Limiting Oxygen Index (LOI) Analysis of Plastic Films: Principles, Methods, and Applications

Limiting oxygen index (LOI) analysis is a fundamental laboratory method for evaluating the flammability of plastic films, quantifying the minimum oxygen concentration required to sustain combustion. Performed in accordance with ISO 4589-2, ASTM D2863, and GB/T 2406.2, LOI data support flame-retardant formulation development, quality control, and fire-safety compliance across packaging, electronics, construction, and transportation industries. This article explains the testing principle, sample preparation, test procedure, and practical applications of LOI analysis for plastic films.

Technical Overview

The limiting oxygen index (LOI), also known as the oxygen index (OI) or critical oxygen index (COI), 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. The LOI is calculated as LOI = 100 × V(O2) / [V(O2) + V(N2)], where V(O2) and V(N2) are the volumetric flow rates of oxygen and nitrogen. Because ambient air contains approximately 20.9% oxygen, a material with an LOI below 21% will continue to burn in normal air once ignited, whereas a material with an LOI well above 21% tends to self-extinguish under ambient conditions.

For plastic films, LOI analysis provides a quantitative, reproducible, and relatively economical means of ranking ignition resistance and flame-spread tendency. It is widely used to screen flame-retardant formulations, benchmark competing materials, and support compliance with fire-safety specifications. It should be noted, however, that LOI is a small-scale screening test; it does not by itself predict behavior in full-scale fires and is typically complemented by tests such as UL 94 vertical or horizontal burning, glow-wire testing (IEC 60695-2 series), or cone calorimetry (ISO 5660-1).

LOI values provide a convenient classification of burning behavior:

• LOI below 21%: readily flammable, sustains burning in normal air, typical of unmodified polyethylene and polypropylene films (LOI approximately 17–19%).
• LOI 21–25%: slow-burning, may self-extinguish depending on specimen geometry and heat feedback.
• LOI 26–34%: generally regarded as flame-retardant; many engineering and halogenated film materials fall in this range.
• LOI above 35%: highly flame-resistant materials, such as polyimide films (approximately 36–38%), PVDF films (approximately 43%), PVC films (approximately 45%), and PTFE films (well above 50%).

Testing Samples

LOI testing is applicable to virtually all thermoplastic and thermoset plastic films, including mono-layer, co-extruded, laminated, coated, metallized, and flame-retardant modified products. Because films are thin and flexible, special specimen preparation is required to produce self-supporting test pieces, as described in the Testing Methods section. Typical sample categories include:

Polyolefin Films

Low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), polypropylene (PP), and cast or biaxially oriented films (CPP, BOPP, BOPE), together with their flame-retardant modifications. These materials are intrinsically flammable, with LOI values of roughly 17–19%, and frequently require halogen-free or halogenated flame-retardant additives to reach LOI values of 26% or higher for construction, agricultural, and electrical applications.

Polyester and Engineering Films

Polyethylene terephthalate (PET/BOPET), polyethylene naphthalate (PEN), polycarbonate (PC), and polyamide (PA) films. PET films typically show LOI values of approximately 20–23%, while PC films reach approximately 25–30%. These films serve as electrical insulation, release liners, and graphic laminates where moderate flame retardancy is required.

Halogenated and High-Performance Films

Polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyimide (PI) films exhibit inherently high LOI values owing to halogen content or aromatic, char-forming molecular structures. PI films are standard in flexible printed circuits and aerospace insulation, while PVDF and PTFE films are used in demanding architectural and chemical environments.

Functional, Coated, and Laminated Films

Photovoltaic encapsulant films (EVA, POE), hot-melt adhesive films, heat-sealable coatings, metallized films, and multilayer barrier laminates. For laminated structures, the LOI of the composite can be determined provided the specimen is representative of the commercial product; coating layers and adhesives can measurably influence the result and should be reported.

Testing Items

The primary measurand is the ambient-temperature limiting oxygen index of the film material. A complete LOI test program typically reports the following items:

• Limiting oxygen index expressed as volume percent of oxygen, reported to the precision specified by the applied method.
• Specimen geometry and preparation method (rolled spiral, folded, or frame-supported configuration).
• Ignition method applied (top-surface ignition or propagation ignition).
• Burning behavior observations, including afterflame time, burning length, dripping, smoke release, and char formation.
• The oxygen concentration sequence and response pattern used to compute the final index.
• Conditioning regime and ambient test conditions (temperature and humidity).
• Optional determinations, such as elevated-temperature LOI per ISO 4589-3 or the temperature index, defined as the temperature at which the LOI falls to 20.9%.

Testing Methods

Applicable Standards

LOI testing of plastic films is performed under the following reference methods:

• 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 (Oxygen Index).
• GB/T 2406.2, the Chinese national standard technically equivalent to ISO 4589-2.
• JIS K 7201-2, the Japanese industrial standard corresponding to ISO 4589-2.
• ISO 4589-3, the extension of the method to elevated temperatures for temperature-index determination.

Specimen Preparation

Because most films are too thin and flexible to stand freely in the test chimney, specimens are converted into self-supporting geometries:

• Rolled spiral specimens: a film strip, commonly on the order of 100–150 mm long and 50–60 mm wide, is wound spirally around a thin mandrel of approximately 6 mm diameter to form a rigid cylinder, which is the preferred approach for flexible films.
• Folded or frame-supported specimens: the film may be folded into a self-supporting strip or mounted on a thin wire support frame where the applied standard permits support fixtures.
• Dimensional control: uniform width and length, cleanly cut edges free of cracks and wrinkles, and consistent rolling tightness across all specimens.
• Conditioning: at least 40 hours at 23 ± 2 °C and 50 ± 5% relative humidity in accordance with ISO 291, unless the material specification dictates otherwise.
• Sample quantity: a minimum of 15 conditioned specimens is generally required for one determination, with spare specimens recommended for verification runs.

Apparatus

A standard oxygen index apparatus comprises a heat-resistant glass chimney mounted vertically, a specimen holder that clamps the lower portion of the specimen, precision flow control for oxygen and nitrogen (or a premixing unit), calibrated flow measurement or a paramagnetic oxygen analyzer, and a small gas-flame igniter applied at the top of the specimen. The total mixed-gas flow is controlled to achieve the nominal upward velocity specified in the standard (approximately 40 mm/s per ISO 4589-2), ensuring a stable and reproducible atmosphere around the specimen.

Test Procedure

• Mount the conditioned specimen vertically in the holder with the exposed length required by the standard, and purge the chimney with the starting gas mixture for at least 30 seconds.
• Apply the ignition flame to the top of the specimen for up to the maximum time allowed by the method, then withdraw the igniter and start timing.
• Record whether the specimen burns for at least the specified period (typically 180 seconds per ISO 4589-2) or over at least the specified length (typically 50 mm); this outcome defines a reaction or no-reaction at that concentration.
• Adjust the oxygen concentration upward after a failure and downward after a pass, using the prescribed concentration steps, and repeat the sequence on fresh specimens until the transition pattern required by the standard is established.
• Compute the LOI from the final oxygen concentration, the increment, and the response-pattern factor using the relationship LOI = cF + k·d, where cF is the final oxygen concentration, d is the concentration increment, and k is a tabulated factor derived from the sequence of results.
• Report the LOI together with the test standard used, the specimen preparation method, conditioning history, and all burning-behavior observations.

Quality Assurance and Influencing Factors

Instrument calibration with certified oxygen and nitrogen mixtures, verified flow measurement, and a clean, undamaged chimney are prerequisites for valid results. Key factors that influence LOI values include film thickness and density, rolling tightness of spiral specimens, additive blooming or plasticizer migration, moisture conditioning, ambient temperature, igniter application time, and gas-flow stability. With controlled specimen preparation, within-laboratory repeatability is typically within about 0.5–1.0% oxygen, while reproducibility between laboratories may differ by 1–2% oxygen, consistent with the precision statements published in ISO 4589-2 and ASTM D2863.

Applications

LOI analysis of plastic films is applied across material development, quality assurance, and regulatory screening in the following areas:

• Flame-retardant formulation development: ranking flame-retardant masterbatches, synergists, and loading levels in polyolefin and EVA films, and verifying that target LOI thresholds (commonly 27–28% or higher) are achieved.
• Electrical and electronics: qualification of insulating films, transformer interleaving films, flexible printed circuit substrates, and components for battery packaging and electronic assemblies.
• Building and construction: fire-performance screening of vapor-barrier films, roofing membranes, façade components, and insulation facing laminates.
• Transportation: interior decorative films, adhesive films, and laminate layers for automotive, railway, marine, and aerospace interiors, where oxygen index data support material selection alongside dedicated fire-safety standards such as EN 45545-2 for railway applications.
• Wire and cable: preliminary evaluation of insulation, sheathing, and wrapping tape materials used in cable constructions.
• Quality control: incoming inspection of film lots, supplier benchmarking, and certification support against agreed material specifications.
• Research and failure analysis: studying the effects of thermal aging, UV exposure, weathering, and additive migration on the long-term flame retardancy of films.

FAQ

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

As a rule of thumb, films with an LOI of at least 26–28% are commonly specified as flame retardant, because they will not sustain combustion in normal air containing 20.9% oxygen. Many procurement specifications for construction and electrical films set minimum LOI requirements in the 27–32% range; the exact threshold depends on the product standard and end-use regulation.

How are thin, flexible films tested if they cannot stand upright?

Thin films are converted into self-supporting specimens, most commonly by rolling a strip spirally around a thin mandrel to form a rigid cylinder, or by folding the film or supporting it on a wire frame where the standard allows. The preparation method must be documented in the test report because it can measurably affect the result.

How does LOI testing differ from UL 94 testing?

LOI testing quantifies the minimum oxygen concentration needed to sustain burning in a controlled atmosphere and yields a continuous numeric value, making it ideal for material ranking and formulation screening. UL 94 (including the VTM classifications for thin films) evaluates burning behavior in ambient air against pass/fail criteria such as afterflame time, flaming drips, and cotton ignition. The two tests are complementary, and LOI is frequently used for screening before UL 94 or IEC 60695-11-10 verification.

Why do LOI results differ slightly between laboratories?

Differences typically arise from specimen preparation details such as rolling tightness and dimensions, conditioning history, calibration of gas flow and oxygen analysis, and operator judgment at the burn/no-burn transition. International precision studies in ISO 4589-2 and ASTM D2863 report within-laboratory repeatability of roughly ±0.5–1% oxygen and larger between-laboratory differences, so minor variation is statistically expected.

Can LOI be measured at elevated temperature?

Yes. ISO 4589-3 extends the ambient-temperature method to elevated temperatures, allowing determination of the temperature at which the LOI falls to 20.9%, known as the temperature index. This is valuable for films used in warm environments, such as electrical insulation operating near heat sources.

How much film is needed for one LOI determination?

A single determination normally consumes about 15–20 prepared specimens, with additional specimens advisable for verification runs. For spiral-rolled strips, this generally corresponds to a film area on the order of 0.1–0.2 square meters, depending on the strip dimensions and rolling configuration used.