Plastic Oxygen Index (LOI) Testing: Technical Overview, Methods, and Applications

The plastic oxygen index test (Limiting Oxygen Index, LOI) is a fundamental laboratory method for evaluating the flammability characteristics of plastic materials, determining the minimum oxygen concentration required to sustain candle-like combustion. This test is widely specified by international standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2 for quality control, material selection, and regulatory compliance across industries. This article provides a comprehensive technical overview of the oxygen index experiment, including testing principles, sample requirements, procedures, applications, and frequently asked questions.

Technical Overview

The oxygen index experiment, also known as the Limiting Oxygen Index (LOI) test, measures the minimum concentration of oxygen in a flowing mixture of oxygen and nitrogen that will just support the flaming combustion of a vertically mounted plastic specimen. Since normal ambient air contains approximately 20.9% oxygen, the LOI value provides an intuitive benchmark for flammability assessment: materials with an LOI below 21% will generally continue to burn in air after ignition, while materials with higher LOI values tend to self-extinguish under normal atmospheric conditions.

During the test, a small test specimen is clamped vertically in a glass chimney and ignited at its top with a small gas flame. The oxygen concentration in the rising gas mixture is progressively adjusted until the critical concentration is found at which the specimen either burns for a defined period or over a defined length. This critical value, expressed as a percentage by volume, is reported as the oxygen index of the material.

The test is governed by several internationally recognized standards, and accredited third-party laboratories typically perform the experiment in strict accordance with one of the following methods:

• ISO 4589-2 / EN ISO 4589-2 — Determination of burning behaviour by oxygen index, 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 aligned with ISO 4589-2
• ISO 4589-3 — Oxygen index test at elevated temperatures (temperature index)
• NES 714 — Determination of the oxygen index of small samples of polymeric materials

Because the test is reproducible, relatively inexpensive, and requires only a small quantity of material, it is one of the most widely used screening tools for flame-retardant plastics in R&D, incoming material inspection, and certification programs.

Testing Samples

Rigid Plastic Specimens

For self-supporting rigid materials such as ABS, PC, PPO, PVC, epoxy laminates, or thermoset moldings, standard specimens are typically rectangular bars of 80–150 mm in length, 10 mm in width, and 4–10 mm in thickness, depending on the specimen type selected in ISO 4589-2. ASTM D2863 describes a comparable geometry of 70–150 mm in length, 6.5 mm in width, and 3 mm in thickness. Specimens must be cut cleanly, free of cracks, burrs, and sharp edges, and surfaces should be smooth and free from residue or contamination.

Flexible Plastic Specimens

For flexible materials such as films, sheets, soft PVC, and rubber compounds that cannot stand vertically on their own, specimens are rolled around a mandrel and secured to form a self-supporting cylinder, typically using a specimen of approximately 140 mm in length and 52 mm in width. The rolled specimen is then mounted in the holder in the same orientation as rigid bars so that the flame application and burn-length criteria remain consistent.

Sample Conditioning and Quantity

Prior to testing, specimens should be conditioned in a controlled atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours (88 hours for certain hygroscopic materials such as polyamides), unless otherwise agreed. Because the bracketing procedure requires testing at several oxygen concentrations, laboratories typically request 15–20 specimens per material to complete a valid determination. Testing different thicknesses separately is strongly recommended, since LOI values are known to vary with specimen thickness.

Testing Items

The core deliverable of the oxygen index experiment is the Limiting Oxygen Index value itself; however, accredited third-party laboratories often report a complete set of supporting information to ensure full traceability and comparability:

• Oxygen index (LOI) value at ambient temperature, expressed as % O2 by volume, reported to the nearest 0.1% or 0.2% depending on the standard
• Specimen geometry, type, thickness, and orientation
• Observed burning behavior, including afterflame time, afterglow, dripping, and whether flaming droplets ignite cotton (if assessed)
• Gas flow conditions, including total volumetric flow rate and linear gas velocity in the chimney
• Conditioning atmosphere and duration before testing
• Optional temperature index (TI) determination per ISO 4589-3, identifying the temperature at which the LOI drops to 20.9%
• Comparative LOI evaluation of different formulations, thicknesses, or processing conditions for R&D purposes

Testing Methods

Test Apparatus

A standard oxygen index apparatus consists of a heat-resistant glass chimney (typically at least 500 mm in height with an internal diameter of roughly 70–100 mm) mounted on a base through which the oxygen/nitrogen mixture is introduced and distributed uniformly through a bed of glass beads. A specimen clamp holds the bar vertically in the center of the chimney. High-accuracy flow meters or mass flow controllers regulate the oxygen and nitrogen streams, and an oxygen analyzer is often used to verify the actual mixture concentration at the specimen position. A small propane or butane burner with a regulated flame serves as the ignition source, and a timer and marked gauge (50 mm burn-length reference) are used to evaluate the burning criterion.

Test Procedure

The experiment follows a well-defined sequence:

• Condition the specimen, measure its dimensions, and mark the 50 mm burn-length reference from the top of the bar.
• Mount the specimen vertically in the clamp with the top of the specimen a defined distance below the top rim of the chimney.
• Establish the oxygen/nitrogen flow at the required total flow rate, producing a linear velocity in the chimney of approximately 40 mm/s, and allow the system to purge and stabilize.
• Apply the ignition flame to the top of the specimen for the prescribed time (typically up to 5 seconds, with brief reapplications if ignition is not achieved) until the specimen is well ignited.
• Record whether the specimen burns more than 50 mm in length or continues burning longer than the specified period after removal of the ignition source (commonly 180 seconds).
• Using the bracketing (index search) method, adjust the oxygen concentration upward or downward and test fresh specimens until the critical concentration bracketing the burning/extinguishing transition is determined within the required tolerance.
• Report the oxygen index together with all relevant test conditions.

Data Interpretation and Classification

Although standards do not impose a single pass/fail threshold, the polymer industry commonly interprets LOI results as follows:

• LOI below 21% — readily flammable; the material continues to burn in normal air
• LOI of 21–24% — relatively slow burning, but still combustible
• LOI of 24–27% — moderately flame retardant, often self-extinguishing in still air
• LOI above 27% — generally classified as flame-retardant or self-extinguishing in air
• LOI above 32% — high flame retardancy, typical of specially engineered compounds

It should be noted that LOI is a small-scale laboratory measurement that describes ease of ignition and continued burning in a controlled atmosphere; it does not by itself represent the behavior of materials in a fully developed fire and is therefore frequently combined with other tests such as UL 94 flame ratings, cone calorimetry, or glow-wire testing in a comprehensive fire-safety assessment.

Applications

Oxygen index testing supports a broad range of industrial and regulatory applications:

• Polymer research and development — comparing flame-retardant formulations, optimizing additive loadings, and evaluating the effect of fillers, plasticizers, and reinforcing agents on combustibility
• Wire and cable industry — quality control of insulation and sheathing compounds for PVC, polyolefins, and halogen-free flame-retardant materials
• Electrical and electronic products — screening engineering plastics used in connectors, enclosures, and structural parts
• Railway and mass transit — supporting fire-safety compliance documentation where LOI values are referenced within material qualification programs
• Construction products — evaluating decorative materials, pipes, and fittings, often alongside standardized reaction-to-fire classifications
• Aerospace and shipbuilding — material qualification and batch acceptance testing for interior components
• Textiles and coated fabrics — screening flame-retardant finishes on flexible polymeric sheet materials using rolled specimens
• Incoming inspection and supplier verification — confirming that received resins or compounds meet contractual LOI specifications

FAQ

What does a higher LOI value mean?

A higher oxygen index indicates that a greater oxygen concentration is required to sustain combustion, which means the material is less flammable. For example, ordinary polyethylene has an LOI of roughly 17–18% and burns readily, whereas well-formulated flame-retardant PVC compounds can exceed 35%.

How is LOI different from the UL 94 test?

LOI quantifies the minimum oxygen concentration needed to support burning and produces a numerical result, while UL 94 grades the burning behavior of a specimen ignited in ambient air (for example V-0, V-2, 5VA) based on afterflame time and dripping. The two methods are complementary and are frequently specified together in material datasheets.

Does specimen thickness affect the result?

Yes. LOI values generally increase with specimen thickness because thicker sections act as a heat sink during combustion. For this reason, the tested thickness must always be stated alongside the reported LOI value, and comparisons between materials should only be made at the same thickness.

Can the test be performed at elevated temperatures?

Yes. ISO 4589-3 defines the elevated-temperature procedure, in which the gas mixture and specimen are preheated inside a heated chimney. This produces a temperature index — the temperature at which the LOI falls to 20.9% — which is valuable for materials used in warm operating environments.

How many specimens are required for one determination?

Because the bracketing procedure requires testing at several oxygen concentrations until the transition point is confirmed, laboratories typically need 15–20 specimens per material condition. Additional specimens may be required if the material shows borderline or erratic burning behavior.

Why is LOI testing often required alongside other fire tests?

LOI is an excellent, highly reproducible screening and quality-control tool, but it reflects combustion in a controlled, oxygen-enriched upward flow rather than a real fire scenario. Combining LOI with methods such as UL 94, glow-wire ignition, and cone calorimetry provides a more complete picture of a material's ignition resistance, flame spread, and heat release characteristics.