When specifying flame retardant (FR) fabric, you’ll encounter terms like “inherent,” “treated,” and various safety standards like NFPA 2112. But one of the most fundamental and scientific metrics is the Limit Oxygen Index (LOI) in FR Fabrics.
Understanding LOI is key to making an informed decision about the true fire-resistant capabilities of a fabric. So, what exactly is it, and why does it matter?
What is LOI? A Simple Definition
The Limit Oxygen Index (LOI) is defined as the minimum concentration of oxygen in an oxygen-nitrogen mixture required to support the candle-like burning of a material.
In simpler terms: LOI measures how easy it is for a fabric to burn. The higher the LOI value, the more oxygen is needed to ignite the material, meaning it is harder to burn.
Low LOI (e.g., 18-21): The fabric will burn easily in normal air (which contains approximately 21% oxygen).
High LOI (e.g., 28+): The fabric will stop burning on its own because normal air doesn’t have enough oxygen to support combustion.
How is LOI Tested? (ASTM D2863)
The LOI test is performed according to the international standard ASTM D2863. Here’s how it works:
- A small sample of the fabric is placed vertically in a glass chimney.
- A controlled mixture of oxygen and nitrogen flows upwards through the chimney.
- The top of the sample is ignited.
- The operator adjusts the oxygen concentration until the minimum level required for the fabric to burn for 3 minutes or longer is found.
This precise percentage is the material’s LOI value.
Why LOI (Limit Oxygen Index) in FR Fabrics is so important？
While standards like NFPA 701 or EN ISO 11612 test fabric performance under intense heat and flame (simulating a flash fire), LOI is a fundamental property test. It tells you about the fabric’s inherent resistance to ignition.
- 1.Predicts Performance: A high LOI is a strong indicator of excellent flame retardancy. It’s a quick and effective way to screen materials.
- 2.Differentiates FR Types:
- (2.1)Treated FR Fabrics: These are typically cellulose-based fabrics (like cotton) that have been chemically treated to achieve flame resistance. Their LOI is usually in the range of 26-32, but this protection can diminish over time with washing and wear.
- (2.2)Inherent FR Fabrics: These are made from synthetic fibers (like meta-aramid, para-aramid, modacrylic, or FR viscose) that have flame resistance built into their molecular structure. They have a naturally high and permanent LOI, often in the range of 28-38.
- 3.Indicator of Self-Extinguishing Properties: A fabric with an LOI significantly above 21% will self-extinguish once the ignition source is removed because surrounding air cannot support continued burning.
LOI Values: A Practical Guide
|
LOI Value (%) |
Interpretation | Example Materials/ Applications |
| ~20.9 | Will burn freely in air. | Untreated cotton, polyester, nylon. |
| 21 – 25 | Some resistance, may burn in air. | Some lightly treated fabrics. |
| 26 – 27 | Good FR properties. Will not burn in normal air. | Many good quality treated cotton FR fabrics. |
| 28 – 31 | Excellent FR properties. Self-extinguishes. | Meta-aramid (e.g., Nomex), Modacrylic blends. Standard for many protective garments. |
| 32 – 35+ | Outstanding FR properties. Very difficult to ignite. |
Para-aramid (e.g., Kevlar), PBI, PBO (Zylon). Used in high-risk industrial, military, and racing applications. |
How to Use LOI When Selecting FR Fabric
- Look for a High LOI: For most industrial applications, an LOI of 28 or higher is a strong benchmark for effective flame retardancy.
- Prioritize LOI for Certain Applications: LOI is especially critical in environments with potential oxygen-enriched atmospheres (e.g., medical, aerospace, certain chemical plants) where the risk of fire is even greater than in normal air.
- Consider it Alongside Other Tests: LOI is a vital data point, but it’s not the only one. Always ensure the fabric also passes the required vertical flame tests (ASTM D6413) and full-scale thermal protective performance (TPP) tests for your specific industry standard (e.g., NFPA 2112, EN ISO 11612).
Conclusion: LOI is Non-Negotiable for True Safety
The Limit Oxygen Index is more than just a number on a spec sheet. It is a direct measure of a material’s intrinsic ability to resist burning. When evaluating flame retardant fabrics, always ask your supplier for the LOI value.
At HENAN SAFE-GUARD CO., LTD., we believe in transparency and education. All of our flame-retardant fabrics have a high LOI value, ensuring your protective garments are certified to the highest standards.
Ready to specify the safest fabric for your application? [Contact our technical experts today] for data sheets and to discuss your needs.
