Oxygen Index Testing at Different Temperatures and Humidity Conditions: Principles, Standards and Ap

Oxygen index testing at different temperatures and humidity conditions determines the minimum oxygen concentration required to sustain combustion of polymeric materials under realistic service environments. By combining elevated-temperature oxygen index determination with controlled humidity conditioning, laboratories can quantify how heat and moisture influence the flammability ranking of plastics, rubbers, composites, textiles and cable materials. This article explains the test principle, sample requirements, applicable standards, test procedures and typical applications of temperature- and humidity-dependent oxygen index testing.

Technical Overview

The oxygen index (OI), also known as the limiting oxygen index (LOI), is the minimum concentration of oxygen, expressed as volume percent in a flowing oxygen and nitrogen mixture, that will just support candle-like combustion of a vertically mounted specimen. Because ambient air contains approximately 20.9 percent oxygen, a material with an oxygen index above about 21 will not continue to burn in still air under the standard test geometry, while values above 26 to 28 are generally associated with flame-retardant or self-extinguishing behavior.

The oxygen index is not a fixed material constant; it depends strongly on temperature, moisture content, specimen geometry and conditioning history. As temperature rises, thermal degradation accelerates, pyrolysis gases are evolved more rapidly, and the measured oxygen index decreases. This behavior is critical for components that operate above room temperature, such as electrical insulation near heat sources.

Absorbed moisture acts as a heat sink and inert diluent, particularly in hygroscopic polymers such as polyamides, polyurethanes and cellulosic materials. Humidified specimens of these materials frequently show oxygen index values several points higher than oven-dried specimens, so the conditioning state must always be reported together with the result.

To quantify temperature dependence, ISO 4589-3 defines the temperature index (TI): the temperature at which the oxygen index of a material falls to 20.9 percent, the oxygen concentration of normal air. The temperature index identifies the maximum ambient temperature at which a self-extinguishing material will still resist candle-like burning in air, and it is widely specified for cable insulation, connector bodies and other electrical components.

Testing Samples

Oxygen index tests can be performed on a broad range of materials, provided specimens can be cut, molded or supported according to the dimension requirements of the applicable standard. Representative sample categories include:

Rigid Thermoplastics and Thermosets

Self-supporting bars, typically 80 to 150 mm long, 10 mm wide and 4 mm thick per ISO 4589-2, or 70 to 150 mm long, 6.5 mm wide and 3 mm thick per ASTM D2863. Specimens may be injection molded, compression molded or machined from sheet, and edges must be smooth and free of cracks.

Films, Sheets and Flexible Materials

Thin or limp materials that cannot stand upright are tested in a supporting frame or holder specified by the standard. Because results for films are strongly thickness dependent, the measured oxygen index must always be reported together with the nominal thickness.

Cellular Plastics and Foams

Flexible and rigid foams are tested as bars of larger cross-section, commonly around 10 mm by 10 mm, with the cell structure oriented consistently between specimens to reduce scatter.

Rubbers, Elastomers, Composites and Cable Materials

Strips cut from cable insulation or sheathing, rubber compounds, textile fabrics and fiber-reinforced laminates can all be evaluated. For composites, fiber orientation and lay-up are recorded since they influence burning behavior.

Testing Items

A typical temperature- and humidity-dependent oxygen index test program includes the following items:

• Ambient-temperature oxygen index after standard conditioning at 23 °C and 50 percent relative humidity
• Oven-dry oxygen index after drying, for example 24 hours at 80 °C or 3 hours at 105 °C, to establish the dry-state baseline
• Elevated-temperature oxygen index at one or more set points, such as 40, 60, 80, 100, 125, 150, 200 or 250 °C
• Temperature index (TI) determined by interpolation to the temperature where the oxygen index equals 20.9 percent
• Oxygen index after humidity conditioning at elevated relative humidity, for example 85 or 95 percent RH for 24 to 96 hours
• Oxygen index after water immersion at 23 °C for 24 hours or to saturation
• Comparative ranking and pass or fail evaluation against material specifications, customer standards or certification requirements

Testing Methods

Standards and Apparatus

Ambient-temperature testing is governed by ISO 4589-2, ASTM D2863 and equivalent national standards such as GB/T 2406.2, while elevated-temperature testing is covered by ISO 4589-3. Conditioning atmospheres are defined in ISO 291 and ASTM D618, and water absorption states in ISO 62 and ASTM D570. The apparatus consists of a vertical heat-resistant glass chimney, a specimen holder, oxygen and nitrogen supplies with calibrated flow control, an oxygen analyzer, a small diffusion flame igniter, and for elevated-temperature work a heated enclosure capable of controlling the chimney atmosphere up to approximately 300 °C.

Ambient-Temperature Oxygen Index Procedure

Conditioned specimens are mounted vertically in the chimney, and the chimney is purged with the oxygen and nitrogen mixture at the specified total flow for at least 30 seconds. The igniter flame is applied to the top of the specimen, and burning is observed against the pass or fail criterion, typically whether flaming reaches a mark 50 mm below the top or persists for 180 seconds. The oxygen concentration is raised or lowered step by step, with increments reduced to 0.2 percent near the limiting value, until the limiting concentration is bracketed. The oxygen index is then calculated from the response series and reported to the nearest 0.1 percent together with specimen dimensions, criterion used and conditioning state.

Elevated-Temperature Testing and Temperature Index Determination

For elevated-temperature work per ISO 4589-3, the heating enclosure is set to the target temperature and each specimen is allowed to reach thermal equilibrium before ignition. The same pass or fail criterion and concentration stepping procedure are applied at each temperature. Oxygen index is measured at several temperatures spanning the expected transition range, the results are plotted as oxygen index versus temperature, and the temperature index is obtained by interpolating the curve to 20.9 percent oxygen. Many specifications instead require the oxygen index at a stated operating temperature, for example 90 or 105 °C for cable insulation classes.

Humidity and Moisture Conditioning Procedures

Hygroscopic materials are tested in defined moisture states: oven-dry, standard atmosphere at 23 °C and 50 percent RH, tropical atmosphere at 27 °C and 65 percent RH, high-humidity exposure at 85 to 95 percent RH, or after immersion per ISO 62. After conditioning, specimens should be transferred to the apparatus promptly, ideally in sealed containers, to prevent moisture loss before ignition. Because identical material can differ by several oxygen index points between dry and humid states, the conditioning history is an inseparable part of the reported result.

Key Factors Affecting Accuracy

• Specimen thickness and geometry, since oxygen index generally increases with thickness for many thermoplastics
• Calibration of flow controllers and the oxygen analyzer, and complete purging of the chimney between trials
• Cleanliness of the chimney, as soot deposits and condensed decomposition products alter the local atmosphere
• Consistent ignition technique and objective application of the pass or fail criterion
• Adequate thermal and moisture equilibration of the specimen before ignition
• Full documentation of the conditioning state, test criterion and specimen dimensions

Applications

Temperature- and humidity-dependent oxygen index testing supports material selection, qualification and quality control across many industries:

• Wire and cable: verification of oxygen index at rated conductor operating temperatures and determination of temperature index for insulation and sheathing compounds
• Electrical and electronics: connectors, switch housings, coil formers and enclosures located near heat sources
• Rail and aerospace interiors: supporting evidence for material selection and certification programs
• Marine and naval engineering: flame-retardance verification of outfitting materials
• Research and development: screening of flame-retardant formulations across the moisture and temperature ranges expected in service
• Quality control: incoming inspection of resins, compounds, fabrics and finished components against specification limits

FAQ

What is the oxygen index and what does the value mean?

The oxygen index is the minimum oxygen volume percentage in an oxygen and nitrogen mixture that just sustains candle-like burning of a specimen for 3 minutes or over 50 mm. A value above 21 means the material will not continue to burn in normal air under the test conditions, and values of 26 to 28 or higher are commonly required for flame-retardant classifications.

Why is oxygen index tested at elevated temperatures?

The oxygen index of virtually all polymers decreases as temperature increases, because heating accelerates pyrolysis and makes ignition easier. Elevated-temperature testing establishes the oxygen index at actual service temperatures and yields the temperature index, the temperature at which the material just continues to burn in air, which is essential for hot-environment applications.

How does humidity affect oxygen index results?

Absorbed water cools and dilutes the flame zone, so humidified hygroscopic materials such as polyamides and cellulosics usually show higher oxygen index values than dry ones. Testing in both dry and conditioned states reveals the full range of flammability behavior expected in service.

Which standards apply to this testing?

Ambient-temperature oxygen index is covered by ISO 4589-2, ASTM D2863 and GB/T 2406.2; elevated-temperature oxygen index and temperature index by ISO 4589-3; conditioning atmospheres by ISO 291 and ASTM D618; and water absorption by ISO 62 and ASTM D570.

How many specimens are required?

Because the limiting concentration is approached by a bracketing procedure, a single oxygen index determination typically consumes 15 to 20 specimens. A full temperature index curve involving five or more temperatures may require roughly one hundred specimens.

What is considered a good oxygen index value?

As reference points, polyethylene and polypropylene are around 17 to 18, polycarbonate around 25 to 27, rigid PVC often exceeds 40, and fluoropolymers can exceed 90. Many industry specifications require a minimum of 26 to 28 at ambient temperature and 21 at the maximum service temperature.

Can results be compared between different laboratories?

Yes, provided the same standard, specimen geometry, pass or fail criterion and conditioning state are used. Interlaboratory studies typically show reproducibility within about one to one and a half oxygen index points when these parameters are controlled and fully reported.