Limiting Oxygen Index (LOI) Testing of Plastic Films: Principles, Standards, and Procedures

Limiting Oxygen Index (LOI) testing is the internationally recognized method for quantifying how readily plastic films and other polymeric materials burn. Performed in accordance with ISO 4589-2, ASTM D2863, and equivalent national standards, the test determines the minimum oxygen concentration in a flowing oxygen–nitrogen mixture that will just support candle-like combustion of a vertically mounted specimen. This article explains the test principle, sample preparation requirements for flexible films, the step-by-step procedure, typical results for common film materials, and the industries that rely on LOI data.

Technical Overview

The limiting oxygen index (LOI), also called the oxygen index (OI) or critical oxygen index, is defined as the minimum concentration of oxygen, expressed as a volume percentage in a mixture of oxygen and nitrogen, that will just support flaming combustion of a material under specified test conditions. Because ambient air contains approximately 20.9% oxygen, the LOI value immediately indicates whether a plastic film will continue to burn once ignited under normal atmospheric conditions.

During the test, a specimen is mounted vertically in a heat-resistant glass column while a precisely controlled oxygen–nitrogen mixture flows upward past it. The top of the specimen is ignited with a small gas flame, and the operator records whether combustion is sustained beyond defined time and distance criteria. By systematically adjusting the oxygen concentration between trials, the boundary between "burning" and "non-burning" behavior is bracketed, and the LOI is calculated from that boundary.

• LOI below approximately 21%: the film continues to burn easily in normal air after ignition.
• LOI between approximately 21% and 27%: the film shows slow-burning or self-extinguishing tendencies in air, but performance is marginal.
• LOI of 27% to 28% and above: the material is generally regarded as flame-retardant and will typically not sustain combustion in ambient air.

Plastic films deserve special attention because their thin cross-section, flexibility, and high surface-to-mass ratio make them behave differently from rigid plastics. Many films melt, shrink, curl, or produce flaming drips during the test, so the relevant standards define specific specimen preparation methods, such as rolling film strips into self-supporting spirals, to obtain reproducible and comparable results.

Testing Samples

Standard Thermoplastic Films

Testing laboratories routinely evaluate LOI on films made from polyethylene (PE), polypropylene (PP, including CPP, IPP, and BOPP), polyethylene terephthalate (PET and BOPET), polyamide (PA6 and PA66), polycarbonate (PC), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), ethylene-vinyl acetate (EVA), thermoplastic polyurethane (TPU), and cellulose acetate. Common film thicknesses range from a few micrometers up to several hundred micrometers.

Flame-Retarded and Modified Films

Flame-retarded film grades, including formulations containing halogenated compounds, phosphorus-based additives, inorganic fillers such as aluminum trihydrate or magnesium hydroxide, and intumescent systems, are frequently submitted to verify whether a target LOI threshold has been achieved. Inherently flame-resistant polymer films such as PVC, PVDF, PTFE, and polyimide are also tested for quality control and datasheet documentation.

Coated, Laminated, and Multilayer Films

Coated films, adhesive or extrusion laminates, metallized films, and multilayer composite constructions can be tested, but the complete construction must be evaluated as a system. The burning behavior of a laminate is not simply the average of its layers, so each commercial construction should be tested in its final form.

Sample Preparation and Quantity

Flexible films that cannot stand upright on their own are prepared by winding narrow strips tightly around a smooth mandrel to form a cylindrical spiral specimen; where necessary, a support rod or wire is retained during the test as permitted by the standard. Specimens are conditioned prior to testing, typically at 23 ± 2 °C and 50 ± 5% relative humidity, until moisture equilibrium is reached. A complete LOI determination normally consumes at least 15 to 20 specimens, so a film sheet of approximately A4 size is usually sufficient. Because molecular orientation can influence burning behavior, machine-direction and transverse-direction specimens should be distinguished and reported.

Testing Items

The core deliverable of the test is a single quantitative result, supported by qualitative observations that help interpret flame performance:

• Determination of the limiting oxygen index at ambient temperature, expressed as a volume percentage of oxygen.
• Optional elevated-temperature oxygen index determination in accordance with ISO 4589-3 for materials used at higher service temperatures.
• Observation of melting, shrinkage, flaming drips, afterflame and afterglow behavior, char formation, and smoke release during the test.
• Assessment of the measured LOI against customer, regulatory, or industry specification limits, such as a requirement of LOI ≥ 28%.
• Comparative ranking of candidate formulations or suppliers for research, development, and quality control purposes.

Testing Methods

Applicable Standards

• ISO 4589-2: the international standard for determination of burning behavior by oxygen index at ambient temperature.
• ASTM D2863: the North American standard for measuring the minimum oxygen concentration supporting candle-like combustion of plastics.
• GB/T 2406.2: the Chinese national standard, technically aligned with ISO 4589-2.
• JIS K 7201-2: the Japanese standard covering the same principle.
• ISO 4589-3: an extension of the method for determining the oxygen index at elevated temperatures.

Results obtained under different standards are broadly comparable, but minor differences in apparatus, ignition procedure, and specimen geometry can cause small discrepancies. The test report must therefore always identify the standard used.

Test Apparatus

A typical oxygen index apparatus consists of a vertical borosilicate glass chimney, a specimen holder, regulated oxygen and nitrogen gas supplies with precision flow control, a calibrated oxygen analyzer or calibrated flow measurement system, and a small igniter tube fed with propane or another suitable fuel. The combined gas flow is controlled to a defined linear velocity through the column, commonly around 40 mm/s, to ensure stable and reproducible combustion conditions. A timing device is used to verify the burning criteria.

Test Procedure

• Condition the specimens and record material identification, thickness, and orientation.
• Prepare the film specimen, for example as a rolled spiral, and mount it vertically in the holder with the specimen top positioned correctly below the top of the chimney.
• Set an initial oxygen concentration based on an estimate of the material's flammability, purge the column thoroughly, and allow the gas flow to stabilize.
• Apply the ignition flame to the top of the specimen for a short interval, typically up to 5 seconds, repeating briefly if ignition does not occur.
• Observe the response: the trial is recorded as "burning" if flaming combustion exceeds the specified time criterion, commonly 180 seconds, or if the specimen is consumed beyond the specified distance criterion, commonly 50 mm.
• Adjust the oxygen concentration upward or downward in response to the previous result, using progressively smaller increments in the final bracketing stage, until the boundary between burning and non-burning is established within the required tolerance.
• Confirm the boundary with repeated specimens and calculate the LOI, normally reported to one decimal place as a percentage of oxygen by volume.

Typical LOI Values for Common Film Materials

• Polyethylene (PE) film: approximately 17–18%.
• Polypropylene (PP) film: approximately 17–18%.
• Polyethylene terephthalate (PET) film: approximately 20–23%.
• Polyamide (nylon) film: approximately 24–27%.
• Polycarbonate (PC) film: approximately 25–28%.
• Rigid PVC film: approximately 40–50%; plasticized flexible PVC is lower due to plasticizer content.
• PVDF film: approximately 43–46%.
• PTFE film: above 95%.

These values are indicative only. Film thickness, additives, plasticizers, fillers, orientation, and thermal history all influence the measured result, which is why LOI must always be determined on the actual commercial film rather than inferred from generic polymer data.

Applications

LOI testing of plastic films supports material qualification, regulatory compliance, and product development across many sectors:

• Building and construction: vapor barrier films, roofing underlayments, membrane materials, and architectural films used in wall and ceiling assemblies.
• Electrical and electronics: insulation films, flexible printed circuit substrates, capacitor films, battery separator films, and cable wrapping tapes where flame performance is specified.
• Transportation: interior trim films, decorative laminates, and adhesive films used in automotive, rail, and aviation applications governed by flammability requirements.
• Renewable energy: photovoltaic encapsulant and backsheet films, where flame retardancy contributes to module safety certifications.
• Public-space products: certain national regulations, such as Chinese requirements for fire-retardant products used in public places, specify minimum LOI levels, for example LOI ≥ 32% for qualifying flame-retardant plastic products.
• Research and quality control: screening flame-retardant formulations, benchmarking suppliers, and monitoring production consistency of flame-retarded film grades.

FAQ

What LOI value is considered flame retardant for a plastic film?

As a practical guideline, films with an LOI of 27% to 28% or higher are generally considered flame-retardant because they will not sustain combustion in ordinary air, which contains about 21% oxygen. Many industry specifications set thresholds between 26% and 32% depending on the application and regulatory context.

Which standards govern LOI testing of plastic films?

The principal standards are ISO 4589-2, ASTM D2863, GB/T 2406.2, and JIS K 7201-2, all of which apply the same fundamental oxygen-index principle at ambient temperature. ISO 4589-3 extends the method to elevated temperatures. The specific standard should always be agreed between the client and the testing laboratory before testing begins.

How are flexible films that cannot stand upright tested?

Thin, limp films are wound into tight spiral cylinders around a mandrel, and the mandrel is either withdrawn before testing or retained as a support where the standard permits it. This preparation gives the specimen sufficient rigidity and geometry for the vertical burning assessment to be carried out reproducibly.

How many specimens are needed, and how much film should be submitted?

A full determination typically requires 15 to 20 specimens, and additional material is recommended for repeat determinations or method-development trials. In practice, a film sheet of approximately A4 size is usually more than enough for a complete test.

Does a high LOI mean the film is safe in a real fire?

No. LOI is a small-scale laboratory ranking tool that measures ease of ignition and sustained burning under controlled conditions. It does not by itself predict full-scale fire hazard, heat release, smoke toxicity, or flame spread. LOI data should be complemented by other tests, such as UL 94 (including the VTM procedures for thin films), cone calorimetry, or application-specific fire performance standards.

How does LOI differ from the UL 94 test?

LOI provides a continuous quantitative value representing the oxygen concentration needed to sustain burning, which is useful for comparing materials and tracking formulation changes. UL 94 is a pass/fail or ranked classification of burning behavior in still air. The two methods measure related but distinct properties, and many specifications request both.

Who should perform the test?

Testing can be carried out by manufacturers with properly equipped in-house laboratories or by accredited third-party laboratories. For regulatory submissions, customer qualification, or certification support, results from an accredited laboratory are generally preferred. Reports should state the standard used, specimen preparation details, thickness, conditioning, and all relevant burning observations.

What factors affect the accuracy of the result?

Key influences include the purity of the gases, calibration of flow controls and the oxygen analyzer, cleanliness of the chimney, consistent specimen preparation and dimensions, ambient temperature control, and consistent operator judgment of the burning criteria. Periodic verification of the apparatus using reference materials with known oxygen index values is recommended good practice.