Green Fire Retardants for Structural Timber

Fire Retardants

Fire retardants are intended to minimise a material’s reaction to fire, not increase a structure’s resistance to fire (7). Fundamentally this will provide a structure with the ability to meet its regulatory requirements, which are designed to enable protection of lives and of property.

For the purpose of this investigation, definitions are as follows(8):
  • Resistance to fire - The ability of a construction element to satisfy the criteria specified in the relevant section of the building regulations
          Effectively how long building elements can resist a fully grown fire for
  • Reaction to fire - The tendency of a material to encourage fire growth
         Effectively how a particular material will encourage propagation and flame spread as a function of its properties

The main fire retardant types used in the construction industry are intumescent paints and impregnations.

Intumescent Paints
These paints are designed such that, at elevated temperatures (but much lower than ignition temperatures) chemical reactions will cause vapour release that will encourage the paint surface to froth, creating an insulating layer. This theoretically offers the timber some protection from heat, delaying charring effects and maintaining a lower thermal gradient through the wood section.

Intumescent paints may be limited somewhat by surface damage during fitting and exposure to weather if uncoated.

Impregnation treatment employs vacuum and pressurised conditions to encourage certain soluble compounds to penetrate the timber section. These are generally compounds that decompose endothermically (9) at relatively low temperatures (200-300°C), creating a heat-sink to blanket flame propagation.

The treatment is applied to the entire section rather than just the surface, reducing the chance of surface damage undermining the retardant.

Drawbacks of impregnation treatments include an expensive and complicated treating process, and inefficient use of materials (ie. Most flame spread occurs nearer the material surface).