Comparative Analysis of the Limiting Oxygen Index of Plastics: Test Methods, Data Interpretation, an

The limiting oxygen index (LOI) test is one of the most widely used bench-scale methods for evaluating the flammability of plastic materials, quantifying the minimum oxygen concentration required to sustain candle-like combustion. This article presents a comprehensive comparative analysis experiment on the oxygen index of common plastics, covering test principles, sample preparation, standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, typical LOI data, and practical applications in material selection and flame-retardant development.

Technical Overview

The limiting oxygen index (LOI), also known as the oxygen index (OI) or critical oxygen index, 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 plastic specimen. Since ambient air contains approximately 20.9% oxygen, the LOI value provides an intuitive benchmark for judging whether a plastic will continue to burn in normal atmospheric conditions.

The oxygen index is calculated using the following relationship:

LOI (%) = [O2] / ([O2] + [N2]) × 100, where [O2] and [N2] are the volumetric flow rates of oxygen and nitrogen at the critical burning condition.

A higher LOI value indicates greater difficulty of ignition and better flame-retardant performance. A common classification convention is summarized below:

• LOI < 21%: readily flammable, the material will continue to burn in normal air after ignition.
• LOI between 21% and 27%: slowly burning or partially self-extinguishing, generally referred to as combustible materials.
• LOI > 27%: considered flame-retardant or self-extinguishing in ambient air.
• LOI > 40%: high-performance flame-resistant materials suitable for demanding applications.

It should be emphasized that LOI is a small-scale laboratory screening result and does not by itself describe the fire behavior of a material in a real large-scale fire. Therefore, LOI data are typically complemented by other flammability tests such as UL 94 vertical and horizontal burning tests, glow-wire tests (IEC 60695-2 series), and cone calorimetry according to ISO 5660.

Testing Samples

Specimen Requirements

For a valid comparative analysis, all specimens must be prepared with consistent geometry and conditioning so that the only variable among groups is the material itself. Typical requirements based on ISO 4589-2, ASTM D2863, and GB/T 2406.2 include:

• Dimensions: 80 mm to 150 mm in length, 10 mm to 15 mm in width, and 4 mm to 10 mm in thickness, depending on the selected test configuration.
• Surface condition: specimens shall be smooth, free of bubbles, cracks, and burrs, with edges free from flash that could influence flame propagation.
• Quantity: at least 15 specimens per material group to allow the incremental oxygen adjustment procedure to converge on the critical concentration.
• Conditioning: 23 °C ± 2 °C and 50% ± 5% relative humidity for a minimum of 88 hours (shorter periods such as 24 hours may be used for hydraulically inert materials if justified).
• Marking: a reference line is drawn 50 mm from the top edge of the specimen to define the burning criterion.

Materials Selected for Comparative Analysis

A representative comparative experiment should cover both commodity thermoplastics with poor intrinsic flame resistance and engineering plastics or flame-retardant grades with improved performance. A typical sample matrix includes:

• Polyolefins: polyethylene (PE), polypropylene (PP), typical LOI approximately 17% to 19%.
• Styrenics: polystyrene (PS), acrylonitrile butadiene styrene (ABS), typical LOI approximately 18% to 21%.
• Engineering plastics: polycarbonate (PC), polyamide 66 (PA66), polyoxymethylene (POM), polyethylene terephthalate (PET), typical LOI approximately 20% to 28%.
• Intrinsically flame-resistant polymers: unplasticized polyvinyl chloride (PVC-U) with LOI around 45%, and polytetrafluoroethylene (PTFE) with LOI exceeding 95%.
• Flame-retardant modified grades: halogen-free flame-retardant PP, brominated ABS, phosphorus-based FR-PC/ABS blends, and mineral-filled compounds.

Testing Items

The core deliverable of the comparative oxygen index experiment is the room-temperature limiting oxygen index of each material, determined in accordance with the selected standard. However, a complete comparative program usually includes the following items:

• Room-temperature LOI: the fundamental index measured at 23 °C, used to rank materials and verify compliance with specification limits.
• Temperature index: the oxygen index measured at elevated ambient temperatures in a heated enclosure according to ISO 4589-3, which reveals how flame retardancy degrades as temperature rises.
• Neat resin versus flame-retardant grade comparison: quantifying the LOI improvement delivered by a flame-retardant package, for example the shift of PP from roughly 18% to 30% or higher after modification.
• Thickness dependence study: evaluating LOI at different specimen thicknesses, since thicker sections generally burn more slowly and often show different indices than thin sections.
• Burning behavior observation: recording afterflame time, dripping, sooting, and char formation, which provide insight into the flame-retardant mechanism of each material.

Testing Methods

Apparatus

The test is performed on an oxygen index apparatus comprising a heat-resistant glass chimney of specified dimensions, a specimen clamp that supports the specimen vertically, a precision gas mixing and flow control system for oxygen and nitrogen, an oxygen analyzer for verifying the mixed gas concentration, and an ignition source consisting of a small premixed propane or hydrogen flame burner applied to the top of the specimen.

Standard Test Procedures

The experiment follows one of the internationally recognized methods, which are technically equivalent in principle:

• 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, Determination of burning behaviour by oxygen index — Part 2: Ambient-temperature test, harmonized with ISO 4589-2.

The general procedure is as follows:

• Mount the conditioned specimen vertically in the clamp so that the top of the specimen is at a defined height below the chimney outlet.
• Purge the chimney with the oxygen-nitrogen mixture at the specified total flow rate 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, removing and reapplying the burner as necessary until the specimen is ignited.
• Observe whether burning reaches the 50 mm reference mark within 180 seconds, or whether the afterflame persists beyond 180 seconds.
• Adjust the oxygen concentration upward or downward following the standard's step increment protocol, testing a fresh specimen at each concentration, until the critical concentration bracketing sustained combustion is identified.
• Calculate the LOI from the final oxygen and nitrogen flow rates and report the result to one decimal place, together with the burning phenomenon description.

Design of the Comparative Experiment

To ensure the comparison is scientifically valid, the following controls must be applied throughout the campaign:

• Use specimens of identical dimensions, orientation, and molding conditions for all materials.
• Perform all tests in the same laboratory environment with the same apparatus, gas sources, and operator to eliminate systematic bias.
• Verify gas mixing accuracy with the calibrated oxygen analyzer before and after each material group.
• Randomize the test sequence among materials to prevent drift in ambient conditions from skewing the ranking.
• Repeat the LOI determination on at least two independent specimen sets per material and report the mean and range to demonstrate repeatability.

Typical Comparative Results and Interpretation

Consistent with published literature and industry data sheets, the comparative experiment generally produces the following ranking: PTFE (LOI > 95%) and PVC (approximately 45%) occupy the top positions, followed by polyimide (approximately 36% to 40%), polyphenylene sulfide (approximately 40% to 46%), flame-retardant engineering grades (30% to 38%), polycarbonate and polyamide (24% to 28%), and finally the commodity resins PE, PP, PS, ABS, and PMMA clustered at 17% to 21%. The results clearly demonstrate that molecular structure, char-forming tendency, halogen content, and the presence of phosphorus or mineral flame retardants are the dominant factors controlling the oxygen index.

Applications

Comparative oxygen index data support a wide range of engineering and regulatory decisions across industries:

• Material selection for electrical and electronic products, where enclosures, connectors, and insulation parts must meet minimum flame-retardancy requirements verified by accredited third-party laboratories together with UL 94 or IEC 60695 based tests.
• Wire and cable industry: screening of insulation and sheathing compounds for improved fire performance in buildings and infrastructure.
• Automotive engineering: evaluation of interior trim, battery module components, and charging system parts, often combined with standards such as GB 8410 and FMVSS 302 for horizontal burning behavior.
• Railway and mass transit: contribution to fire safety assessment under EN 45545-2, where LOI is one of several accepted measurement parameters.
• Research and development: rapid iteration of halogen-free flame-retardant formulations by comparing LOI increments achieved with different loading levels of phosphorus, nitrogen, or mineral additives.
• Incoming quality control: verification that delivered resin lots meet the LOI specification agreed between suppliers and converters.

FAQ

What is considered a good oxygen index value for plastics?

As a practical guideline, plastics with LOI above 27% are regarded as flame-retardant in ambient air, while values above 32% are typically required by demanding applications such as rail vehicles, mining equipment, and aircraft interiors. Any material with LOI below 21% will continue to burn freely in ordinary air once ignited.

How does LOI relate to the UL 94 test?

The two methods measure different aspects of flammability. LOI quantifies the sensitivity of combustion to oxygen concentration, whereas UL 94 evaluates ignition, flame spread rate, afterflame time, and dripping under a specific small-flame exposure. A high LOI generally correlates with better UL 94 ratings, but the correlation is not absolute, so both tests should be performed for full material characterization.

Why do different thicknesses give different LOI results?

Thicker specimens conduct heat away from the combustion zone more effectively and present a smaller surface-to-mass ratio, which typically raises the measured oxygen index. For this reason, the specimen thickness must always be reported together with the LOI value, and comparisons are valid only between specimens of the same geometry.

What accuracy can be expected from the test?

With properly calibrated flowmeters and a verified oxygen analyzer, the repeatability of LOI within a single laboratory is normally within ±0.5% to ±1.0% oxygen. Interlaboratory reproducibility is somewhat wider, which is why comparative studies should be conducted on the same equipment under controlled conditions.

Can LOI results alone prove fire safety compliance?

No. LOI is a screening and characterization tool. Regulatory acceptance usually requires a combination of tests, such as UL 94 flame ratings, glow-wire ignition temperature, smoke density, and toxicity assessments, depending on the end-use standard and the product certification scheme applied.