Plastic Oxygen Index Testing Under Temperature and Humidity Conditions: Standards, Methods, and Appl

Plastic oxygen index testing under controlled temperature and humidity conditions measures the minimum oxygen concentration required to sustain flaming combustion of polymeric materials, providing a quantitative indicator of flame retardancy across realistic service environments. This technical guide explains the limiting oxygen index (LOI) method, elevated-temperature and humidity-conditioned test procedures, applicable international standards such as ISO 4589, ASTM D2863, GB/T 2406, and JIS K 7201, and how the results support material selection, quality control, and compliance in the electrical, construction, and transportation industries.

Technical Overview

The limiting oxygen index (LOI, also called the oxygen index or OI) is defined as the minimum concentration of oxygen, expressed as a volume percentage in a mixture of oxygen and nitrogen flowing upward through a vertical test chimney, that will just support candle-like flaming combustion of a plastics specimen. In the standard room-temperature configuration described by ISO 4589-2, ASTM D2863, GB/T 2406.2, and JIS K 7201-2, a specimen is considered to "burn" when at least one specimen in a set ignites and the flame persists for more than 3 minutes or consumes a length of more than 50 mm after removal of the ignition source.

Because ambient air contains approximately 20.9 percent oxygen, a material with an LOI below 21 will generally continue to burn freely once ignited in normal air, while materials with LOI values above 21 tend toward self-extinguishing behavior. As a practical classification convention often applied in industry, plastics with an LOI of 22 to 27 are regarded as slow-burning or self-extinguishing, and materials above 27 are frequently described as flame retardant. These thresholds are indicative rather than regulatory pass/fail criteria, and LOI data should always be interpreted alongside other flammability tests such as UL 94 or IEC 60695-11-10.

Importantly, the LOI is not an immutable material constant. It decreases measurably as the ambient temperature rises, because preheated polymers require less additional heat input to sustain pyrolysis and gas-phase combustion. ISO 4589-3 and GB/T 2406.3 therefore specify an elevated-temperature oxygen index method in which the chimney, the gas mixture, and the specimen are preheated to a controlled temperature, allowing construction of an oxygen-temperature curve and derivation of the temperature index (TI), i.e., the temperature at which the LOI falls to a specified oxygen concentration, commonly 21 percent. Humidity conditioning is equally significant: hygroscopic polymers such as polyamide 6, polyamide 66, cellulose-based plastics, and compounds containing hydrolysable or migratory flame retardants can show LOI shifts of several percentage points between the dry state and the equilibrium-moisture state. Combining ambient testing, elevated-temperature testing, and defined temperature/humidity conditioning therefore gives a far more realistic picture of fire performance in service than a single room-temperature value.

Testing Samples

Rigid Sheets, Bars, and Molded Specimens

Rigid thermoplastics and thermosets such as PVC, ABS, polycarbonate, PBT, PET, polyoxymethylene, PMMA, epoxy, and phenolic molding compounds are typically tested as rectangular bars with a preferred length of 80 to 150 mm, a width of 10 mm, and a thickness of 4 mm, although thicknesses up to approximately 10 mm can be evaluated. Injection-molded or machined plaques must be free of flash, sink marks, and surface contamination, and specimens should be marked so that testing direction relative to processing flow can be reported.

Flexible Films

Thin flexible films, including cable wrapping tapes and packaging films, are tested using the rolled or folded specimen fixtures defined in ISO 4589-2 and ASTM D2863. Films intended for comparison must be tested at the same nominal thickness, since oxygen index results for thin gauges are strongly thickness-dependent.

Rods and Tubes

Extruded rods, pipes, and cable insulation or sheathing samples are cut to the standard length and tested with appropriate support so that the specimen remains vertical in the gas stream. For tubing and cable compounds, the as-extruded geometry is generally retained because wall thickness and drawing orientation influence the measured LOI.

Foam Materials

Rigid and flexible polymeric foams, such as polyurethane, polystyrene, and polyolefin foams used in insulation and cushioning, are cut to the standard bar dimensions with clean, parallel faces. Because foam cell structure and density dominate burning behavior, density and cell orientation should be documented with the LOI result.

Specimen Conditioning and Quantity

A typical test series requires 15 to 20 specimens per condition to complete the up-and-down (bracketing) oxygen concentration sequence. Unless otherwise specified, specimens are conditioned in a standard atmosphere of 23 °C ± 2 °C and 50 % ± 5 % relative humidity in accordance with ISO 291, GB/T 2918, or ASTM D618 prior to testing, with extended conditioning or drying agreed for moisture-sensitive materials.

Testing Items

Accredited third-party laboratories offering temperature- and humidity-related oxygen index programs typically report the following items:

• Ambient-temperature limiting oxygen index (LOI) at 23 °C, determined per ISO 4589-2, ASTM D2863, GB/T 2406.2, or JIS K 7201-2.
• Elevated-temperature LOI at selected temperatures between approximately 40 °C and 200 °C, determined per ISO 4589-3 or GB/T 2406.3.
• Full oxygen-temperature index curves plotting LOI against test temperature, generated from multi-temperature test sequences.
• Temperature index (TI), defined as the temperature at which the LOI drops to a specified oxygen concentration, commonly 21 percent, reflecting the maximum service temperature at which the material remains self-extinguishing in air.
• LOI after humidity conditioning, comparing dry-as-molded specimens with specimens equilibrated at 23 °C/50 % RH or immersed per ISO 62, to quantify moisture sensitivity of the flame-retardant system.
• Comparative LOI screening for flame-retardant formulation development, halogen-free versus halogenated systems, and incoming quality control of compounds.
• Burning behavior observations, including afterflame time, dripping, and charring, recorded as supplementary information to the numerical LOI.

Testing Methods

Ambient-Temperature Oxygen Index (ISO 4589-2 / ASTM D2863 / GB/T 2406.2 / JIS K 7201-2)

The specimen is clamped vertically in the center of a heat-resistant glass chimney. A metered mixture of oxygen and nitrogen flows upward through the chimney at a defined total flow rate, and the top of the specimen is ignited with a small premixed gas flame for a prescribed application time. The oxygen concentration is adjusted stepwise using the bracketing (Dixon up-and-down) procedure until the critical concentration separating "burns" from "does not burn" responses is bracketed, and the LOI is calculated as LOI = 100 × V(O2) / (V(O2) + V(N2)), reported to the nearest 0.1 percent. Results from ISO 4589-2 and ASTM D2863 are broadly comparable, although minor differences in ignition procedure, chimney dimensions, and flow control can produce small systematic offsets, so a single standard should be specified throughout a test program.

Elevated-Temperature Oxygen Index (ISO 4589-3 / GB/T 2406.3)

ISO 4589-3 employs a chimney fitted with electrical heating elements and a gas preheater so that the rising oxygen/nitrogen mixture, the chimney wall, and the specimen are all stabilized at the selected test temperature. After thermal equilibration, the ignition and bracketing procedure follows the same logic as the ambient method, yielding an LOI value valid at that temperature. Repeating the sequence at several temperatures produces the oxygen-temperature curve from which the temperature index is derived. This method is particularly valuable for wire and cable compounds, connector housings, and engineering plastics serving near engines, heaters, or current-carrying conductors, where elevated ambient temperature can substantially erode the flame-retardancy margin observed at room temperature.

Temperature and Humidity Conditioning (ISO 291 / ASTM D618 / GB/T 2918 / ISO 62)

Conditioning is an integral part of the test, not an optional preliminary. Standard atmospheres of 23 °C/50 % RH or 27 °C/65 % RH are used for general conditioning, while dry-state conditioning in a desiccator or oven and water-immersion conditioning per ISO 62 are used when the hygroscopic effect itself is under investigation. Polyamides, cellulosics, and formulations based on phosphorus-nitrogen intumescent or hydrolysable brominated flame retardants should always be tested in both the dry and moisture-equilibrated states, and the conditioning history must be reported with the LOI value, because absorbed water can act as an inert diluent and heat sink that raises the LOI, while hydrolysis or migration of the flame retardant can lower it.

Applications

Oxygen index testing under temperature and humidity conditions is applied across the polymer value chain, from resin producers and compounders to OEM design and compliance teams. Representative application areas include:

• Wire and cable insulation and sheathing compounds, where the temperature index supports thermal endurance and fire-safety assessment of installations near heat sources.
• Railway rolling stock and mass transit interior materials, where LOI data complement flame spread, smoke density, and toxicity requirements in the EN 45545-2 framework.
• Housings and structural parts of electrical and electronic equipment, including connectors, breakers, and appliance enclosures, evaluated alongside UL 94 and IEC 60695 series tests.
• Building and construction products such as wall panels, pipe insulation, and foams, for which LOI serves as a formulation screening and quality-control indicator.
• Automotive and aerospace interior components, where combined thermal and flammability margins must be demonstrated for materials exposed to engine-bay or avionics-bay heat.
• Research and development of halogen-free flame-retardant systems, where LOI curves versus temperature provide rapid feedback on synergist loading, charring behavior, and durability after humidity aging.
• Incoming material inspection and supplier qualification, where LOI drift from a certified reference value signals compounding errors, flame-retardant depletion, or improper moisture management.

FAQ

What is considered a good LOI value for a flame-retardant plastic?

Values above 21 indicate the material will not sustain combustion in normal ambient air, and industry convention commonly treats LOI values above 27 as flame retardant. Regulatory acceptance, however, is governed by product-specific requirements such as UL 94 ratings or IEC 60695-11-10 results, and LOI should be treated as a comparative and screening parameter rather than a standalone pass/fail criterion.

Why does the LOI decrease at elevated temperatures?

Raising the specimen and gas temperature preheats the polymer toward its decomposition range, so less heat feedback from the flame is needed to generate combustible pyrolysis products. The flame can therefore be sustained at lower oxygen concentrations, which is why the temperature index test in ISO 4589-3 is essential for materials used in hot environments.

How does humidity conditioning affect oxygen index results?

For most non-hygroscopic polymers, the effect is small. For polyamides, cellulosics, and materials with moisture-sensitive flame retardants, absorbed water can shift the LOI by several percentage points in either direction, so dry-state and humidity-equilibrated results should both be reported whenever moisture sensitivity is a concern.

What is the difference between LOI and temperature index (TI)?

LOI is an oxygen concentration at a stated temperature, whereas the temperature index is the temperature at which the LOI falls to a chosen oxygen concentration, typically 21 percent. The TI effectively defines the maximum ambient temperature at which the material remains self-extinguishing in air and is derived from the oxygen-temperature curve measured under ISO 4589-3.

Are ISO 4589-2, ASTM D2863, GB/T 2406.2, and JIS K 7201-2 results interchangeable?

The methods are technically aligned, and results are generally comparable, but small differences in apparatus, ignition, and flow conditions can cause offsets of a few tenths of a percent oxygen. For contractual or certification purposes, a single standard should be specified and held constant across all comparisons.

Who can perform these tests?

Oxygen index testing at ambient and elevated temperatures, including humidity conditioning programs, is routinely offered by accredited third-party laboratories holding ISO/IEC 17025 accreditation for ISO 4589-2, ISO 4589-3, ASTM D2863, and the corresponding GB/T and JIS standards, and such laboratories can also advise on specimen preparation, conditioning history, and result interpretation.