The Limiting Oxygen Index (LOI) test is a fundamental small-scale flammability evaluation used to determine the minimum oxygen concentration required to sustain candle-like combustion of packaging foam materials. This article provides a comprehensive technical overview of LOI testing for packaging foams, covering the testing principle, applicable standards such as ASTM D2863, ISO 4589-2, and GB/T 2406.2, specimen requirements, test procedures, result interpretation, and typical industrial applications.
Technical Overview
The Limiting Oxygen Index (LOI), also known as the Oxygen Index (OI), 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 flaming combustion of a vertically mounted specimen. Since ambient air contains approximately 20.9% oxygen, a foam material with an LOI value above approximately 21% will tend to self-extinguish after removal of the ignition source under normal atmospheric conditions.
In the LOI experiment, a conditioned foam specimen is clamped vertically in a glass chimney, and a precisely controlled oxygen/nitrogen mixture is passed upward through the column. The top of the specimen is ignited with a small diffusion flame, and the gas composition is adjusted incrementally until the critical concentration at which burning is just sustained is bracketed. The LOI value is calculated as follows:
- LOI = 100 × [O₂] / ([O₂] + [N₂]), where the gas volumes are expressed at the same temperature and pressure
Higher LOI values indicate greater flame retardancy. As a general guideline for polymer foams, materials with an LOI below 21% are considered easily ignitable, values between 22% and 26% indicate moderately resistant materials, and values of 27% or above are typically classified as flame-retardant grades suitable for demanding packaging applications.
The LOI method for plastics is standardized internationally, and the commonly referenced standards 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 (Oxygen Index)
- GB/T 2406.2 — Chinese national standard, technically aligned with ISO 4589-2
- JIS K 7201-2 — Japanese industrial standard for oxygen index of plastics
Testing Samples
Expanded Polystyrene (EPS) Foam
EPS is widely used in electronics and appliance packaging. Because unmodified polystyrene has a relatively low LOI (typically around 18%), flame-retardant grades containing additives such as brominated compounds with synergists are commonly evaluated. Specimens are cut from molded foam blocks with careful control of density and cell structure, as both parameters measurably influence the LOI result.
Expanded Polyethylene (EPE) and Expanded Polypropylene (EPP) Foam
These closed-cell polyolefin foams are used for cushioning, corner protection, and insert trays in export packaging. Cross-linked PE foams and EPP foams used in automotive and logistics packaging are frequently specified with a minimum LOI requirement, commonly 26% to 30%, particularly for applications involving air freight or rail transport regulations.
Flexible Polyurethane (PU) Foam
PU foams used in case inserts and cushioning packaging are highly combustible in their untreated form. LOI testing is applied to melamine-treated or halogen/phosphorus flame-retardant PU foams to verify compliance with flame-retardant specifications, with target values often set at 26% or higher depending on the end use.
Ethylene-Vinyl Acetate (EVA) Foam
EVA foams are used in precision instrument packaging, tool case inserts, and medical device packaging. Specimens are typically die-cut from sheet stock, and density along with vinyl acetate content must be reported as it affects the measured oxygen index.
Specimen Dimensions and Conditioning
Standard specimen geometry for rigid and semi-rigid foams generally follows the requirements of ISO 4589-2 and ASTM D2863:
- Length: 80 mm to 150 mm (100 mm to 150 mm is typical for foams), measured along the burning direction
- Width: 10 mm to 20 mm depending on the material category
- Thickness: 10 mm to 20 mm for cellular materials; sheet products may be tested in multiple layers where permitted
- Conditioning: minimum of 88 hours at 23 ± 2 °C and 50 ± 5% relative humidity prior to testing, unless otherwise specified
- Specimens must be free of skin, mold defects, densified edges, and cut burrs, which can significantly alter burning behavior
Testing Items
A typical LOI test program for packaging foam includes the following evaluation items:
- Limiting Oxygen Index value (volume % O₂) determined at ambient temperature
- Verification of compliance with specified minimum LOI requirements (for example, ≥26% or ≥28%)
- Observation of afterflame time and afterglow behavior after removal of the ignition source
- Assessment of flaming drips, molten collapse, or shrinkage away from the flame during testing
- Supporting properties that influence flammability interpretation, such as apparent density and compression set, when required by the specification
Testing Methods
Apparatus
The test is performed using an oxygen index apparatus consisting of a heat-resistant glass chimney of specified internal diameter, a specimen holder that supports the specimen vertically in the center of the chimney, precision flow meters or mass flow controllers for oxygen and nitrogen, a gas mixing chamber, and a small propane or butane diffusion flame igniter. The gas supply must be of high purity, and the system must be capable of controlling the total volumetric flow rate within the tolerance specified by the standard.
Test Procedure
The general procedure in accordance with ISO 4589-2 / ASTM D2863 / GB/T 2406.2 is as follows:
- Mount the conditioned specimen vertically in the holder so that at least the specified exposed length projects above the top of the chimney
- Purge the chimney with the oxygen/nitrogen mixture at the selected initial concentration for at least 30 seconds to establish a stable atmosphere
- Apply the ignition flame to the top of the specimen for up to 30 seconds, withdrawing and reapplying as required, until the specimen is ignited across its full width
- Observe the burning behavior and record whether combustion meets or fails the criterion for the particular specimen geometry — typically burning extent of 50 mm as marked on the specimen, or a specified burning time
- Adjust the oxygen concentration upward or downward in increments and repeat until the critical concentration is bracketed, then refine the interval using the reduced increment method
- Calculate the LOI from the final oxygen concentration series, commonly using the statistical estimation method described in the standard
Result Expression and Reporting
The test report should include the material identification, foam type and density, specimen dimensions, conditioning details, the standard applied, the measured LOI value to one decimal place, the number of specimens tested, and observations of burning behavior such as dripping, glowing, or collapse. If a material shows inconsistent results due to melt dripping or severe shrinkage, this should be noted, as such behavior affects the repeatability characteristic of cellular materials.
Applications
LOI testing of packaging foams supports a wide range of industrial and regulatory requirements:
- Flame-retardant verification for EPS packaging used in consumer electronics, appliances, and audio-visual equipment export shipments
- Compliance demonstration for air cargo and logistics specifications that require non-combustible or flame-retardant cushioning materials
- Quality control of flame-retardant masterbatch dosing during foam extrusion and molding production
- Material qualification for military, aerospace, and rail transport packaging programs where small-scale flammability screening is mandated
- Supporting data for fire performance classification of foam materials, such as classification under GB 8624 for building applications where the same foam grade is used
- Research and development screening of halogen-free flame retardant formulations for polyolefin and polyurethane foams
- Supplier verification and incoming material inspection programs for manufacturers of protective packaging systems
FAQ
What LOI value is considered flame retardant for packaging foam?
While thresholds depend on the specification, packaging foams with an LOI of 27% or greater are generally regarded as flame-retardant grades. Many procurement specifications for electronics and transport packaging set a minimum requirement of 26% to 30%. Any foam with an LOI above 21% will typically not continue burning in normal air after the ignition source is removed.
Which standard should be selected for the test: ASTM D2863 or ISO 4589-2?
The two methods are technically similar and produce comparable results for most materials, but they differ in specimen geometry details, flow control tolerances, and calculation conventions. The choice is usually dictated by the end market or the customer specification: ISO 4589-2 and GB/T 2406.2 are common in international and Chinese contexts, while ASTM D2863 is frequently referenced in North American specifications. Testing should be performed by accredited third-party laboratories familiar with cellular material behavior.
How is LOI different from other flammability tests such as UL 94?
LOI is a quantitative measure that reports a numerical oxygen concentration threshold, making it well suited to comparing formulations and tracking flame-retardant loading. Tests such as UL 94 (for example, the V-0, V-1, or V-2 classifications) assess the ability of a specimen to extinguish after defined flame applications and evaluate flaming drips. The two methods are complementary, and many specifications require both for full material qualification.
Does foam density affect the LOI result?
Yes. For cellular materials, the apparent density, cell size, and cell structure significantly influence the measured oxygen index. Lower-density foams generally burn more readily due to greater surface area and heat feedback characteristics. For this reason, the density of the tested foam should always be reported alongside the LOI value, and comparisons should only be made between materials of similar density and structure.
How many specimens are required for one determination?
A complete LOI determination typically requires between 10 and 20 specimens, since the bracketing and refinement procedure involves repeated testing at incremental oxygen concentrations. Specimens that exhibit non-standard behavior such as collapse or full-width melt burning may require additional samples to obtain a statistically valid result.