Determination of Limiting Oxygen Index (LOI) of Wood-Plastic Composites

The limiting oxygen index (LOI) test is one of the most widely used laboratory methods for evaluating the flammability of wood-plastic composites (WPC), quantifying the minimum oxygen concentration required to sustain candle-like combustion of a vertically mounted specimen. Performed in accordance with ISO 4589-2, ASTM D2863, or GB/T 2406.2, this small-scale test provides a reproducible, quantitative indicator of flame-retardant performance for WPC decking, cladding, railing, and other products. This article explains the test principle, specimen requirements, apparatus, procedure, result interpretation, and typical applications of LOI determination for wood-plastic composite materials.

Technical Overview

Wood-plastic composites (WPC) combine thermoplastic matrices such as HDPE, PP, or PVC with 30-70 wt% of wood flour or natural fibers. Because both the lignocellulosic filler and the polyolefin matrix are combustible, fire performance is a critical quality attribute for WPC used in construction and interior applications.

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 at 23 ± 2 °C, that will just support flaming combustion of a vertically held specimen. The pass/fail criterion is candle-like burning for 180 seconds or longer, or propagation of the flame front to a mark 50 mm below the top of the specimen.

Since ambient air contains approximately 20.9% oxygen, the LOI value provides an immediate indication of ignition behavior:

• LOI below 21%: the material continues to burn readily in normal air;
• LOI between 21% and 26%: the material burns slowly or shows a tendency to self-extinguish;
• LOI of 26% or higher: the material is generally considered to have flame-retardant, self-extinguishing characteristics.

The primary standards governing the method are 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), and GB/T 2406.2, which is technically aligned with ISO 4589-2. ISO 4589-3 extends the technique to elevated temperatures and defines parameters such as the temperature index. The test is well suited to formulation screening and quality control, although it is not by itself a fire hazard classification and is typically complemented by reaction-to-fire tests such as EN ISO 11925-2, EN 13823, or ASTM E84 depending on the regulatory context.

Testing Samples

Standard Laboratory Specimens

For ISO 4589-2 and GB/T 2406.2, specimens are typically 80-150 mm long, 10 ± 0.5 mm wide, and 4-10 mm thick, with 100 mm × 10 mm × 4 mm being a common configuration. ASTM D2863 uses specimens of 70-150 mm in length, 6.5 ± 0.5 mm in width, and 3 ± 0.25 mm in thickness for rigid materials under Method A. Specimens must be free of cracks, burrs, and ragged edges, and surfaces should not be altered beyond the necessary cutting or machining, since surface condition can influence ignition behavior.

Specimens Taken from Finished WPC Products

Samples may be sawn or milled directly from extruded decking boards, cladding panels, railing profiles, or injection-molded parts. Where the product thickness exceeds the standard range, the specimen is machined down to a thickness within 4-10 mm, retaining the original exposed surface when the evaluation targets the product face. The sampling location, orientation, and machining history should be recorded in the report.

Conditioning Prior to Test

Specimens are conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity, generally for a minimum of 88 hours per ISO 4589-2 or at least 40 hours per ASTM D618 practice referenced by ASTM D2863. Conditioning is especially important for WPC because absorbed moisture can measurably raise the apparent LOI; comparing oven-dried and conditioned specimens is a useful supplementary check. Typically 10-15 specimens per material are required to complete the full bracketing sequence.

Testing Items

The core deliverable of the test is the ambient-temperature limiting oxygen index value; accredited third-party laboratories typically report the following:

• Limiting oxygen index (LOI) at 23 °C, expressed to one decimal place as a volume percentage;
• Oxygen index at elevated temperatures and the temperature index (optional, per ISO 4589-3);
• Qualitative burning observations, including afterflame behavior, flaming drips, char formation, smoke release, and whether the flame front reached the 50 mm mark;
• Complete specimen documentation: dimensions, source, surface condition, conditioning regime, and the standard applied;
• Conformity statements against specification limits, for example a required LOI of 26% or higher for flame-retardant WPC grades.

Testing Methods

Apparatus

A dedicated LOI analyzer consists of a vertical heat-resistant glass chimney (typically 450-500 mm high with an internal diameter of 75-100 mm), a specimen holder that clamps the specimen vertically on the chimney axis, and precise mass flow controllers for oxygen and nitrogen with an oxygen analyzer accurate to approximately ±0.1 vol%. The mixed gas flow is regulated to the linear velocity specified by the standard (approximately 40 ± 10 mm/s in the chimney per ISO 4589-2). A small, controlled diffusion flame, usually propane, serves as the ignition source.

Test Procedure

• Calibrate the gas flow controllers and oxygen analyzer, then mount the specimen so that its top edge is at least 100 mm below the top of the chimney;
• Select an initial oxygen concentration based on experience or a quick scoping test, and allow the gas mixture to purge and stabilize the chimney atmosphere, typically for at least 30 seconds;
• Apply the igniter flame to the top of the specimen for the specified duration (up to 5 seconds under ISO 4589-2 Method A), then withdraw it and observe the burning behavior;
• Judge the result: if flaming persists for 180 seconds or more, or the flame front reaches the 50 mm mark, the oxygen concentration is too high; if the specimen self-extinguishes beforehand, the oxygen concentration is too low;
• Adjust the oxygen concentration up or down in progressively smaller increments (typically narrowing to 0.2-0.5%) and repeat on fresh specimens until the pass/fail transition is bracketed;
• Verify the final concentration and calculate the LOI.

Calculation and Expression of Results

The limiting oxygen index is calculated from the volumetric flow rates of the two gases:

LOI (%) = 100 × V(O2) / [V(O2) + V(N2)]

where V(O2) and V(N2) are the volume flow rates of oxygen and nitrogen at the limiting condition. The result is reported to one decimal place together with the test standard, specimen geometry, conditioning details, and any relevant burning observations. Because LOI values depend on specimen thickness, density, and moisture content, comparisons between materials are only valid when these parameters are matched.

Applications

LOI determination supports the wood-plastic composite industry across the full product lifecycle:

• Formulation development: screening flame retardants such as ammonium polyphosphate (APP), aluminum trihydroxide (ATH), magnesium hydroxide, zinc borate, and intumescent systems, and optimizing their loading against cost and mechanical property targets;
• Matrix selection: comparing PE-based, PP-based, and PVC-based WPC, where PVC-based composites often achieve LOI values above 30% without additional retardants, while unmodified PE/PP-based grades typically fall in the range of 19-23%;
• Quality control: monitoring incoming raw materials and extrusion consistency, including the dispersion of flame-retardant masterbatches;
• Regulatory and specification support: providing supporting flammability data for WPC decking and railing qualified under ASTM D7032 or EN 15534, and for building products assessed under GB 8624 or EN 13501-1 classification frameworks;
• Material datasheets and export documentation: supplying the quantitative oxygen index value commonly requested by architects, specifiers, and OEM buyers;
• Academic and industrial research: benchmarking novel bio-fillers, coupling agents, and hybrid fire-retardant packages.

FAQ

What LOI value is considered flame retardant for WPC?

An LOI of 26% or higher is generally regarded as indicating self-extinguishing, flame-retardant behavior. Unmodified PE- and PP-based wood-plastic composites commonly measure 19-23%, while PVC-based composites and grades containing effective retardant packages can reach 28-35% or higher.

Does moisture in the wood fiber affect the result?

Yes. Absorbed water acts as a heat sink and diluent, artificially raising the measured LOI. This is why strict conditioning at 23 ± 2 °C and 50 ± 5% relative humidity is required, and why conditioned and oven-dry values should be distinguished when comparing data.

Are ISO 4589-2, ASTM D2863, and GB/T 2406.2 interchangeable?

The three standards are harmonized in principle and use the same oxygen-index concept, but they differ in default specimen dimensions and procedural details, so results should always be cited together with the standard applied. GB/T 2406.2 is aligned with ISO 4589-2, while ASTM D2863 is preferred in North American specifications.

How many specimens are needed and how long does a test take?

A complete bracketing sequence normally requires 10-15 specimens. Depending on the number of concentration steps, a single material determination typically takes 1-2 hours after specimen conditioning is complete.

Can an LOI result be used as a fire rating?

No. The LOI is a relative flammability indicator obtained under controlled small-scale conditions. Official fire performance classes, such as those under GB 8624 or EN 13501-1, require additional reaction-to-fire testing, including flame spread and heat release measurements, conducted by accredited third-party laboratories.

Why do thicker specimens sometimes show higher LOI values?

Greater specimen thickness increases the thermal mass and heat-sink effect, which can slow flame propagation and raise the oxygen demand. This is one reason the standards limit the nominal thickness range and why geometry-matched comparisons are essential for meaningful interpretation.