Determination of the Limiting Oxygen Index (LOI) of PVC Cable Compounds: Principles, Standards, and

The limiting oxygen index (LOI) is the key flammability parameter used to evaluate the flame-retardant performance of PVC cable compounds, defining the minimum oxygen concentration required to sustain combustion. This technical article explains the LOI determination principle, applicable standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, specimen preparation, step-by-step testing procedure, and typical acceptance values for insulation and sheath grades.

Technical Overview

The limiting oxygen index (LOI), also called the oxygen index (OI), 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 candle-like downward burning of a vertically mounted specimen. In the standard LOI test, a specimen is considered to have burned if flaming combustion persists for 180 seconds or longer, or if the flame front travels more than 50 mm from the point of ignition.

Because normal air contains approximately 21% oxygen, a material with an LOI above 21 will tend to self-extinguish in air, and materials with an LOI of 27 or higher are generally classified as flame retardant. PVC benefits from the chlorine atoms released during decomposition, which promote char formation and gas-phase flame inhibition, so even plasticized PVC cable compounds typically achieve LOI values well above those of polyolefin compounds. However, the large quantities of plasticizers, processing aids, and combustible fillers used in soft cable compounds significantly reduce the LOI, which is why flame-retardant formulations rely on additives such as antimony trioxide, aluminum hydroxide, or zinc borate to restore high oxygen index values.

LOI determination for PVC cable compounds is most commonly performed in accordance with ISO 4589-2, ASTM D2863, or the Chinese national standard GB/T 2406.2, all of which share the same vertical burning principle. Product standards such as GB/T 8815 for PVC compounds used in wires and cables, together with cable material datasheets, specify minimum LOI requirements for individual compound grades.

Testing Samples

Compression-Molded Plaques from Compound Pellets

The preferred sample form is a flat strip cut from a compression-molded plaque prepared from the PVC compound pellets or granules. Typical strip dimensions are 80–150 mm in length, 10 mm in width, and 3–4 mm in thickness, with the exact dimensions recorded and reported since LOI depends on thickness. Molding must be carried out at the recommended temperature, pressure, and cooling conditions so that the plaque is free of voids, bubbles, and residual stresses that could distort the result.

Specimens Taken from Finished Cables

For verification purposes, LOI may also be measured on strips cut directly from extruded PVC insulation or sheath. Because tubular or curved samples cannot stand upright on their own, they are usually slit, flattened, or supported on a small wire frame so that the specimen remains vertical and the top edge is accessible to the igniter. Results on finished-cable samples should be compared only with data obtained on the same geometry, since wall thickness and curvature influence the measured value.

Conditioning Before Testing

All specimens must be conditioned prior to testing in a standard atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for at least 40 hours, in accordance with ISO 291 or ASTM D618. Moisture content and thermal history affect ignition behavior, particularly for highly plasticized compounds.

Testing Items

A standard LOI test report for PVC cable compound includes the following items:

• Limiting oxygen index value, expressed as a volume percentage to the nearest 0.1%
• Specimen dimensions, thickness, and form (molded plaque, cable strip, or supported specimen)
• Conditioning atmosphere and duration
• Observations of burning behavior, including afterflame time, dripping of flaming particles, sooting, and char formation
• Test standard applied (ISO 4589-2, ASTM D2863, or GB/T 2406.2) and ignition method
• Optional: temperature index (TI) determination at elevated temperatures in accordance with ISO 4589-3, used to assess flame retardancy at cable operating temperatures

Testing Methods

Apparatus

The test is performed in an oxygen index apparatus consisting of a heat-resistant glass chimney (typically 75–100 mm in internal diameter and 450–600 mm high), a specimen clamp that holds the specimen vertically at the center of the column, a gas mixing and metering system with calibrated flow controllers for high-purity oxygen and nitrogen, and a small propane or butane igniter. Non-metallic bead beds at the base of the column ensure uniform, laminar gas flow. Gas purity, flow meter calibration, and periodic verification with certified gas mixtures are essential for maintaining accuracy of ±0.1 vol% in oxygen concentration.

Test Procedure

The total gas flow is set to give a nominal upward velocity of approximately 40 mm/s through the column, and the column is purged for at least 30 seconds before each ignition attempt so that the atmosphere around the specimen is fully stabilized. The igniter flame is then applied briefly to the top edge of the specimen, typically for 5-second intervals repeated as necessary up to a total of about 10 seconds, and the flame is withdrawn to observe the burning response.

• If the specimen burns for 180 seconds or longer, or the burn front exceeds 50 mm, the oxygen concentration is too high and is reduced for the next specimen.
• If the specimen self-extinguishes before either criterion is reached, the oxygen concentration is increased.
• Large steps of 0.5–1.0 vol% are used first to bracket the critical concentration, followed by fine steps of 0.2 vol% to converge on the limiting value.
• A full determination normally consumes 10–15 specimens, and the final LOI is the lowest oxygen concentration at which the balance of results is achieved according to the statistical evaluation defined by the standard.

Calculation

The limiting oxygen index is calculated directly from the gas flow rates as follows: LOI = 100 × V(O2) / [V(O2) + V(N2)], where V(O2) and V(N2) are the volumetric flow rates of oxygen and nitrogen at the limiting concentration. The result is rounded to the nearest 0.1% and reported together with the specimen geometry and conditioning state.

Factors Affecting Results

Several formulation and procedural variables influence the measured LOI of PVC cable compound. Increased plasticizer loading lowers the oxygen index, while antimony trioxide synergists, mineral flame retardants such as aluminum hydroxide, and chlorinated or phosphate ester plasticizers raise it. Thicker specimens generally give higher LOI values, so thickness must be controlled and stated. Gas purity errors, incomplete purging, improper igniter positioning, and inadequate conditioning are the most common causes of poor reproducibility, which should remain within roughly ±0.5–1.0 vol% between competent laboratories.

Applications

LOI determination is applied throughout the life cycle of PVC cable compounds:

• Incoming inspection and outgoing quality control of insulation, sheath, and flame-retardant compound batches against datasheet and purchase-specification limits
• Research and development of flame-retardant formulations, including plasticizer substitution and optimization of antimony trioxide, ATH, and zinc borate loadings
• Conformity assessment of compound grades to product standards such as GB/T 8815, where minimum oxygen index values are specified for designated grades
• Screening and supporting data for cable flame performance classifications, complementing finished-cable tests such as IEC 60332-1 and IEC 60332-2 vertical burning tests
• Comparative evaluation of materials in certification submissions, supplier qualifications, and failure investigations

Typical reference values for interpretation are as follows: general-purpose soft PVC cable compound generally falls in the range of 22–27%, flame-retardant sheath compounds commonly meet minimum requirements of 30–32%, and highly filled specialty grades can exceed 35–40%.

FAQ

What LOI value is considered flame retardant for PVC cable compound?

An LOI of 27% or above is broadly recognized as the threshold for flame-retardant classification. In cable industry practice, flame-retardant PVC sheath compounds are usually specified with minimum LOI requirements of 30–32%, while ordinary non-flame-retardant soft PVC typically measures between 22% and 26%.

Which standards govern LOI testing of PVC cable materials?

The principal standards are ISO 4589-2, ASTM D2863, and GB/T 2406.2, which use essentially the same vertical top-ignition principle. Related methods include ISO 4589-3 for temperature index measurement at elevated temperatures.

Why is specimen preparation so important for soft PVC compounds?

Highly plasticized PVC is flexible and cannot always stand upright in the test column. Specimens must therefore be cut from flat compression-molded plaques or supported on a wire frame, and their thickness must be uniform and reported, because both support conditions and thickness directly affect the measured oxygen index.

How many specimens are required for one determination?

A complete LOI determination typically requires 10–15 specimens: several for coarse bracketing of the critical oxygen concentration with large concentration steps, and the remainder for fine adjustment in 0.2 vol% increments to establish the final reported value.

Can LOI be tested directly on insulation or sheath taken from finished cable?

Yes, provided the curved tube is slit or flattened and, where necessary, supported so the specimen remains vertical. Results are valid for comparison against data obtained on the same geometry, since cable wall thickness and curvature cause systematic deviations from molded-plaque values.

What does a low LOI result indicate about the compound formulation?

An unexpectedly low LOI usually points to excessive plasticizer content, insufficient flame-retardant loading, poor dispersion of antimony trioxide or mineral fillers, or material contamination. It signals elevated flammability risk and typically triggers reformulation or investigation of raw material quality before the compound is released for cable production.