Flammability behaviour of wood and a review of the methods for its reduction

Wood is one of the most sustainable, aesthetically pleasing and environmentally benign materials. Not only is wood often an integral part of structures, it is also the main source of furnishings found in homes, schools, and offices around the world. The often inevitable hazards of fire make wood a very desirable material for further investigation. As well as ignition resistance and a low heat release rate, timber products have long been required to resist burn-through and maintain structural integrity whilst continuing to provide protection when exposed to fire or heat. Various industry standard tests are thus required to ensure adequate protection from fire is provided. When heated, wood undergoes thermal degradation and combustion to produce gases, vapours, tars and char. In order to understand and alter the fire behaviour of wood, it is necessary to know in as much detail as possible about its processes of decomposition. Various thermal analysis and flammability assessment techniques are utilised for this purpose, including thermogravimetric analysis, cone calorimetry and the single burning item test. The results of such tests are often highly dependent on various parameters including changes to the gas composition, temperature, heating rate, and sample shape size. Potential approaches for fire retarding timber are reviewed, identifying two main approaches: char formation and isolating layers. Other potential approaches are recognised, including the use of inorganic minerals, such as sericrite, and metal foils in combination with intumescent products. Formulations containing silicon, nitrogen and phosphorus have been reported, and efforts to retain silicon in the wood have been successful using micro-layers of silicon dioxide. Nano-scale fire retardants, such as nanocomposite coatings, are considered to provide a new generation of fire retardants, and may have potential for wood. Expandable graphite is identified for use in polymers and has potential for wood provided coating applications are preferred.