ASTM D2863-19: Oxygen Index

In the late 1990s, NASA scientists ran several space shuttle experiments to see how candle flames behaved in microgravity. On Earth, gravity-driven buoyant convection causes a candle flame to be teardrop-shaped and carries soot to the flame’s tip, which makes it yellow; in microgravity, where convective flows are absent, the flame is spherical, soot-free, and blue. This is because without gravity, there is no up direction for warm air to rise and create a convention current. ASTM D2863-19: Standard Test Method For Measuring The Minimum Oxygen Concentration To Support Candle-Like Combustion Of Plastics (Oxygen Index) is a test method for calculating minimal oxygen concentrations in an oxygen/nitrogen mixture in a plastic sample to sustain a flaming burn.

The Colors of A Candle Flame

When you look closely at a candle flame, there is a blue area at the base of the flame, a small dark orange-brown section above it, and a large yellow region at the top of the flame.The steam is made in the blue part of a candle flame, where the wax burns cleanly with lots of oxygen. Here, the flame heat vaporizes wax into molecules of hydrogen and carbon. The dark or orange/brown region has relatively little oxygen. This is where the various forms of carbon continue to break down and small, hardened carbon particles start to form. Lastly, the smoke is made in the bright, yellow part of the flame, where there is not enough oxygen for perfect combustion to take place.

What Is ASTM D2863?

ASTM D2863-19 describes a procedure for measuring the minimum concentration of oxygen, expressed as percent volume, that supports flaming combustion in a flowing mixture of oxygen and nitrogen. This fire-test-response standard provides three testing procedures for its test method:

1. Procedure A involves a complete assessment of the oxygen index conducted using top surface ignition
2. Procedure B involves a complete assessment of the oxygen index conducted using propagating ignition
3. Procedure C is a short procedure involving the comparison with a specified minimum value of the oxygen index and can be conducted using top surface ignition or propagating ignition.

How Do Candles Burn?

A candle needs fuel (made out of some sort of wax), a wick (made out of absorbent twine), and activation energy (a burning match) to make the wax burn thereby producing light. All waxes are essentially hydrocarbons: they are largely composed of hydrogen (H) and carbon (C) atoms. When a candle is lit, the heat of the flame melts the wax near the wick. This liquid wax is then drawn up the wick by capillary action (the absorbency of a wick is important because the wick needs to absorb liquid wax and move it upward while the candle is burning).

The heat of the flame vaporizes the liquid wax (turns it into a hot gas) and starts to break down the hydrocarbons into molecules of hydrogen and carbon. These vaporized molecules are drawn up into the flame, where they react with oxygen from the air to create heat, light, water vapor (H₂O), and carbon dioxide (CO₂). In other words, this chemical reaction is known as combustion in which the wax (made from carbon-based chemicals typically derived from petroleum) reacts with oxygen in the air to make a colorless gas called carbon dioxide. It takes a few minutes when a candle is first lit for this combustion process to stabilize. The flame may flicker or smoke a bit at first, but once the process is stabilized, the flame will burn cleanly and steadily in a quiet upwards teardrop shape, giving off carbon dioxide and water vapor.

Summary of the Test Method in ASTM D2863

The test method in ASTM D2863-19 provides for the measuring of the minimum concentration of oxygen in a flowing mixture of oxygen and nitrogen that will just support flaming combustion of plastics. To conduct testing, a small test specimen is supported vertically in a mixture of oxygen and nitrogen flowing upwards through a transparent chimney. Next, the upper end of the specimen is ignited and the subsequent burning behavior of the specimen is observed to compare the period for which burning continues, or the length of specimen burnt, with specified limits for each burning. Lastly, by testing a series of specimens in different oxygen concentrations, the minimum oxygen concentration is determined.

ASTM D2863-19: Standard Test Method For Measuring The Minimum Oxygen Concentration To Support Candle-Like Combustion Of Plastics (Oxygen Index) is available on the ANSI Webstore.