Oxygen index is defined as the minimum concentration of oxygen, expressed as volume percent, in a mixture of oxygen and nitrogen that will just support flaming combustion of a material initially at room temperature under specified conditions.
The oxygen index test is considered one of the most useful flammability tests because it allows one to precisely rate the materials on a numerical basis and simplifies the selection of plastics in terms of flammability. The oxygen index test overcomes the serious drawbacks of conventional flammability tests. These drawbacks are variation in sample ignition techniques, variation in the description of the endpoint from test to test, and operation of tests under non equilibrium conditions.
The test determines the minimum concentration of oxygen in a mixture of oxygen and nitrogen flowing upward in a test column that will just support combustion. This process is carried out under equilibrium conditions of candle like burning. It is necessary to establish equilibrium between the heat removed by the gases fl owing past the specimen and the heat generated from the combustion. The equilibrium can only be established if the specimen is well ignited and given a chance to reach equilibrium when the percent oxygen in the mixture is near limiting or critical value. The equipment used for measuring the oxygen index consists of a heat-resistant glass tube with a brass base. The bottom of the column is filled with glass beads, which allows the entering gas mixture to mix and distribute more evenly. A specimen-holding device to support the specimen and hold it vertically in the center of the column is used. A tube with a small orifice having propane, hydrogen, or other gas flame, suitable for inserting into the open end of the column to ignite the specimen, is used as an ignition source. A timer, flow measurement, and control device are also used. The test specimen used in the experiment must be dry since the moisture content of some materials alters the oxygen index. Four different types of specimens are specified. They are physically self-supporting plastics, flexible plastics, cellular plastics, and plastic film or thin sheet. The dimension of the specimen varies according to the type. The specimen is clamped vertically in the center of the column. The flow valves are set to introduce the desired concentration of oxygen in the column. The entire top of the specimen is ignited with an ignition flame so that the specimen is well lighted. The specimen is required to burn in accordance with set criteria, which spell out the time of burning or the length of specimen burned. The concentration of oxygen is adjusted to meet the criteria. The test is repeated until the critical concentration of oxygen, which is the lowest oxygen concentration that will meet the specified criteria, is determined.
The oxygen index is calculated as follows:
Oxygen index percent = (100 × O2)/ (O2 + N2)
where O2 = volumetric flow of oxygen at the concentration determined;
N2 = volumetric flow of nitrogen.
Factors Affecting the Test Results
1. Thickness of Specimen. As the specimen thickness increases, the oxygen index also increases steadily.
2. Fillers. Fillers such as glass fibers tend to increase the oxygen index up to a certain percentage loading. In case of polycarbonate, the oxygen index peaks at about 25 percent loading. Higher loading beyond this point subsequently decreases the oxygen index.3. Flame Retardants. Flame retardants increase the oxygen index, making polymers more suitable for applications requiring improved flammability.