Carpet Oxygen Index (LOI) Flame Retardancy Testing: Standards, Methods and Applications

Carpet oxygen index flame retardancy testing measures the limiting oxygen index (LOI) of carpet fibers, backing materials and finished textile floor coverings, providing a quantitative indicator of how readily a carpet will burn. Performed in accredited third-party laboratories in accordance with standards such as GB/T 5454, ISO 4589-2 and ASTM D2863, the test is widely used for fire safety compliance, flame-retardant product development and quality control. This article explains the testing principle, sample requirements, procedures, evaluation criteria and typical applications of oxygen index testing for carpets.

Technical Overview

The limiting oxygen index (LOI), also called the oxygen index (OI), 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. Since ambient air contains approximately 20.9% oxygen, a material whose LOI is well above this level will tend to self-extinguish once an external ignition source is removed, because the oxygen available in normal air is insufficient to sustain flaming combustion.

For carpets and other textile floor coverings, oxygen index testing serves as a small-scale, quantitative and highly reproducible screening method for flame retardancy. It is used to evaluate face fibers, yarns, primary and secondary backing materials, binder compounds and flame-retardant finishes. While full fire classification of a finished carpet assembly generally requires additional reaction-to-fire tests (such as critical radiant flux or surface flammability tests), the LOI value remains one of the most widely quoted indicators for comparing the intrinsic ignitability of carpet materials and for verifying the effectiveness of flame-retardant treatments.

As a general interpretation framework used across the industry:

• LOI below 21%: the material burns readily in ambient air and is considered easily flammable.
• LOI between 21% and 27%: the material burns slowly or shows partial self-extinguishing behavior.
• LOI above 27%: the material is generally regarded as flame retardant, since it cannot sustain combustion in normal air.

Acceptance thresholds of 26% to 28% are commonly specified in procurement contracts, technical specifications and flame-retardant product requirements, depending on the jurisdiction and end use.

Testing Samples

Synthetic Fiber Carpets

Carpets made from polypropylene (PP, including BCF and solution-dyed fibers), polyamide (nylon 6 and nylon 6,6), polyester (PET, including recycled variants) and acrylic fibers represent the largest share of the commercial market. These polymers have relatively low natural oxygen indices, typically ranging from approximately 17% for polypropylene to around 21% to 24% for nylon, so flame-retardant masterbatches, finishes or structural modifications are frequently evaluated through LOI testing.

Natural Fiber and Wool Carpets

Wool and wool-rich carpets exhibit comparatively favorable behavior, with wool typically showing an oxygen index of around 25% to 26% due to its high nitrogen and moisture content. Wool-nylon blends, cotton-containing products and plant-fiber carpets such as jute or seagrass can also be characterized to document their burning behavior and the effect of durable flame-retardant treatments.

Carpet Components and Raw Materials

Beyond finished carpet, oxygen index testing is routinely applied to individual components, including face yarns, primary woven or nonwoven backing, secondary backing fabrics, latex or polyurethane binder compounds, adhesives and flame-retardant additives. Testing raw materials separately helps identify which layer governs the overall flammability of the finished floor covering.

Specimen Preparation and Conditioning

Specimens are cut in specified dimensions according to the applicable standard, typically around 140 mm to 150 mm in length and 52 mm to 58 mm in width for flexible textile materials, with thickness recorded as received. Flexible specimens may require a support frame or specimen holder to keep them vertical during the test. Unless otherwise agreed, samples are conditioned to equilibrium in a standard atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity before testing. A sufficient number of coupons, commonly fifteen or more, should be submitted because the bracketing procedure consumes multiple specimens.

Testing Items

The core deliverable of the test is the limiting oxygen index value, but accredited third-party laboratories typically record supporting observations that complete the flammability picture:

• Limiting oxygen index (LOI) expressed as a volume percentage of oxygen in the O2/N2 mixture.
• Ignition behavior, including ease of ignition and flame propagation along the specimen.
• After-flame time and after-glow time following removal of the ignition source.
• Melting, sputtering, shrinking and flaming drip behavior during combustion.
• Char formation, damaged length and burn-through characteristics.
• Comparative LOI results before and after flame-retardant treatment, or after washing and durability conditioning when required by the specification.

These items together allow manufacturers to demonstrate compliance, benchmark formulations and support claims of flame-retardant performance in technical documentation.

Testing Methods

Applicable Standards

• GB/T 5454: the Chinese national standard for the determination of burning behavior of textiles by the oxygen index method, applied to carpet fibers and textile components.
• ISO 4589-2 and its Chinese counterpart GB/T 2406.2: determination of burning behavior by oxygen index for plastics and polymeric materials, applicable to backing sheets, binder compounds and polymer granules.
• ASTM D2863: the North American standard method for measuring the minimum oxygen concentration to support candle-like combustion.

Apparatus and Test Principle

The test is conducted in an oxygen index apparatus consisting of a heat-resistant glass chimney (typically 450 mm in height), a specimen holder that supports the sample vertically in the center of the column, a precision gas mixing and flow control system for oxygen and nitrogen, and a small ignition burner. Gas flows upward through the column at a controlled velocity, creating a stable, adjustable atmosphere around the specimen. By systematically varying the oxygen concentration, the operator identifies the exact borderline condition at which combustion is just sustained, which defines the LOI.

Test Procedure

• Condition and measure the specimens, then mount one vertically in the holder within the chimney.
• Purge the column with the oxygen and nitrogen mixture at the specified flow rate to establish a stable atmosphere.
• Apply the ignition flame to the top of the specimen for the prescribed period and withdraw the burner.
• Observe whether burning meets the judgment criterion, typically sustained flaming for a set duration such as 180 seconds or flame spread over a defined distance such as 50 mm, depending on the standard.
• Adjust the oxygen concentration upward or downward using a bracketing sequence across multiple specimens until the critical concentration interval is narrowed.
• Calculate the LOI as 100 multiplied by the oxygen volume divided by the total gas volume, and report the result to one decimal place.

Complementary Flammability Tests for Carpets

Because LOI is a screening method on material specimens, finished carpets are usually also assessed with full-scale reaction-to-fire tests. Commonly paired methods include the critical radiant flux test for flooring (GB/T 11785, aligned with ISO 9239-1), the methenamine pill ignition test for textile floor coverings (GB/T 11049, aligned with ISO 6925, and the corresponding US regulatory requirement under 16 CFR 1630), smoke density testing, and classification under GB 8624 or EN 13501-1 for building applications. Combining LOI data with these tests provides a complete flammability profile for regulatory submission.

Applications

Oxygen index testing of carpets supports a broad range of regulatory, commercial and technical objectives:

• Fire safety compliance for carpets installed in public assembly and high-occupancy locations such as hotels, theaters, cinemas, hospitals, schools, airports and high-rise buildings, where flame-retardant performance is mandated.
• Flame-retardant product marking and conformity documentation demonstrating that materials meet specified LOI thresholds in purchase contracts and engineering specifications.
• Export compliance support, where buyers in different markets require reaction-to-fire data aligned with EN 13501-1, GB 8624 or North American flammability requirements.
• Research and development of flame-retardant fiber modifications, halogen-free and phosphorus-based finishes, intumescent coatings and FR-loaded backing formulations.
• Incoming quality control and supplier verification, ensuring that flame-retardant masterbatches and treated yarns consistently meet defined oxygen index targets across production batches.
• Fire hazard assessments, insurance evaluations and tender documentation requiring objective, laboratory-measured flammability data.

FAQ

What oxygen index value qualifies a carpet as flame retardant?

As a widely accepted convention, materials with an LOI above 27% are considered flame retardant because they will not sustain combustion in ambient air. Many project specifications and flame-retardant product requirements set acceptance thresholds between 26% and 28%, so the applicable limit should always be confirmed against the specific standard, contract or regulation governing the installation.

Is oxygen index testing alone sufficient to certify a carpet?

No. LOI is a small-scale material screening test. Regulatory classification of a finished carpet normally requires additional reaction-to-fire tests such as critical radiant flux (GB/T 11785 / ISO 9239-1), the methenamine pill test (GB/T 11049 / ISO 6925) and, for building products, classification under GB 8624 or EN 13501-1. LOI data complement these tests by characterizing the intrinsic flammability of the constituent fibers and backings.

How are specimens taken from a finished carpet?

Coupon sets are usually cut with the pile direction recorded, and testing can be performed on the full carpet construction or on separated layers such as face yarn and backing, depending on the purpose of the evaluation. Separating layers is particularly useful for diagnosing which component limits the overall flame-retardant performance.

How much sample is needed and how long does testing take?

A typical submission consists of enough material to cut fifteen or more specimens of the required dimensions, which usually corresponds to a few dozen grams of fiber or a small rectangle of carpet. After standard conditioning, testing and reporting by an accredited third-party laboratory generally take three to seven working days.

Why do different fibers show very different LOI values?

Oxygen index is governed by polymer chemistry and composition. Polypropylene naturally sits near 17% to 18% and burns readily, nylon and polyester fall roughly between 20% and 24%, while wool achieves around 25% to 26% due to its nitrogen and moisture content. Flame-retardant additives, finishes and fiber modifications are specifically designed to raise these values above the self-extinguishing threshold, which is exactly what the test verifies.

Are LOI results comparable between laboratories?

When testing is performed under the same standard with properly calibrated gas mixing systems and conditioned specimens, results from accredited third-party laboratories are closely comparable, typically agreeing within one to two percentage points. Differences in specimen orientation, conditioning history and judgment criteria are the main sources of variation, which is why reporting should always state the standard and specimen details.