Fire Retardancy: What, Why, and How

Reading time ( words)

I. What are flame retardants?

Flame retardants are compounds, which when added to materials during or after manufacture, inhibit or suppress the combustion process.

They interfere with combustion at various stages of the process, e.g., during heating, decomposition, ignition or flame spread. Their primary function is to suppress the spread of fires or delay the time of flashover so that people can escape.

Flame retardants used in plastic materials fall broadly into two categories, namely additive and reactive. Additive flame retardants are incorporated and dispersed into the plastic prior to, during, or most commonly following polymerisation. If they are chemically compatible with the plastic they act as plasticisers, otherwise they are considered as fillers. Reactive flame retardants are chemically bound to the polymer molecule by incorporating them into the polymer backbone or by grafting them onto the backbone as branches. As reactive flame retardants are chemically bound to the host polymer they are prevented from bleeding out and thus generally exert greater flame retardancy than additive compounds due to their greater availability throughout the life cycle of the polymer into which they are incorporated.

II. Why do we need flame retardants?

On average there are more than 4,500 fatalities annually in the EU-27 as a result of fires; this accounts for 2% of all fatal injuries. Fires develop from inception through build-up until a stage where the total thermal radiation from the fire-plume, hot gases and hot compartment boundaries cause the radiative ignition of all exposed combustible surfaces within the compartment. This sudden and sustained transition of a growing fire to a fully developed fire is called flashover.

At this point the radiation of energy to the contents of the room raises all the contents to their ignition temperature whereby the contents of the room suddenly and simultaneously ignite. It is estimated that in a domestic dwelling fitted with working fire alarms on all levels where the occupants are asleep upstairs and a fire starts on the main level of the residence the occupants have about three minutes to escape if they are to have any chance of survival. The presence of flame retardants in otherwise combustible materials has two possible effects;

  • The flame retardant may prevent the fire from developing altogether or;
  • The flame retardant may slow down the build-up phase of the fire thus delaying the onset of flash over thus extending the escape time window.

In either case, the flame retardant serves its primary purpose of reducing the risk of fire related fatalities.

The efficacy of the use of appropriate flame retardants can be seen by way of example from the introduction of “The Furniture and Furnishings Fire Safety Regulations" in the UK in 1988. These imposed a fire resistance requirement on all upholstered furniture supplied in the UK. Between 1988 and 2002 a Government commissioned report estimated that the "Furniture and Furnishings Fire Safety Regulations" played a direct role in saving 1,150 lives and preventing 13,442 injuries.

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Editor's Note: This article originally appeared in the March 2015 issue of The PCB Magazine


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