Green Fire Retardants for Structural Timber

The Charring Process

If the temperature of the wood is increased to 300°C after the onset of pyrolysis, it will begin to burn, producing charcoal which acts as insulating layer. Frangi & Fontana (2003) pg91+92 (22) explain this clearly: "A surface layer of char is then formed, which because of its low thermal conducitivity, protects the interior off the timber cross-section against heat.”  
When the outer layer reaches its burning point (universally accepted as 300°C (20)(30)(32)(34)) the wood ignites and burns rapidly. The burned char layer loses all its strength but retains a role as an insulating layer albeit with greater porosity, protecting and delaying the temperature rise in the virgin wood. The whole process is shown on the diagram to the left (27 - Janssens 2004 Fig 1 pg201).

Low conductivity of the char will cause a steep thermal gradient as charcoal only has a thermal conductivity between a third and a half that of cellulose (the main constituent of wood). Charcoal is actually very effective in retarding penetration of heat and delays the attainment of the exothermal point of wood underneath it. Greater yields of charcoal occur when pyrolysis is slow (Browne 1958 - 30). 

Some of the exothermic heat of combustion is lost through convection and radiation to surroundings; the rest however is conducted back through the cracks in the char layer to the pyroysis zone, thus the combustion continues.

Charring rate for most woods are more or less constant (depends on moisture content and density) which most current design methods assume. However  variability in charring rates is examined in the following section.

Why is charring important to timber in terms of a structural context?

 “Load carrying capacity is reduced partly by charring which is considered to have no strength and partly by strength degradation of the burnt region.” ( Lau et al (1999) pg 209 (21))

Therefore charring is an important parameter for fire safety engineering timber design, i.e. when calculating mechanical resistance of the residual section, it is imperative to know the charring rate the size of residual section after a certain period of time can be determined. This concept is explored in greater depth in Timber Pyrolysis.
(Buchanan 2000 pp284 Fig. 8 20)

Frangi & Fontana (2003) (22) on pg92
 state that cracks in the char layer markedly increase the penetration of heat into the pyrolysis zone. The time-dependent thermal degradation of wood is quantified by the charring rate, which is defined as the ratio between the distance of the char-line from the original wood surface and the fire duration time. The charring rate of wood, which is the main parameter to describe the fire behaviour of timber structures, is mainly determined by the type of wood.” 

Generally the charring rate is measured in (g/s) or (mm/s) with the latter being more widely used due to its convenience in designing structural members i.e. residual cross section area.

The char front itself which is measured by the rate at which it propagates (i.e. mm/s) is usually estimated by assuming the interface temperature is 300°C ( pg209 -21). This is confirmed by 
Eurocode 5, which states the “Position of the char-line should be taken as the position of the 300°C isotherm. Valid for most softwoods and hardwoods,” (pg20 3.4.1 29).

Below is a graph showing the effect of fire retardant treatment on char formation. The increasing linear progression below shows that char formation is increased as fire retardant concentration goes up, this is due to slower pyrolysis occurring which generates more charcoal (see above/Browne 1958 (30)). Although this may not be directly applicable for our protected timber section (the carbonific in the paint chars, adding an insulative layer) it is a good example of fire retardants benefiting structural timber via increased char formation. This principle is reinforced by Buchanan (2000) who states that intumescent products can be used for reducing the charring rate of wood (pg279 (20)).

(Garba 1999 pg 521 Fig 5 (25)