Oxygen Index Testing: What It Is, Method & Flame Retardancy Guide
What Is the Oxygen Index Test?
The Oxygen Index test measures the minimum concentration of oxygen in an oxygen-nitrogen mixture required to sustain the combustion of a vertically oriented test specimen. The result is expressed as the Limiting Oxygen Index (LOI) — the percentage of oxygen by volume in the gas mixture at which the material just barely sustains burning under the defined test conditions.
The standard atmospheric oxygen concentration is approximately 21%. Materials with an LOI greater than 21% require an oxygen-enriched environment to sustain combustion and are therefore classified as self-extinguishing in normal air. Materials with an LOI below 21% burn readily in ambient air and are considered more flammable.
The test is standardized by:
- ASTM D2863 — Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics
- ISO 4589-2 — Plastics — Determination of Burning Behaviour by Oxygen Index — Part 2: Ambient-Temperature Test
How the Oxygen Index Test Is Performed
The test apparatus consists of a vertical glass combustion tube through which a controlled mixture of oxygen and nitrogen is passed from the bottom. A standardized test specimen (typically 80–150 mm long, 10 mm wide, and 4 mm thick per ASTM D2863) is mounted vertically in the center of the tube.
The specimen is ignited at the top (top ignition) or at the end (end ignition), depending on the standard and material type. The oxygen concentration in the gas mixture is then adjusted, increased or decreased, until the minimum concentration that allows the specimen to burn for at least 3 minutes (or consume more than 50 mm of its length) is found. This concentration, expressed as a percentage, is the Limiting Oxygen Index.
Multiple tests are performed at different oxygen concentrations using a step-wise bracketing method (ASTM D2863 uses Staircase or Dixon methods) to converge on the LOI value with statistical confidence.
Interpreting LOI Values
LOI Range | Flammability Classification |
< 21% | Burns readily in air (highly flammable) |
21–26% | Burns in air; marginally self-extinguishing |
26–34% | Difficult to ignite; self-extinguishing in air |
> 34% | Highly flame-retardant; difficult to sustain combustion |
Standard untreated polypropylene has an LOI of approximately 17–18%; polyethylene is around 17–18%; polystyrene is around 18%. Flame-retardant-formulated materials can achieve LOI values above 30–35%, and high-performance flame-retardant polymers such as PTFE can exceed 95%.
Flame Retardant Formulation and LOI Testing
LOI testing is a primary tool for the development of flame-retardant compounds. Formulators adding halogenated flame retardants, phosphorus-based systems, aluminum hydroxide, or intumescent agents to polymers use LOI measurements as a rapid, quantitative feedback metric for formulation optimization. The relationship between flame retardant loading level and LOI value guides efficient formulation development.
LOI vs. UL 94 Flammability Classification
LOI and UL 94 are complementary, not equivalent, flammability tests. UL 94 classifies materials based on the behavior of a burning specimen in specific orientations (vertical or horizontal), while LOI provides a quantitative, continuous-scale measure of combustion sustainability. Both are needed for comprehensive flammability characterization:
- UL 94 V-0 classification indicates superior flame resistance in vertical specimen testing
- LOI > 28% is often (but not always) associated with UL 94 V-0 performance, but exceptions exist depending on material system and geometry
Industry Applications of Oxygen Index Testing
Electronics and Electrical: Polymer housings, connector bodies, PCB substrates, and wire insulation must meet LOI requirements in addition to UL 94 classifications for electrical safety certification.
Aerospace: Aircraft interior panels, seat materials, and cabin component polymers are tested for LOI as part of FAR 25.853 and other airworthiness flammability requirements.
Automotive: Interior trim materials, wire harness insulation, and under-hood polymer components are LOI-tested to meet vehicle safety and insurance standards.
Building and Construction: Polymer materials used in building facades, insulation, roofing, and structural applications are LOI-tested to meet building code flammability requirements.
Conclusion
The Oxygen Index test is a critical method for evaluating the flammability of materials by determining the minimum oxygen concentration required to sustain combustion. By providing a quantitative measure through the Limiting Oxygen Index (LOI), it helps manufacturers assess fire resistance, optimize flame-retardant formulations, and ensure compliance with safety standards across industries such as electronics, automotive, aerospace, and construction.
Infinita Lab’s Oxygen Index Testing Services
Infinita Lab provides ASTM D2863 and ISO 4589-2 Limiting Oxygen Index testing through its nationwide accredited laboratory network. Testing covers all standard specimen geometries and material types — thermoplastics, thermosets, rubber compounds, foams, films, and fiber-reinforced composites. Results include LOI value, statistical uncertainty, and comparative assessment against industry requirements. Expert flammability engineers provide interpretation and guidance on the implications of flame-retardant formulations
Contact Infinita Lab: (888) 878-3090 | www.infinitalab.com
Frequently Asked Questions (FAQs)
What is the Limiting Oxygen Index (LOI) test? The LOI test measures the minimum oxygen concentration (in an O₂/N₂ mixture) required to sustain combustion of a vertically oriented plastic specimen. Materials with LOI > 21% self-extinguish in normal air; lower LOI materials burn readily.
What standards govern oxygen index testing? ASTM D2863 governs the test in the United States. ISO 4589-2 is the corresponding international standard. Both define specimen geometry, ignition method, test procedure, and LOI calculation methodology.
How does LOI relate to UL 94 flammability classification? LOI and UL 94 are complementary measures. LOI provides a quantitative, continuous-scale value; UL 94 provides a categorical classification (V-0, V-1, V-2, HB) based on specific specimen burning behaviors. Both are typically required for comprehensive flammability characterization.
What materials can be evaluated by oxygen index testing? ASTM D2863 is applicable to thermoplastics, thermosets, rubber compounds, cellular foams, films, and fiber-reinforced composites. The test is not limited to plastics — it is also used for textile fibers, coatings, and other combustible materials.
How is LOI testing used in flame retardant compound development? LOI measurements provide rapid, quantitative feedback on the effectiveness of flame retardant additives in polymer formulations — allowing formulators to optimize retardant type and loading level against target LOI values and downstream UL 94 or building code requirements.
