on the applicable reaction-to-fire test suites. LOI data are best used for material selection, development, and quality control alongside the required classification tests.