Plastic Oxygen Index Combustion Test Analysis: Principles, Methods, and Applications

The oxygen index test is one of the most widely used methods for evaluating the flammability of plastics, expressing flame retardancy as the minimum oxygen concentration required to sustain candle-like combustion. This article provides a professional analysis of plastic oxygen index combustion testing, covering the test principle, specimen requirements, applicable standards such as ISO 4589-2 and ASTM D2863, the test procedure, result interpretation, and typical industrial applications. It serves as a practical reference for design engineers, quality managers, and accredited third-party laboratories involved in flame-retardant material development and compliance verification.

Technical Overview

The oxygen index (OI), also known as 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 flaming combustion of a vertically mounted specimen for a specified time or over a specified length. Because normal ambient air contains approximately 20.95% oxygen, any plastic with an LOI below this level will burn readily in air, while materials with higher oxygen index values tend to self-extinguish once an external ignition source is removed.

The oxygen index test provides a numerical, repeatable, and quantitative measure of combustibility, which makes it particularly valuable for comparing material formulations, verifying the effect of flame-retardant additives, and supporting quality control. Unlike pass/fail small-flame tests, the LOI method yields a continuous result, allowing researchers to rank candidate materials and track incremental improvements in flame-retardant performance.

The test is governed by several internationally recognized standards, including:

• ISO 4589-2 and 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 — Chinese national standard, technically equivalent to ISO 4589-2
• JIS K 7201-2 — Japanese industrial standard based on the same principle
• ISO 4589-3 — Elevated-temperature oxygen index determination (temperature index)

Testing Samples

Specimen geometry has a significant influence on oxygen index results, so samples must be prepared strictly according to the requirements of the selected standard. ISO 4589-2 defines several specimen types (Types I through IV) covering different material forms, while ASTM D2863 specifies its own dimensional ranges. Typical sample categories are described below.

Rigid, Self-Supporting Plastics

For moulded, machined, extruded, or compression-moulded rigid materials such as PP, PE, PA, PC, ABS, PBT, and PVC, the standard rigid specimen is typically 80–150 mm in length, 10 mm in width, and 4 mm in thickness. Edges should be smooth and free from burrs, cracks, and stress marks, as surface defects can alter ignition behaviour. Specimens of other thicknesses may be tested when evaluating thickness dependence, but the actual dimensions must be reported.

Flexible Films and Thin Sheets

Thin polymer films that cannot stand upright on their own are tested as a rolled specimen, in which a strip of film is wound helically around a small rod to produce a self-supporting specimen of the required length. This configuration prevents collapse of the material during the test and ensures that the flame front propagates along a defined path.

Cellular Materials and Foams

Rigid and flexible plastic foams are tested using the special specimen configurations specified in the standard for cellular materials. Density, cell structure, and specimen orientation relative to the foaming direction strongly influence the result, so these parameters must be documented. Low-density foams often show lower oxygen index values than their solid counterparts due to enhanced oxygen access to the combustion zone.

Pipes, Tubing, and Profiles

For pipes, cable insulation, and extruded profiles, specimens are typically cut as longitudinal strips of the dimensions required by the standard, ensuring that the tested geometry represents the wall thickness of the actual product. Where the product geometry cannot be reduced to a standard specimen, close agreement between the laboratory and the client on specimen preparation is essential.

All specimens should be conditioned prior to testing, commonly at 23 ± 2 °C and 50 ± 10% relative humidity for at least 40 hours, unless the material specification states otherwise. A minimum of 15 specimens is generally required for one full determination, and additional spares are recommended.

Testing Items

Typical testing items offered within an oxygen index combustion test program include:

• Ambient-temperature limiting oxygen index (LOI) determination per ISO 4589-2, EN ISO 4589-2, ASTM D2863, GB/T 2406.2, or JIS K 7201-2
• Elevated-temperature oxygen index and temperature index determination per ISO 4589-3
• Thickness-series LOI evaluation to characterize the geometric dependence of flammability
• Comparative formulation screening for halogenated and halogen-free flame-retardant systems
• Verification of flame-retardant masterbatch or compound consistency between production lots
• Observation and recording of after-flame behaviour, afterglow, dripping, melting, and char formation
• Supporting data for pre-assessment of performance in other flammability tests such as UL 94 small-flame testing

Testing Methods

Test Principle and Apparatus

The test is performed in a vertical glass chimney, typically 450–500 mm high with an internal diameter of at least 75 mm. The specimen is clamped vertically in the centre of the column on a specimen holder. High-purity oxygen and nitrogen are metered through calibrated flow controllers into a mixing chamber and introduced at the base of the chimney at a controlled linear velocity, normally 40 ± 10 mm/s. A small propane flame from an igniter is used to ignite the top of the specimen. By systematically varying the oxygen concentration of the inlet gas mixture, the operator identifies the exact balance point at which combustion is just sustained.

Specimen Conditioning and Preparation

Before testing, specimens are conditioned in a standard atmosphere, typically 23 °C and 50% relative humidity for a minimum of 40 hours. Specimens must be marked with reference lines so that the burned or damaged length can be measured after each trial. Any surface contamination, release agents, or machining residues should be removed, as they can noticeably shift the ignition and burning behaviour.

Test Procedure

The general procedure is as follows:

• Set the initial oxygen concentration based on experience or prior data for the material class
• Establish the required total gas flow and allow the system to stabilize
• Mount the conditioned specimen vertically in the chimney
• Apply the ignition flame to the upper end of the specimen in brief on/off cycles until ignition occurs, with the total application time limited (typically not exceeding 30 seconds)
• Start timing when the igniter is removed, and record the burning time and damaged length
• Classify the trial: the specimen is considered to have burned excessively at that concentration if it burns for more than 180 seconds or if the damaged length exceeds 50 mm for standard-length specimens
• Adjust the oxygen concentration using progressively smaller steps, commonly down to 0.2% increments near the balance point, and repeat trials on fresh specimens until the pass/fail boundary is bracketed

Calculation and Reporting

The limiting oxygen index is calculated from the volume flow rates of oxygen and nitrogen at the balance point:

LOI = 100 × V(O₂) / [V(O₂) + V(N₂)]

The result is reported as a volume percentage of oxygen, normally rounded to 0.1%. The test report should also state the standard used, the specimen type and dimensions, the conditioning method, the ignition mode, the gas flow conditions, and any visual observations such as dripping, flaming drips, afterglow, cracking, or swelling. Under repeatability conditions, results within the same laboratory are typically reproducible to within about ±0.5% to ±1% oxygen, while wider scatter may occur between laboratories due to differences in specimen preparation and equipment calibration.

Result Interpretation

Although the LOI is a continuous measurement rather than a pass/fail criterion, the following indicative classification is widely used as an engineering rule of thumb:

• LOI below 21%: the material burns readily in normal air and is considered highly combustible
• LOI between 21% and about 27%: the material is slow-burning and may continue to burn under favourable conditions
• LOI above approximately 27% to 28%: the material is generally regarded as flame-retardant and tends to self-extinguish in air

Many product and industry specifications, for example in railway, marine, electrical equipment, and wire-and-cable sectors, define their own minimum LOI requirements, and those contractual values always take precedence over general guidelines. It should also be noted that the oxygen index generally decreases with increasing temperature, which is why ISO 4589-3 temperature index data are valuable for applications involving sustained heat exposure.

Applications

Oxygen index combustion testing supports a broad range of engineering and regulatory activities, including:

• Development and optimization of flame-retardant polymer formulations, including halogen-free systems based on metal hydroxides, phosphorus compounds, intumescent agents, and nitrogen-based synergists
• Quality control of incoming resins, compounds, and masterbatches, and lot-to-lot consistency verification in production
• Wire and cable insulation and sheathing materials, where minimum LOI values are frequently specified by purchasers
• Electrical and electronic product enclosures, connectors, and internal components requiring combustibility characterization
• Railway, marine, automotive, and aerospace interior materials, where fire safety specifications often reference oxygen index performance
• Building and construction plastics, decorative materials, and textile-reinforced polymer sheets
• Academic research and failure analysis, where LOI data help explain fire incidents and material behaviour
• Compliance programs in which manufacturers engage accredited third-party laboratories to obtain standardized LOI reports for customer submission or certification support

FAQ

What oxygen index value qualifies a plastic as flame retardant?

As a common rule of thumb, plastics with an LOI above approximately 27–28% are considered self-extinguishing in air, while values below 21% indicate the material burns readily at ambient oxygen levels. However, flame-retardant qualification is always defined by the applicable product standard or specification, and a minimum LOI requirement stated in a contract or regulation takes precedence over general guidelines.

How does the oxygen index relate to UL 94 ratings?

The two tests measure different things. The oxygen index quantifies the minimum oxygen concentration needed to sustain burning, producing a numerical value, while UL 94 evaluates after-flame time, dripping, and afterglow under a defined small-flame ignition source and assigns a classification such as V-0, V-1, or V-2. Experience shows a broad correlation—materials with a higher LOI generally perform better in UL 94 testing—but the relationship is not exact, and a good LOI result cannot substitute for an actual UL 94 test when a specific rating is required.

Which factors most often cause LOI results to vary?

Key influences include specimen thickness, geometry and surface condition, moulding parameters and internal stresses, moisture content, dispersion quality of flame-retardant additives, material density in the case of foams, gas purity and flow calibration of the apparatus, ignition technique, and ambient draughts. Standardized conditioning and careful specimen preparation are essential for reproducible results.

Are ISO 4589-2, ASTM D2863, and GB/T 2406.2 results directly comparable?

All three standards are based on the same principle, and results for the same material are usually in good agreement. However, the standards differ in some details of specimen dimensions, criteria, and procedures, so when reporting or comparing data it is important to identify which standard was used and to confirm that both sets of results were generated with equivalent specimen geometry and conditioning.

Can the oxygen index test be performed at elevated temperature?

Yes. ISO 4589-3 extends the ambient-temperature method to elevated temperatures and determines a temperature index, reflecting the fact that most plastics burn more readily when hot. This information is particularly relevant for materials used near heat sources or in enclosed electrical equipment.

How many specimens should be submitted for testing?

A single full LOI determination typically consumes at least 15 specimens, and additional specimens are needed if the initial oxygen concentration estimate is far from the final balance point or if elevated-temperature testing is requested. Clients are advised to consult with the testing laboratory in advance and provide conditioned, well-prepared specimens together with complete material information to ensure efficient and accurate testing.