The oxygen index method refers to the minimum oxygen concentration required for the flame retardant fabric in the oxygen-nitrogen mixture to maintain the combustion state under the specified test conditions, expressed as the volume percentage of oxygen.
Before the test, the flame retardant fabric should be drawn at a distance of 50mm from the ignition source, and then vertically placed on the flame retardant fabric clip. The distance from the upper end to the combustion cylinder is greater than 10mm. The initial oxygen concentration is estimated and adjusted to make oxygen. The nitrogen mixed gas flow flows from bottom to top, and the flow velocity in the combustion cylinder should be maintained at (40 ± 10) mm/s at any time. Let the conditioned air flow flow for 30 seconds to clean the combustion cylinder. Then use an igniter to ignite the top of the flame retardant fabric and confirm that the top of the flame retardant fabric is fully ignited, remove the igniter and start timing. Do not arbitrarily change the flow rate and oxygen concentration at this time.
During the test, if the flame retardant fabric burns for more than 3 minutes, or the flame front exceeds the marking line, the oxygen concentration should be lowered before testing; otherwise, the oxygen concentration should be increased. When the difference between the increase or decrease in the oxygen concentration value is adjusted to be less than 0.5%, the oxygen index (OI) of the flame retardant fabric should be calculated with the reduced oxygen concentration value, and three tests are performed within this range.
The method for determining the high temperature oxygen index is used to test the oxygen index of the same flame retardant fabric at different temperatures above room temperature.