Oxygen index (Limiting Oxygen Index, LOI) testing is a widely recognized qualitative method for evaluating the combustion behavior and flame-retardant performance of plastic materials. By determining the minimum oxygen concentration required to sustain flaming combustion, manufacturers can verify material safety, optimize flame-retardant formulations, and demonstrate compliance with ISO, ASTM, and GB standards. This article explains the technical principles, sample requirements, testing procedures, and typical applications of oxygen index qualitative analysis for plastics.
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. Since normal ambient air contains approximately 20.9% oxygen, the LOI value provides an immediate qualitative indication of how a plastic will behave in air: materials with an LOI below 21% burn readily, while materials with higher values burn with increasing difficulty or self-extinguish.
The LOI is calculated as follows:
- LOI (%) = [O2 / (O2 + N2)] × 100, determined at the critical oxygen concentration
For qualitative interpretation, the following classification is commonly applied in engineering practice:
- LOI < 21%: readily flammable in normal air (typical of PE, PP, PS, and most hydrocarbon-based polymers)
- LOI 21% to 27%: slow-burning or self-extinguishing tendency (typical of PC, PA, and modified compounds)
- LOI > 27%: generally classified as flame-retardant or self-extinguishing (typical of PVC, phenolics, and halogen- or phosphorus-modified compounds)
- LOI > 40%: highly flame-resistant materials, including PTFE and certain engineering fluoropolymers
Because each polymer family exhibits a characteristic LOI range, oxygen index qualitative analysis is also used as a screening tool to verify polymer identity, confirm the presence and effectiveness of flame-retardant additives, detect batch-to-batch variation, and support root-cause investigations following fire incidents. Accredited third-party laboratories routinely perform this test in accordance with ISO, ASTM, and GB standards.
Testing Samples
Rigid and Semi-Rigid Thermoplastics
Typical specimen dimensions are 80 mm to 150 mm in length, 10 mm in width, and 4 mm in thickness, in accordance with ISO 4589-2 and GB/T 2406.2. Specimens may be injection-molded, compression-molded, or cut from finished sheets or parts. Edges should be smooth and free of burrs, cracks, or stress concentrations that could influence ignition behavior.
Thermosetting and Molded Materials
Phenolic, epoxy, unsaturated polyester, and melamine-based materials are tested using similar specimen geometry. Cure state, filler content, and reinforcement type (such as glass fiber) should be documented, as these factors measurably influence the LOI result.
Films and Flexible Sheets
Thin films cannot be tested free-standing and require special specimen preparation, such as winding the film onto a rod or supporting it in a frame as described in the relevant standard annexes. Flexible materials such as PVC film, polyolefin films, and cable insulation compounds fall into this category.
Foamed and Cellular Plastics
Rigid and flexible foams are tested with larger cross-sections, typically 10 mm to 15 mm in thickness, because cell structure and density strongly affect combustion behavior. Specimens must be cut cleanly without collapsing or sealing the cells.
Sample Conditioning Requirements
Unless otherwise specified, specimens should be conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 88 hours prior to testing. A minimum of 15 to 20 specimens is generally required for a full quantitative LOI determination, while a reduced number may be used for qualitative pass/fail screening at a specified oxygen concentration.
Testing Items
Oxygen index qualitative analysis typically covers the following evaluation items:
- Limiting Oxygen Index (LOI) at ambient temperature (23 ± 2 °C)
- Oxygen index at elevated temperatures in accordance with ISO 4589-3
- Qualitative flammability classification of the material
- Comparison of measured LOI against datasheet values, material specifications, or contractual requirements
- Observation of combustion behavior, including melting, dripping, char formation, afterflame time, and smoke generation
- Verification of flame-retardant modification or identification of unknown polymer samples based on characteristic LOI ranges
- Batch-to-batch consistency assessment for incoming material inspection
Testing Methods
ISO 4589-2 Method
ISO 4589-2 specifies the determination of the burning behavior of vertical specimens by the oxygen index method at ambient temperature. The specimen is clamped vertically in a heat-resistant glass chimney, a controlled mixture of oxygen and nitrogen flows upward through the chimney, and the top of the specimen is ignited with a small propane flame. The test determines whether the specimen burns for more than 3 minutes or beyond a 50 mm burn mark after removal of the igniter.
ASTM D2863 Method
ASTM D2863 is the North American equivalent procedure and follows the same fundamental principle: the minimum oxygen concentration is determined that supports candle-like burning of a vertical specimen for a defined period or over a defined length. Results from ASTM D2863 and ISO 4589-2 are generally comparable, though minor differences in apparatus and acceptance criteria may exist.
GB/T 2406.2 Method
The Chinese national standard GB/T 2406.2 is technically aligned with ISO 4589-2 and is widely used for domestic regulatory compliance and product certification within the Chinese market. Laboratories accredited to GB/T 2406.2 can issue results that are directly comparable to ISO 4589-2 data.
General Test Procedure
- Condition the specimens and record material identification, dimensions, and mass where required
- Set an initial oxygen concentration based on expected material behavior or supplier data
- Ignite the specimen top and observe whether it burns beyond the criterion (time or length) at the set concentration
- Adjust the oxygen concentration upward or downward in a bracketing sequence across successive specimens
- Determine the final oxygen concentration using the prescribed calculation based on the last series of results
- Report the LOI value, specimen orientation, thickness, and any observed phenomena such as dripping or glowing combustion
For qualitative screening purposes, the test may be simplified to a single-point verification: the specimen is tested at a contractually specified oxygen concentration, such as 26% or 28%, and is reported simply as burning or self-extinguishing at that concentration.
Applications
Oxygen index qualitative analysis is applied across the full plastics value chain, including the following areas:
- Flame-retardant formulation development and optimization of halogenated, phosphorus-based, mineral filler, and intumescent additive systems
- Incoming quality control of resins, masterbatches, and compounded pellets
- Compliance verification for building and construction materials, railway and mass transit components, marine interiors, and aerospace parts
- Wire and cable insulation and sheathing compounds, in support of IEC 60332 series flame propagation requirements
- Verification and correlation with UL 94 horizontal and vertical burn classifications, and with IEC 60695-based fire hazard testing programs
- Failure analysis and forensic investigation following fire incidents, where LOI comparison helps establish whether the correct flame-retardant grade was installed
- Material screening and reverse engineering, where LOI fingerprints assist in confirming polymer identity and additive content
FAQ
What LOI value is considered flame retardant?
While classification thresholds vary by industry and specification, materials with an LOI of 27% or above are generally regarded as flame-retardant or self-extinguishing. Many rail, marine, and cable specifications require LOI values of 30% to 32% or higher for critical applications.
Can an LOI result alone certify a material as flame retardant?
No. The oxygen index is a comparative and qualitative indicator measured under controlled laboratory conditions. Regulatory approval usually requires supplementary tests such as UL 94 burning tests, IEC 60695 fire hazard testing, or IEC 60332 cable tests, depending on the end-use application.
What is the difference between LOI and UL 94 testing?
LOI quantifies the minimum oxygen concentration needed to sustain combustion in a flowing gas mixture, whereas UL 94 evaluates ignition and flame spread characteristics in ambient air, classifying materials as HB, V-2, V-1, V-0, or 5VA/5VB. The two methods are complementary, and LOI is often used for screening before formal UL 94 assessment.
How many specimens are needed and how long does testing take?
A full LOI determination typically requires 15 to 20 conditioned specimens. The bench testing itself usually takes one to two hours, and accredited third-party laboratories generally issue reports within three to five working days after sample conditioning.
Do fillers, reinforcements, and colorants affect the LOI?
Yes. Glass fiber, mineral fillers, halogenated compounds, phosphorus agents, and even some pigments can measurably shift the oxygen index. For meaningful comparisons, samples should be tested in the same formulation, thickness, and color as the intended production material.