Probably the single most frequently occurring and most misunderstood issue in electronic product safety is electrically-caused fire. I thought I would review fire processes in plastics materials (the most common flammable construction material in electronic products).
The kind of fire that we are concerned with can be defined as an uncontrolled combustion process, usually accompanied by flames.
There are two combustion processes, flaming and flameless. This discussion is limited to flaming combustion. Flaming combustion is the one with which we are most familiar, where luminescent flames appear directly above the fuel material.
Flaming combustion is a chemical chain-reaction process which takes place exclusively in the gas phase. Solids and liquids do not burn.
Only gasses burn. If you look carefully at any flame, you will see that it appears ABOVE the fuel material. This demonstrates that a gas is burning. However, the gas is evolved from the solid or liquid by heating the solid or liquid to a suitable temperature.
Let’s examine the process by which a solid or liquid contributes to a fire. Consider igniting paper with a match (presuming the match is already lit).
To initiate the fire (flaming combustion), the solid material (paper) must be heated by a source (the match) whose temperature is greater than the combustible material’s flame-ignition temperature.
As the material is heated and the material temperature increases, chemical decomposition (pyrolysis) occurs. Pyrolysis is a chemical change (decomposition) within the material and is indicated by a change in color and by evolution of smoke. Typical products of pyrolysis are flammable gasses, non-flammable gasses, liquids, and carbonaceous residues.
As heat is continued to be applied to the evolved gas, the pyrolytic gas temperature continues to rise to the flame-ignition temperature. At this temperature, the flammable gasses, mixed with atmospheric oxygen, will self-ignite, and flaming combustion occurs. For the purposes of this discussion, the flame-ignition temperature of paper is taken as 451° F or 232° C.
Note that, for ignition, the GASSES must attain the flame-ignition temperature, not the solid material (although the solid material must be heated sufficiently to evolve flammable gasses).
Consider that gasoline produces gasses at room temperature, but those gasses do not burn spontaneously. Instead, the gas above a container of gasoline must be heated to its flame ignition temperature before a fire occurs. Such a gas is very dangerous since a small spark is sufficient to achieve flame-ignition temperature.
To sustain flaming combustion, the production of flammable gasses (pyrolysis), must be sustained.
Heat for pyrolysis must be continually supplied to the solid or liquid material, either from an external source or from the flame.
Sustaining flaming combustion after the external heat source stops generating heat (i.e., the match is taken away) depends on the heat of the remaining flame.
If the heat from the flame exceeds the heat necessary for pyrolysis, flaming will be sustained.
Likewise, if the heat from the flame is small, there will be less pyrolysis, the temperatures will decrease, and the fire will die.
So, a sustained fire is dependent on sustained thermal feedback from the flame to the solid material.
Considering the situation in terms of the law of conservation of energy, pyrolysis is an endothermic (heat-absorbing) chemical process; flaming is an exothermic (heat-producing) chemical process.
If the exothermic reaction produces more energy than that required by the endothermic reaction, and there is adequate thermal coupling between the two processes, then fire will be sustained.
In addition to producing heat, the flame also produces combustion products, usually carbon monoxide, carbon dioxide, water vapor, other gasses produced by pyrolysis and flaming of the particular material, and solid carbonaceous residues.
Some of the solids are carried off by convection currents, and some are left behind as ash.
Now, let’s consider ignition of a plastic material from a non-flame source, i.e., electrical heating.
Electrically-caused fire is electrical heating of a material to ignition temperature followed by combustion. So, we’ll now consider the situation of non-flame heating and ignition.
In the case of plastics, as the material is heated, first the plastic softens, often changes color, then produces smoke, then boils.
Published decomposition temperatures of common plastic materials range from 200° C for PVC to 500° C for PTFE, with most others in the 300° C range. My measurements suggest a decomposition temperature of 225° C for polyester.
If heating is continued, and since we have no flame from an electrical heat source, the combustible gasses evolved from the heated plastic will self-ignite at a temperature slightly higher than the flame-ignition temperature. Typical plastic self-ignition temperatures range from about 350° C for polyethylene and polypropylene to 580° C for PTFE.
Sustained flaming combustion after the external heat source stops generating heat (e.g., the fuse opens) depends on the heat of the remaining flame.
For plastic materials rated HB, most of the pyrolytic gasses are flammable. The heat in the flame of such plastic materials usually is more than the heat necessary for sustained pyrolysis.
After ignition, when the external heat source is removed from the material, flaming is sustained.
For materials rated V2, V1, V0, or 5V, most of the pyrolytic gasses are non-flammable. The heat in the flame of the remaining flammable gasses of such plastic materials is less than the heat necessary for sustained pyrolysis. After ignition, when the external heat source is removed from the material, the rate of combustion will decrease until extinction occurs.
The fire process is:
- Fuel material
- Pyrolysis (an endothermic reaction requiring heat)
- Pyrolytic products
- Combustible gasses
- Non-combustible gasses
- Liquid products
- Solid products
- Mixing of pyrolytic products with atmospheric oxygen
- Flame (an exothermic reaction producing heat)
- Combustion products
- Sufficient thermal feedback from the flame to the fuel material to sustain pyrolysis
(Smoke is an indicator of incomplete combustion. Incomplete combustion is due to insufficient pyrolysis or insufficient oxygen.)
With this background in the process of fire, we can address control and prevention of fire in electronic products.
Acknowledgments
Some of the material presented here is the result of a collaboration of Dave Adams, Ray Corson, Kevin Cyrus, Richard Pescatore, and Brady Turner, all of Hewlett-Packard, and Bob Davidson and Don Mader of Underwriters Laboratories.
A good basic explanation of fire is a videotape entitled “Fire Concepts and Behavior,” FL-46 VH, available from the National Fire Protection Association, Batterymarch Park, Quincy, Massachusetts.
A good reference on burning processes of plastics is the “International Plastics Flammability Handbook,” Second Edition, by Jurgen Troitzsch, published by Hanser Publishers, Munich. Distributed in the USA and Canada by Oxford University Press, New York, ISBN 0- 19-520797-1.
Richard Nute is a product safety consultant engaged in safety design, safety manufacturing, safety certification, safety standards, and forensic investigations.
