What Is Limiting Oxygen Index (LOI)? A Core Indicator of Flame-Retardant Performance

Limiting Oxygen Index (LOI) is one of the most critical indicators for evaluating the flame retardancy of textiles and polymer materials. This article aims to deconstruct the safety significance behind this professional value from the perspective of a textile R&D expert, covering physical definitions, calculation logic, testing standards, and industrial applications. For organizations procuring high-safety flame-retardant fabrics, understanding LOI is not only about regulatory compliance but also about finding the optimal balance between performance and sustainability. Simply put, the Limiting Oxygen Index refers to the minimum oxygen concentration required to maintain steady combustion of a material in an oxygen-nitrogen mixture; the higher the value, the stronger the flame retardancy of the material.

What is Limiting Oxygen Index?

The Limiting Oxygen Index refers to the minimum concentration of oxygen in an oxygen-nitrogen mixture required for a material to just maintain a state of combustion under specified experimental conditions. It serves as a physical judgment benchmark for evaluating the ease of ignition of textiles and is typically expressed as a volume percentage of oxygen.

The core physical nature of LOI

In the atmospheric environment, the concentration of oxygen is approximately 21%. If a material’s LOI is below 21%, it means it ignites easily and burns continuously in natural air. Conversely, if the LOI is significantly higher than 21%, the material requires additional oxygen to maintain combustion, a characteristic that provides self-extinguishing properties under normal atmospheric conditions.

• Self-extinguishment: The material stops flaming combustion once the fire source is removed due to insufficient oxygen supply.
• Stability: As a quantitative analysis method, LOI provides a more precise grade classification than traditional horizontal burning methods.

The mathematical calculation logic of the Limiting Oxygen Index

In a laboratory environment, LOI is not determined by visual estimation but through precise control of oxygen and nitrogen flow rates using mathematical calculation logic. This value directly reflects the dynamic balance between the rate of combustible gas generation after thermal decomposition and oxygen consumption.

Description of the physical calculation formula

According to relevant technical standards, the calculation formula for the Limiting Oxygen Index is as follows:

1. [O2] represents the volumetric flow rate of oxygen in the mixed gas.
2. [N2] represents the volumetric flow rate of nitrogen in the mixed gas.

By adjusting the flow meters in the glass chimney, technicians find a critical point where the specimen can continue burning for 2 minutes or the damaged length is exactly 40 mm. The oxygen percentage recorded at this moment is the final result.

What does the LOI level mean?

The magnitude of the LOI value directly determines the ignition difficulty and the ability to sustain combustion of a material under specific environmental conditions. A higher numerical value indicates that a material is more resistant to ignition in low-oxygen environments and exhibits superior self-extinguishing properties once the external heat source is removed; conversely, a lower value signifies a higher dependency on lower oxygen levels, making the material highly prone to continuous combustion in standard air. This quantitative metric eliminates the qualitative ambiguity of “fireproof” and provides a precise classification for safety-critical venues.

Table 1: Material Combustion Performance Classification via LOI Intervals

Classification	LOI Range (%)	Physical Judgment	Application Example
Highly Flammable	< 20%	Rapidly ignites and cannot self-extinguish in air.	Untreated Cotton / Rayon fabrics.
Combustible	20% – 26%	Supports steady combustion in standard atmosphere.	Standard Industrial Polyester.
Flame Retardant (FR)	26% – 34%	Self-extinguishes; provides high safety margins.	Begoodtex IFR Blackout Fabrics.
Non-combustible	> 35%	Extremely difficult to ignite; primarily chars.	Aramid / Glass Fiber composites.

Procurement logic behind the numbers

In vertical industry procurement, minor differences in LOI values represent fundamental material distinctions:

• 21% LOI: The combustion boundary. Below this, materials act as fuel for fire in normal air.
• 28% LOI: The “Safety Threshold” often required by international standards like BS 5867 to ensure anti-dripping and limited flame spread.
• 32% LOI: Begoodtex Internal Safety Standard. This level of oxygen dependency ensures that even in complex fire scenes with elevated temperatures or air convection, the fabric remains physically stable.

Technical comparison of international standards ISO 4589 and ASTM D2863

While the basic principles of LOI are universal, specific testing standards formulated for textiles in different regions vary in specimen size, ignition methods, and judgment criteria. Flame-retardant products developed and produced by Begoodtex usually pass multiple standard verifications to ensure global market compatibility.

Technical parameters of ISO 4589 and ASTM D2863

ISO 4589 is currently the internationally recognized general principle for oxygen index testing of plastics and fabrics, while the American standard ASTM D2863 focuses more on dynamic monitoring of the ignition process. In China, GB/T 5454-1997 “Textiles – Burning behaviour – Oxygen index method” is typically implemented.

Standard execution detail comparison

• Specimen Specifications: Different standards require varying specimen sizes, usually 150 mm × 58 mm.
• Igniter Requirements: Propane or butane burners with flame heights strictly controlled between 15 mm and 20 mm.

Table 2: Brief Comparison of Technical Parameters for Mainstream LOI Standards

Standard Number	Scope of Application	Core Judgment Indicator	Environmental Requirements
ISO 4589-2	Plastics, fabrics, films	Oxygen concentration for 180s burn	Room temp 23±2 degrees Celsius
GB/T 5454	Chinese textile standards	Oxygen for 2min burn or 40mm damage	Humidity 65±5% pre-treatment
ASTM D2863	Polymers, plastics	Minimum oxygen for specific burn rate	Extremely high flow control precision

How is a standard LOI test performed?

Executing a complete oxygen index test requires a rigorous laboratory process. Any neglect in operational details, such as environmental humidity control or verticality of specimen clamping, can lead to a deviation in the final LOI value by 0.5% to 1.5%.

Standardized testing steps

1. Specimen Pre-treatment: Specimens are treated in a constant temperature and humidity box for over 24 hours to eliminate moisture interference with combustion heat.
2. Mixed Gas Input: Adjust oxygen and nitrogen valves to set an estimated initial oxygen concentration.
3. Top Ignition: Use a specialized igniter to ignite the top center of the specimen downward.
4. Observation and Recording: Monitor the duration and length of the specimen’s combustion.
5. Stepwise Approximation: Adjust oxygen concentration in 0.2% increments based on performance until the critical equilibrium point is found.

An overview of LOI data for different textile materials

Understanding the LOI values of various base fibers helps buyers identify the flame-retardant potential of fabrics from the source. The Inherent Flame Retardant (IFR) technology focused on by Begoodtex injects efficient flame-retardant elements during the polymerization stage, making its values far exceed those of ordinary synthetic fibers.

Table 3: Comparison of Measured LOI Values for Common and Functional Fibers

Fiber Type	Measured LOI Range (%)	Combustion Morphology
Virgin Cotton	17.0 – 18.5	Burns rapidly with red-hot smoldering
Standard Polyester	20.0 – 22.0	Melts while burning, produces droplets
Wool	24.0 – 25.0	Natural flame resistance, foams and chars
Aramid	28.0 – 30.0	Heat resistant, non-melting, self-extinguishing
Begoodtex IFR Fabric	32.0 – 35.0	Highly stable, self-extinguishes, no droplets

Will environmental factors interfere with test results?

The micro-environment of the laboratory has a significant intervention effect on LOI test results. In actual R&D and production, the technical team at Begoodtex must exclude variables like temperature and humidity to ensure the authenticity and authority of the factory data.

Microscopic effects of temperature and humidity

When the laboratory temperature rises, the thermal motion of molecules within the fiber intensifies, and the activation energy for cracking into combustible gases decreases, leading to a lower measured LOI. Similarly, high humidity increases moisture content in the specimen; as moisture evaporates, it absorbs substantial combustion heat, causing the measured LOI to be higher.

Environmental control points

• Temperature Control: The laboratory must be maintained at a standard 23 degrees Celsius.
• Humidity Balance: Specimens must reach moisture equilibrium at a specific humidity to prevent moisture from acting as a “temporary flame retardant.”

The safety logic behind Begoodtex’s pursuit of high LOI

Why does Begoodtex insist on producing flame-retardant fabrics with an LOI maintained above 32%? This is not just to meet testing standards but is based on safety redundancy considerations for extreme fire scenarios. High oxygen index means the fabric has a longer self-rescue time and lower risk of secondary fire in fire environments with fluctuating oxygen concentrations.

Advantages of high-performance IFR fabrics

Traditional after-finish flame-retardant fabrics may have an initial LOI of over 30%, but as wash cycles increase and flame retardants leach out, the LOI can quickly drop below the 24% safety threshold. Begoodtex’s core technology lies in molecular-level modification:

• Permanence: After more than 50 industrial washes, the LOI value fluctuation is less than 1%.
• Environmental Safety: No toxic halogen gases are produced during combustion, with extremely low smoke emission, meeting green environmental export requirements.

How does fabric structure affect the oxygen index?

Even with the same flame-retardant fiber, the physical structure of the fabric (weave, density, weight) can cause differences in LOI. This is a comprehensive manifestation of physical barrier effects and air permeability in textile engineering.

The dual role of weight and density

Generally, for fabrics of the same material, the greater the weight and thickness, the higher the LOI value. This is because thicker fabrics have larger heat capacities, capable of absorbing more feedback heat, and dense structures limit the rate of oxygen penetration into the fabric’s interior.

Structural factor analysis

• Plain vs. Satin: Satin weaves may burn faster than stable plain weaves in some cases due to long float lines.
• Loft/Pile: Pile fabrics often face stricter challenges than flat fabrics because they contain a large amount of air (oxygen source).

Necessary indicators for comprehensive assessment of flame retardancy

While the Limiting Oxygen Index is a key indicator, it is a laboratory test conducted in a static environment and cannot fully represent the full scope of a fire outbreak. In Begoodtex’s quality system, LOI must be analyzed collaboratively with other testing indicators.

Multi-dimensional evaluation system

Simply pursuing an extremely high LOI (e.g., over 40%) might sacrifice fabric hand-feel and strength. Therefore, professional flame-retardant evaluation must combine the following parameters:

1. Vertical Burn Test: Evaluates char length, afterflame time, and afterglow time (e.g., NFPA 701 standard).
2. Smoke Toxicity Test: Evaluates the concentration of toxic substances emitted during combustion.
3. Heat Release Rate (HRR): Evaluates the energy released per unit time.

Summary

The Limiting Oxygen Index (LOI) is the technical “yardstick” of the flame-retardant textile industry. By maintaining this value at a high level of 32% or above, Begoodtex has successfully established a solid technical barrier in the field of public safety protection. For buyers, while focusing on LOI, more attention should be paid to the durability of this value and the comprehensive performance of the material in actual fire scenarios, thereby providing more reliable safety protection for vertical application scenes.

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