Limiting Oxygen Index (LOI) Testing of Silicone Coatings: Principles, Standards, and Procedures

The limiting oxygen index (LOI) test is the primary bench-scale method used to quantify the flame-retardant performance of silicone coatings applied to fabrics, cables, electronics, and building products. This article explains the LOI principle, the governing standards such as ASTM D2863, ISO 4589-2, and GB/T 2406.2, specimen preparation and test procedures, and how LOI results are interpreted for quality control and certification. It serves as a practical reference for engineers and quality managers evaluating the combustion behavior of polysiloxane-based coating systems.

Technical Overview

Silicone coatings are polysiloxane-based materials built on a silicon–oxygen backbone, which gives them outstanding thermal stability, flexibility, weathering resistance, and an inherently low level of flammability compared with most organic coatings. When exposed to flame, the silicone matrix tends to form a protective silica-like char layer that slows heat and mass transfer, suppresses smoke, and limits dripping. These characteristics make silicone coatings a preferred choice for fire-sensitive applications in cable insulation, protective textiles, aerospace interiors, and electrical encapsulation.

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 candle-like combustion of a vertically mounted specimen for a specified time or over a specified distance. Because ambient air contains approximately 21% oxygen, a material with an LOI above 21% will not continue to burn easily under normal atmospheric conditions. The higher the LOI value, the greater the flame-retardant performance of the material.

LOI testing of silicone coatings is typically performed in accordance with the following internationally recognized standards:

• ASTM D2863 – Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics.
• ISO 4589-2 – Plastics: Determination of burning behaviour by oxygen index, Part 2: Ambient-temperature test.
• ISO 4589-3 – Ambient-temperature and elevated-temperature LOI testing for higher service temperature conditions.
• GB/T 2406.2 – Chinese national standard, technically aligned with ISO 4589-2.
• JIS K 7201-2 – Japanese industrial standard equivalent for oxygen index determination.

Typical unfilled silicone elastomers exhibit LOI values in the range of 25% to 30%, while formulations incorporating flame-retardant fillers such as aluminum hydroxide, magnesium hydroxide, platinum compounds, or ceramifiable additives can achieve LOI values above 35%. Testing is commonly carried out by accredited third-party laboratories or in-house quality control facilities equipped with calibrated oxygen index apparatus.

Testing Samples

Free-Standing Coating Films

Where the cured silicone coating can be produced or peeled as a self-supporting film, specimens are prepared directly from the film. This approach isolates the intrinsic combustion behavior of the coating formulation without influence from the substrate, which is useful for comparing raw material grades, evaluating filler loading effects, or verifying supplier datasheet claims.

Coated Substrates (As-Received Condition)

In most practical cases, the coating must be evaluated in combination with its real substrate, such as glass-fiber fabric, aramid textile, aluminum or steel sheet, or cable insulation. Standards refer to this as testing in the "state of combination," and the LOI result then reflects the performance of the complete composite system, including any interfacial or penetration effects between the coating and the substrate.

Specimen Dimensions and Conditioning

Representative specimen geometries include the following:

• ISO 4589-2 / GB/T 2406.2 rigid self-supporting specimens: 80 mm to 150 mm in length, 10 mm wide, with a thickness of 4 mm or 10 mm depending on the mold type.
• ISO 4589-2 flexible film specimens (Type IV): 140 mm × 52 mm films wound onto a mandrel for support.
• ASTM D2863 specimens: typically 70 mm to 150 mm long, 6.5 mm to 20 mm wide, and 3 mm to 12.7 mm thick, or as agreed between parties for coated fabrics.
• Conditioning: minimum 40 hours at 23 ± 2 °C and 50 ± 5% relative humidity prior to testing.
• Sample quantity: normally 15 to 20 specimens are required to complete the concentration bracketing sequence and obtain a statistically valid LOI value.

Testing Items

The core deliverable of the LOI test is a single numerical result, but a complete test report for silicone coatings typically covers the following items:

• Limiting oxygen index value, expressed as a volume percentage of oxygen in the O2/N2 mixture.
• Assessment of whether the coating system meets the specified flame-retardant index threshold (for example, LOI ≥ 28%) required by the customer specification or end-product standard.
• Observed burning behavior, including afterflame time, extent of burning, char formation, dripping of flaming particles, and smoke evolution.
• Comparison of ambient-temperature LOI versus elevated-temperature LOI for coatings used near heat sources.
• Verification of formulation consistency between production batches or between suppliers.

Testing Methods

Apparatus

The oxygen index apparatus consists of a heat-resistant glass chimney (typically at least 450 mm high with an internal diameter of 75 mm to 100 mm), a specimen holder that clamps the specimen vertically, a precision gas mixing and flow control system for oxygen and nitrogen, a small ignition burner producing a non-luminous flame, and instrumentation for monitoring gas concentrations and total flow rate. Gas purity of at least 99.5% is required for both oxygen and nitrogen, and flowmeters must be calibrated against certified references before testing.

Test Procedure

The general procedure under ASTM D2863 and ISO 4589-2 proceeds as follows:

• Condition the specimens and record their dimensions, mass, and any visible coating defects.
• Mount a specimen vertically in the holder so that the top of the specimen extends approximately 10 mm below the top of the chimney.
• Purge the chimney with a gas mixture at an estimated starting oxygen concentration and allow the flow to stabilize.
• Apply the ignition flame to the top surface of the specimen for up to 30 seconds, withdrawing and reapplying the flame as necessary until the specimen ignites.
• Observe whether burning is sustained for at least 3 minutes or extends at least 50 mm down the specimen, as defined by the pass/fail criterion.
• Adjust the oxygen concentration upward or downward in progressively smaller increments, testing a fresh specimen at each concentration, until the critical bracket of concentrations supporting and not supporting combustion is established.
• Calculate the LOI as the oxygen volume fraction at the boundary condition, expressed as LOI = 100 × VO2 / (VO2 + VN2), and report the result to the precision required by the standard.

Quality Assurance Considerations

Because LOI results are sensitive to specimen geometry, coating cure state, thickness uniformity, and gas flow calibration, laboratories must verify that specimens are fully cured, free of trapped solvent, and prepared consistently. Repeat testing on at least three independently prepared specimens is recommended when results are close to a specification limit. For silicone coatings on textiles, particular attention must be paid to specimen orientation, edge fraying, and the add-on weight of the coating, since these factors measurably influence the measured index.

Applications

LOI testing of silicone coatings supports material qualification and product certification across a wide range of industries:

• Wire and cable: verifying the flame retardance of silicone-coated glass braid and ceramifiable fire-resistant cable wraps used in power plants, tunnels, and mass transit systems.
• Building and construction: evaluating fire-protective and weatherproof silicone coatings for architectural membranes, curtain-wall fabrics, and penetration seals.
• Transportation: qualifying interior textile coatings, insulation wraps, and engine-compartment protective layers for rail and aerospace applications.
• Electronics: assessing conformal coatings, potting compounds, and thermal interface coatings where combustion behavior and low smoke emission are critical.
• Industrial textiles: confirming the flame-retardant performance of silicone-coated glass-fiber and aramid fabrics used in welding blankets, expansion joints, and thermal insulation jackets.
• Research and development: screening flame-retardant filler packages, platinum synergists, and ceramification additives in new silicone formulations.

FAQ

What LOI value is considered flame retardant for a silicone coating?

Materials with an LOI above 21% will not readily burn in normal air. In industry practice, coatings with an LOI of 26% to 28% are generally regarded as flame retardant, while values above 32% are typical of highly flame-retardant or ceramifiable silicone systems. Specific thresholds depend on the end-product specification.

Why do silicone coatings achieve higher LOI values than organic coatings?

The strong silicon–oxygen backbone of polysiloxanes is thermally stable, and upon combustion it converts to a coherent silica residue that shields the underlying material. Combined with halogen-free flame-retardant fillers, this yields low heat release, reduced dripping, and high LOI values without generating corrosive or highly toxic combustion gases.

Can a coating on a substrate be tested directly, or must a free film be prepared?

Both approaches are valid. Testing the coated substrate measures the performance of the real composite system, while testing a free-standing film isolates the intrinsic properties of the coating formulation. Standards allow specimens to be tested in their "state of combination" when the final use involves the same combination.

How does LOI testing relate to other flammability tests such as UL 94?

LOI is a quantitative measure of ease of extinction, whereas UL 94 ratings classify burning behavior in specific specimen geometries and ignition scenarios. The two tests are complementary; many specifications require both an LOI value and a UL 94 rating to fully characterize a flame-retardant coating.

How many specimens and how much time are required for a typical LOI determination?

A complete determination normally consumes 15 to 20 specimens and can be completed within one working day, including conditioning of representative samples, calibration checks, and the concentration bracketing sequence. Complex coated fabrics or borderline results may require additional replicate testing.