Limiting Oxygen Index (LOI) Testing of Reinforced Plastics: Principles, Standards, and Methods

The limiting oxygen index (LOI) test is one of the most widely used methods for evaluating the flammability of reinforced plastics, measuring the minimum oxygen concentration required to sustain candle-like combustion of a specimen. This article provides a comprehensive technical overview of LOI testing for fiber-reinforced thermoset and thermoplastic composites, covering relevant standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, specimen preparation, test procedures, and typical applications across automotive, railway, electronics, and construction industries.

Technical Overview

The limiting oxygen index (LOI), also known as the oxygen index (OI) or critical oxygen index, 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. Because ambient air contains approximately 21% oxygen, materials with an LOI above 21% will tend to self-extinguish after removal of the ignition source, while materials with an LOI below 21% will continue to burn in normal air. In general, the higher the LOI value, the better the flame-retardant performance of the material.

For reinforced plastics—polymer matrices combined with glass fibers, carbon fibers, aramid fibers, or mineral fillers—the LOI test provides a sensitive, quantitative, and reproducible measure of burning behavior. It is frequently used for quality control, material qualification, flame-retardant formulation development, and compliance demonstration. Because reinforced plastics are often anisotropic, the measured LOI can be influenced by fiber content, fiber orientation, laminate lay-up, resin chemistry, and specimen thickness, all of which must be documented when reporting results.

LOI testing of reinforced plastics is typically performed in accordance with the following standards:

• ISO 4589-2 – Plastics – Determination of burning behaviour by oxygen index – Part 2: 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 equivalent to ISO 4589-2, commonly specified for the Chinese market.
• ISO 4589-3 – Elevated-temperature oxygen index determination (typically 25 °C to 150 °C).
• ISO 291 / ASTM D618 – Conditioning atmospheres for specimen preparation prior to testing.

Testing Samples

Reinforced plastic materials suitable for LOI determination cover a broad range of product forms. Specimens are cut from sheets, laminates, molded plaques, pultruded profiles, or finished parts, with edges smoothed and free of cracks to avoid premature ignition or flash-over along cut edges.

Thermoset Fiber-Reinforced Composites

Typical samples include glass-fiber-reinforced polyester, vinyl ester, and epoxy laminates; sheet molding compound (SMC) and bulk molding compound (BMC) parts; pultruded FRP profiles; phenolic and polyurethane composites; and carbon-fiber prepreg laminates cured under controlled conditions.

Thermoplastic Composites

Examples include short- and long-glass-fiber-reinforced polyamide (PA6, PA66), polypropylene (PP), PBT, PET, PC, and PPS compounds, as well as glass-mat thermoplastics (GMT) and continuous-fiber-reinforced thermoplastic laminates, both with and without halogen-free flame-retardant additives.

Sandwich and Multi-Layer Structures

Facing skins of FRP honeycomb and foam-core sandwich panels can be tested individually, provided the core material is removed and the facing thickness is within the permissible range of the test method.

Specimen Geometry and Conditioning

For ISO 4589-2, the most common rigid self-supporting specimen is 80–150 mm long, 10 mm wide, and 10 mm thick (thinner Type II/III geometries are permitted and the actual thickness must be reported). ASTM D2863 typically uses specimens 70–150 mm long, 6.5 mm wide, and 3 mm thick. Edges should be smooth, and specimens must be marked at the burn-length reference distance. All specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours (or as agreed) before testing, and at least 15 specimens are normally required to complete a full determination.

Testing Items

Accredited third-party laboratories offering LOI testing of reinforced plastics typically report the following:

• Ambient-temperature LOI value – the oxygen index at 23 °C, expressed as volume percent oxygen to the nearest 0.1%.
• Elevated-temperature LOI – oxygen index measured at temperatures up to 150 °C per ISO 4589-3, reflecting the well-known decrease of LOI as material temperature rises.
• Burning behavior observations – afterflame time, burning distance, char formation, glowing combustion, dripping, delamination, and fiber exposure.
• Comparative material evaluations – ranking of resin systems, flame-retardant packages, fiber loadings, and laminate constructions.
• Verification of flame-retardant treatments – confirmation of the effectiveness of halogen-free or intumescent additives in reinforced compounds.

Testing Methods

The test is conducted in an oxygen index apparatus consisting of a heat-resistant glass chimney (typically at least 75 mm in internal diameter and 450 mm high), a specimen holder that clamps the specimen vertically at its lower portion, a precision oxygen/nitrogen gas mixing and metering system with an oxygen analyzer, and a small propane-fueled igniter.

The general test procedure is as follows:

• Mounting – the conditioned specimen is clamped vertically so that the top of the specimen is at least 100 mm below the top of the chimney.
• Gas flow adjustment – the total volumetric flow of the oxygen/nitrogen mixture is set to give a linear velocity of approximately 40 mm/s through the chimney, starting from an oxygen concentration estimated to be near the material's limit.
• Ignition – the propane flame is applied to the top edge of the specimen for 5-second intervals, repeated for up to 15 seconds, until the specimen is ignited and burns with a stable flame front.
• Observation of pass/fail criteria – the specimen "passes" at a given concentration if it either burns for at least 3 minutes or consumes a burned length of at least 50 mm; otherwise it "fails" (self-extinguishes).
• Bracketing of the limit – the oxygen concentration is progressively increased after a failure or decreased after a pass, using coarse steps initially and fine steps (0.2% or smaller) near the critical concentration, until the concentration interval bracketing the transition is established.
• Calculation and reporting – the LOI is calculated from the volume fraction of oxygen in the final gas mixture, LOI = 100 × V(O₂) / [V(O₂) + V(N₂)], and reported together with the standard used, specimen dimensions, fiber content, conditioning, and all burning observations.

For reinforced plastics, particular attention is paid to edge effects and to the tendency of exposed fibers to support glowing combustion after the visible flame extinguishes; such phenomena are recorded because they influence the interpretation of the result in end-use risk assessments. LOI results should always be interpreted alongside complementary fire tests such as UL 94, IEC 60695, and cone calorimetry when full fire-performance characterization is required.

Applications

LOI determination for reinforced plastics supports material selection, regulatory compliance, and product development across many industries:

• Transportation – interior and structural components for rail vehicles, buses, ships, and automobiles, where reinforced composites must meet flame-spread and smoke requirements.
• Electrical and electronics – glass-fiber-reinforced enclosures, connectors, and structural parts specified with flame-retardant grades.
• Construction – FRP panels, pultruded profiles, gratings, cable trays, and sandwich panels used in buildings and infrastructure.
• Renewable energy – wind turbine blade skins, nacelle covers, and photovoltaic mounting components.
• R&D and quality control – screening flame-retardant formulations, incoming resin and compound inspection, and batch-to-batch consistency verification.
• Regulatory submission – providing quantitative flammability data for product data sheets, certifications, and tender documentation.

FAQ

What LOI value is considered flame retardant for reinforced plastics?

As a common industry guideline, materials with an LOI greater than 21% will not continue to burn in normal air, and LOI values of 27–28% or above are generally regarded as indicating self-extinguishing or flame-retardant behavior. Contractual and regulatory requirements should always define the specific acceptance threshold for the intended application.

How does glass fiber reinforcement affect the LOI result?

Glass fibers are non-combustible and act as a diluent of the combustible resin, often increasing the LOI and promoting a protective char or ash crust. However, in some thermoplastics the added fibers suppress melt dripping that would otherwise remove heat, and exposed fibers may sustain glowing combustion, so the effect of reinforcement on burning behavior is material-specific and must be verified by testing.

What is the difference between the LOI test and the UL 94 test?

The LOI test quantifies the minimum oxygen concentration needed to sustain burning under controlled gas flow, yielding a numerical index, whereas UL 94 evaluates the ability of a specimen to extinguish after ignition in ambient air, assigning ratings such as V-0, V-1, V-2, 5VA, or 5VB. The two methods are complementary and are frequently specified together in material data sheets.

How many specimens are required for one LOI determination?

At least 15 conditioned specimens are normally needed to complete the bracketing procedure, and additional specimens are recommended for anisotropic reinforced laminates or when confirmation of reproducibility at the critical concentration is required.

Can thick laminates be tested?

The maximum specimen thickness is limited by the standard geometry; thick laminates are usually machined or sectioned to the standard dimensions, and the actual tested thickness, together with the laminate construction and fiber volume fraction, must be clearly reported because thickness influences the measured index.

Why does the LOI decrease at elevated temperatures?

As the ambient temperature rises, polymers pyrolyze more readily and less oxygen is needed to sustain combustion, so the oxygen index falls. ISO 4589-3 provides the procedure for determining this temperature dependence, which is important for applications where components operate in hot environments.