ASTM D2863 outlines the standard test for measuring the minimum Oxygen Concentration to Support Candle-Like Combustion, referred to as the Limited Oxygen Index (LOI). This value has differing roles in the development and assessment of FR fibres. It is used to characterise the fibres, polymers, textiles and other material components. As this is a test method alone, it is essential to judge the resulting value in light of performance requirements from other standards for specific garments.
Defines the oxygen level needed for the test material to support combustion. It is important to note that this test method is used on various plastic-based materials, products, or assemblies. It measures and describes their response to heat and flame under controlled conditions.
The test method involves three procedures:
- Procedure A: Top surface ignition
- Procedure B: Propagating ignition
- Procedure C: Minimum value of oxygen index
The Limited Oxygen Index (LOI) value is measured in SI base units and expressed as the percentage of oxygen in the atmosphere to support combustion. LOI does not indicate the behaviour of fabric in regards to liquid metal or metal droplets from welding.
The test is performed regularly within the research and development process of blended fabric engineering, especially those intended for constructing an FR garment. When calculating blend ratios between certain fibers - cotton and modacrylic - to achieve an LOI that lies over 25%, performing this test will aid engineers in the fine-tuning process, to factor in an indication of fibre combustibility.
The resulting value of this test LOI is often used as an indicator for whether the garment's fabric will support combustion. However, an LOI must be put into context when assessing an FR garment and not be used as a sole source of judgment.
The LOI is just one piece of data that informs an accurate fire hazard assessment of the materials or products. It is essential to consider this value in light of other considerations which contribute to whether the garment will support combustion. Therefore, the results from this test should be used in conjunction with other standard tests for an accurate assessment of fire hazard.
The below table indicates industry estimations of what LOI percentage would support the best performance for an FR fabric. However, it is unproven whether fabric with an LOI of 45% provides a significant leap in safety compared to a fabric with an LOI of 28%.
|Liquid Oxygen Index (LOI)||Description|
|> 25%||Flame retardant fabric, self-extinguishing|
|22-25%||Slow burning, difficult to ignite, could potentially self-extinguish|
|18-22%||Burnable, burns without self-ignition|
|< 18%||Easy ignition, burns and flame is rapidly spreading|