**5. Wood**

zone. The function of fire retardant coatings is to protect the substrate that depends on the mode of operation of flame retardants [7]. For example, the coatings formulated with halogenated compounds are more effective in gas phase, and they act in the flame zone by forming a blanket of halogen vapor that interferes with the propagation of the flame by interrupting the generation of highly reactive free radicals, thus helping flame extinguishments. However, the release of toxic and corrosive gases from halogenated compounds while burning is ecologically unsafe. The action of phosphorus in coatings varies with the type of flame retardants and the polymer binders. They mostly operate in condensed phase to form a protective char layer that acts as physical barrier to heat transfer from the flame to the substrate and to diffusion of gases. The vapor phase action is also found to be effective in phosphorus flame retardants, which are capable of controlling the high energy radicals in the flame. Intumescent flame retardant is a combination of an acid source, a char former and a gas source, and sometimes, it is available in a single compound including all three functions. The mechanism of intumescent coatings is to undergo an endothermic decomposition reaction at an elevated temperature that causes the coating to swell and form into a highly porous, thick, and thermally stable char layer that has a very low thermal conductivity (heat insulation). Other flame retardants such as metal oxides and hydroxides are operated by cooling with the release of water and by

Typically, the flammable materials are easily combustible and rapidly growing in a fire (reaction-to-fire), in terms of the spread of fire or propagation of fire, up the stage when flashover occurs in a compartment. Flashover can occur quickly in seconds or slowly depending on the speed of fire growth rate. Wood is the most frequently used combustible products in addition with polymers (plastics and rubbers), foams, textiles, cables, and fire reinforced composites. Three different kinds of methods, such as clear or transparent varnish paints, pigmented intumescent reactive coatings, and surface impregnations are utilized in wood to enable the restriction of growth and/or spread of fire. The European fire classification for reaction-to-fire is based on fire growth rate index (FIGRA), which indicates the time to reach flashover in the standardized reference test as per BS EN 14390. Other test methods, the single burning item (SBI) test (BS EN 13823; 2002), radiant panel test for flooring (EN ISO 9239-1; 2002) and either the small flame test (BS EN 11925-2; 2002) or the bomb calorimeter (BS EN ISO 1716), are also relevant to the fire retardant system in Euro-class [8]. Two standards, such as fire propagation test (BS476, part 6:1989) and surface spread of flame test (BS476, part 7:1987), were applicable to the fire test methods in the UK for flammable materials. The BS476, part 6 test method is intended to provide a comparative measure of the contribution to the growth of fire of a product. The test result is expressed as fire propagation index (I) and three sub-indices, i1, i2, and i3. The higher the fire propagation index means the greater the growth of fire. On the other hand, the BS476, part 7 measures the lateral spread of flame along the surface of a specimen, which is mounted at right angles to a high intensity radiation panel. The extent and rate of flame spread of specimen are used to determine the classification, which can range from Class

diluting or removing the flammable fuels and oxygen.

104 New Technologies in Protective Coatings

**4. Fire retardancy of flammable materials**

Wood is one of the most versatile, sustainable, aesthetically pleasing, and environmentally benign materials. It can be classified into hard and soft wood, which can have different percentages of cellulose, hemicellulose, and lignin. Different types of wood products including solid wood-based panels (particleboard, hardboard, fiberboard, fir Douglas plywood), structural timbers, glued laminated timbers, cladding and wood floorings are widely used for structural purposes in building construction, flooring and furnishing materials that found in homes, schools, and offices around the world. The basic deficiencies of wood products are flammability, poor dimensional stability, and low resistance to micro-biological decay that must be addressed when used as a construction material. Because of easily flammable and contribute fuel to fires, wood is considered to poor construction materials. The flaming combustion of wood is mostly supported by cellulose [9].

The various categories of action are typically described in wood treated with flame retardants: (1) an acceleration of dehydration and carbonization that provides thermal insulation, (2) chemical modification of wood pyrolysis, (3) absorb the surrounding heat by endothermic reactions, (4) inhibition of the flaming combustion in the gas phase, and (5) increase the thermal conductivity of wood in order to dissipate the heat from the wood surface [10]. The fire retardation of wood is typically treated with flame retardant chemicals that are coated onto the surface of wood by painting, spraying or dipping methods and/or impregnated into the wood structure using vacuum pressure technique or plasma treatments [11], which inhibits ignition and do not contribute to the spread of flame. Three components of wood have quite different decomposition range; for instance, temperature from 200 to 260°C is for hemicellulose, temperature from 240 to 350°C is for cellulose, and temperature from 280 to 500°C is for lignin. When heated, wood undergoes degradation and combustion to produce volatile gases, tars (levoglucosan), and carbonaceous chars. The fire performance of wood-based products and test methods has been reviewed and studied extensively [12–22]. Traditional flame retardants, such as boron compounds, mineral acids, and inorganic salts, (monoammonium phosphate, diammonium phosphate, guanylurea phosphate, guanidine phosphate, ammonium polyphosphate, and melamine phosphate), may considerably improve the fire retardant properties of wood [23]. However, the use of boron and formaldehyde-based systems is likely to be declined in accordance with the growing awareness of environmental issues and consumer safety. In addition, inorganic salts may also affect the performance of wood in various ways by increasing hygroscopicity, reducing strength that leads to dimensional instability, wood degradation, corrosion of metal fasteners, adhesion problems, and increased abrasiveness. The flame retardants based on phosphorus, nitrogen, silicone, and char forming additives are still prominent solution to address the flammability and environmental issues [24].

Cone calorimeter is a most commonly used bench-scale method to evaluate the flammability of wood. Shi and Chew have investigated the carbon monoxide (CO) yield of six species of wood samples under different external heat fluxes and moisture content by spontaneous ignition in a cone calorimeter. Spontaneous ignition is a complex phenomenon that combustible materials are ignited by internal heating, without the spark plug. As compare to piloted ignition, process of spontaneous ignition is much closer to the development of real fire. Results observed that thickness of wood has little effect to peak CO release rate, but the time to peak is postponed with a higher thickness. The peak CO release rate decreases with a higher external heat flux, but the decrease is not obvious when heat flux increases from 50 to 75 kW/ m2 . Average CO yield is inversely proportional to external heat flux, thickness, and density. They concluded that both flame and moisture can also reduce CO release rate because energy used for water evaporation increases with high moisture content [25]. The effect of variable heat flux and oxygen concentrations (20.9, 18, 16 and 15%) on ignition time and mass loss rate of wood was investigated to obtain the kinetic parameters, activation energy and frequency factor. It was found that with increasing the oxygen concentration, the mass loss rate was increased, but the ignition time, the activation energy and the frequency factor were decreased [26]. Critical heat flux for ignition has been calculated to be between 10 and 13 kW/ m2 for a range of wood products. Density, thickness and moisture content have a large influence on the material dependent properties [27].
