Limiting Oxygen Index (LOI) Testing of Low Smoke Halogen-Free Cable Compounds

The limiting oxygen index (LOI) test is the primary laboratory method used to quantify the flame-retardant performance of low smoke halogen-free (LSZH) cable compounds before cable-level fire testing. Conducted in accordance with ISO 4589-2, ASTM D2863, and GB/T 2406.2, this test determines the minimum oxygen concentration required to sustain candle-like combustion of a molded compound specimen. This article explains the test principle, sample preparation, procedure, typical acceptance values, and how the results are applied in cable material development and quality control.

Technical Overview

Low smoke halogen-free (LSZH or LSOH) cable compounds are typically polyolefin-based materials—such as polyethylene, ethylene-vinyl acetate copolymer, or polyolefin elastomers—heavily loaded with metal hydroxide flame retardants, most commonly aluminum trihydrate (ATH) or magnesium hydroxide. When exposed to fire, these fillers decompose endothermically and release water, which dilutes combustible gases and raises the oxygen concentration needed to sustain burning. Many grades are additionally cross-linked by peroxide, silane, or electron-beam methods to improve thermal and mechanical performance.

The limiting oxygen index (LOI) is defined as the minimum concentration of oxygen, expressed as a volume percentage in a flowing oxygen and nitrogen mixture, that will just support flaming combustion of a vertically mounted specimen after ignition from its top, under specified test conditions. The test is performed at ambient temperature following ISO 4589-2, ASTM D2863, or GB/T 2406.2. Because normal air contains approximately 20.9% oxygen, a material with an LOI above 21 tends to self-extinguish in air, while a higher LOI indicates stronger flame retardancy.

For reference, ordinary polyethylene exhibits an LOI of roughly 17 to 18, and unfilled polyolefin elastomers generally fall between 19 and 21. Properly formulated LSZH cable compounds typically achieve LOI values of 32 to 38, and high-performance grades used in rail transit, ships, and tunnels can exceed 40. The oxygen index test therefore serves as an efficient screening and quality-control tool for confirming that flame-retardant loading and dispersion meet the intended compound design targets.

Testing Samples

Molded Compound Specimens

Because cable compounds are supplied as pellets or granules, test specimens must first be prepared by compression molding. A representative quantity of compound is melted and pressed into a flat sheet of uniform thickness under controlled temperature and pressure, then cooled under defined conditions. Specimens are cut from the sheet with clean, parallel edges, and any burrs or surface defects are removed, as edge quality can affect burning behavior.

Specimen Dimensions

Under ISO 4589-2 and GB/T 2406.2, the standard specimen for self-supporting rigid materials measures 80 to 150 mm in length, 10 mm in width, and approximately 4 mm in thickness. ASTM D2863 commonly uses strips of 70 to 150 mm in length, 6.5 mm in width, and around 3 mm in thickness. For thin or flexible compounds that cannot support themselves, larger sheet specimens supported on a wire frame are prescribed. A gauge line is marked 50 mm below the top of each specimen so the burned length can be assessed during testing. A full determination generally requires at least 15 to 20 specimens.

Specimen Conditioning

Prior to testing, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 88 hours in accordance with ISO 291, unless the material specification states otherwise. Specimens that have undergone thermal ageing are cooled to room temperature and reconditioned before measurement, so that the result reflects the aged state of the compound.

Testing Items

The core deliverable of the test is the ambient-temperature limiting oxygen index of the LSZH compound, reported as a volume percentage of oxygen and rounded to the nearest 0.1%. Typical test items include:

• Ambient-temperature LOI of insulation, bedding, or sheathing compounds per ISO 4589-2, ASTM D2863, or GB/T 2406.2.
• LOI after thermal ageing, to verify long-term retention of flame-retardant performance.
• Temperature index (TI) per ISO 4589-3, in which the oxygen index is determined at elevated temperatures to characterize performance under realistic fire conditions.
• Comparative LOI measurements of formulation variants during compound development.

The test report includes the standard applied, specimen dimensions and preparation method, ignition procedure, the pass and fail oxygen concentrations observed, the calculated LOI value, and any relevant observations such as flaming drips, afterglow, or intumescence.

Testing Methods

Applicable Standards

• ISO 4589-2 / EN ISO 4589-2: Determination of burning behaviour by oxygen index, ambient-temperature test.
• ASTM D2863: Standard test method for measuring the minimum oxygen concentration to support candle-like combustion of plastics.
• GB/T 2406.2: Chinese national standard, technically aligned with ISO 4589-2.
• JIS K 7201-2: Japanese industrial standard for the oxygen index test of plastics.

Test Apparatus

The test is performed in an oxygen index chamber comprising a heat-resistant glass chimney of at least 75 mm internal diameter and roughly 450 to 500 mm height, a vertical specimen holder that positions the specimen on the central axis, a gas mixing and metering system with calibrated flow controls or mass flow meters, and a small propane or butane diffusion-flame igniter. The chamber is supplied with oxygen and nitrogen of at least 98% purity. A paramagnetic or zirconia-based oxygen analyzer is frequently integrated to verify the actual oxygen concentration delivered to the chimney, which improves measurement traceability.

Test Procedure

• Condition the specimens, then verify chamber ambient temperature, gas purity, and flow calibration.
• Mount a specimen vertically in the holder with its top a defined distance below the chimney top, and purge the chimney with the oxygen and nitrogen mixture at a linear gas velocity of approximately 40 mm/s.
• Set an initial oxygen concentration based on experience, apply the igniter flame to the top of the specimen, and withdraw it once the specimen is ignited.
• Record whether the specimen satisfies the burning criterion, typically sustained flaming for 180 seconds or a combustion front reaching the 50 mm gauge mark, with the exact criterion defined by the governing standard and specimen geometry.
• Adjust the oxygen concentration in small increments, commonly 0.2% volume steps, using the nearest-bracketing technique until a concentration is found at which the specimen just continues to burn and, one step lower, self-extinguishes.
• Repeat the bracketing sequence with fresh specimens to complete the required series and calculate the final LOI.

Calculation

The oxygen index is calculated from the gas flow rates as LOI = 100 × V(O₂) / [V(O₂) + V(N₂)], or taken directly from the calibrated analyzer reading at the bracketed concentration. With properly conditioned specimens and calibrated equipment, within-laboratory repeatability is generally within 0.2 to 0.5 percentage points of oxygen.

Applications

Oxygen index testing of LSZH cable compounds is used across the cable industry value chain:

• Compound manufacturing quality control, where each production batch is released against a specified minimum LOI.
• Research and development, where ATH or magnesium hydroxide loading, synergist packages, and cross-linking systems are optimized to reach target LOI values without sacrificing mechanical and processing properties.
• Incoming inspection by cable makers, verifying that purchased insulation and sheathing compounds meet contractual flame-retardant specifications.
• Qualification support for flame-retardant and fire-performance cable designs intended for rail transit, marine, building, data center, and tunnel applications, where low smoke halogen-free materials are mandated.
• Ageing and reliability studies that track the retention of flame retardancy after long-term thermal exposure.

In procurement and certification contexts, testing is typically carried out by accredited third-party laboratories or manufacturer laboratories operating under ISO/IEC 17025 management systems, so that reported LOI values are internationally comparable.

FAQ

What oxygen index value is required for LSZH cable compounds?

Requirements vary by specification and application, but most LSZH insulation compounds are specified at LOI of 30 or above, while sheathing compounds commonly require 32 to 35 or higher. Premium grades for rail and marine use may specify 37 or more. The applicable purchase specification or cable standard should always be consulted for the exact limit.

Is a high LOI sufficient to prove that a finished cable is flame retardant?

No. The LOI is a material-level screening test. Flame retardance of finished cables must be demonstrated with cable-level fire tests such as IEC 60332-1-2 or IEC 60332-3 series for flame propagation, IEC 61034-2 for smoke density, and IEC 60754-1 and IEC 60754-2 for halogen acid gas content and corrosivity. A high LOI strongly supports good performance in these tests but does not replace them.

How are pellet samples converted into test specimens?

Pellets are compression molded into flat sheets at a temperature appropriate for the compound, using controlled pressure and cooling to avoid voids and degradation. Specimens of the required dimensions are then cut from the sheet. Consistent molding conditions are essential, because porosity, cure state, and filler dispersion all influence the measured oxygen index.

What factors affect the accuracy and repeatability of LOI results?

Key factors include specimen thickness and geometry, molding quality, conditioning state, gas purity and flow calibration, chimney drafts, ignition technique, and operator judgment when timing the burning criterion. Interlaboratory differences are minimized by strict adherence to the specified conditions in ISO 4589-2, ASTM D2863, or GB/T 2406.2.

What is the difference between LOI and temperature index?

The LOI is measured at ambient temperature, while the temperature index per ISO 4589-3 is the temperature at which the oxygen index of the material drops to a specified value, often 21, corresponding to normal air. The temperature index better reflects real fire scenarios, in which materials heat up before and during combustion, and is therefore used as a complementary evaluation for demanding applications.