**3. Fire retardant coatings**

support the combustion, or release the flammable vapors when subject to fire. However, it is important to consider these materials can withstand a fire for a specified period of time. In this regard, the strength and deformation of these materials deteriorate significantly at high temperature. As a result, the structural elements and assemblies may deform or even collapse when exposed to fire conditions. The allowable or extra time in a fire situation depends largely on the anticipated temperature development of the fire, which depends on the type and amount of combustible materials present and the ventilation condition [1–3]. Although protection of materials against fire by the use of coatings for indefinite periods is impossible, it can delay the spread of fire or keep a structure intact against fire, thereby allowing sufficient time for safety measures to be taken. This chapter covers with a discussion of the fire retarda-

There is always misconception about inhibition of fire by using coating of either fire retardant or fire resistance. **Figure 1** shows the development of fire in a compartment, which distinguishes the usage of coating in a particular stage of fire. For instance, the material properties play an important role in prior to flashover, which will be controlled by fire retardant coatings (ignition, flame spread, release of heat, smoke, and gases), whereas fire resistant coatings are mainly predominated in protection of structures that occur after flashover, that is, phase of fully developed fire. Post-flashover fires (temperature above 600°C) are considered as hazardous fires for structures [4]. Fire retardant coatings are mainly involved for reaction-to-fire to retard or inhibit the combustion of flammable materials (wood, foam, textile fabrics, electric cables, and fiber reinforced composites) whereas fire resistant or fire protective coating for resistance-to-fire to protect the non-flammable materials. Different test parameters, such as oxygen index (OI), flame spread rate, ignition time, heat intensity, smoke generation, and

tion of wood products that can be available in scientific literatures.

**2. Fire retardant vs. fire resistant**

102 New Technologies in Protective Coatings

**Figure 1.** Phases of fire development in an enclosure.

In general, conventional organic surface coating is easily ignitable, melts, drips, and it may cause severe injury and damage to the substrates in the event of fire. Therefore, coatings that are designed to formulate should not contribute a significant amount of fuel to the fire and, at the same time, limit the flame spread and smoke development. Fire retardant coating is one of the easiest, oldest, and most efficient ways to protect the materials against fire. This approach does not cause chemical modification of the substrate, but rather the formation of a protective layer which alters the heat flux to the substrate and can inhibit its thermal degradation, ignition, or combustion [5]. The ideal fire retardant coatings should have minimum flame spread, negligible or low release of smoke and/or toxic gases, be easy to apply, show good wear resistance, adhere to the underlying substrate and offer low cost. Typically, they are based on chlorinated alkyds or brominated epoxy resin and filled aluminum hydroxide or a combination of chlorinated paraffin and antimony oxide system. However, the wide range of flame retardants in the form of reactive or additive and different classes (halogen, phosphorus, nitrogen, minerals, oxides, intumescent, and nanofillers) are available commercially that are incorporated into coating formulations to inhibit or retard the burning of materials. The selection of flame retardants can be tailor-made for a specific polymer binder in coating formulations. Fire retardant coatings look like architectural paints and mainly available in solvent form and are applied by conventional methods, brush, roller, and spray.

Fire retardant coatings can be classified into two groups: non-intumescent and intumescent coatings [6]. Non-intumescent coatings are basically decorative, architectural coatings that contain flame retardant additives designed to reduce the rate of flame spread and smoke development of combustible substrates. They are rated as Class A, B or C based on the ability to not contribute to fire and smoke. Rate of flame spread depends on both substrate and thickness of the film. On the other hand, intumescent coatings swell under the influence of heat to form a multicellular charred layer, which acts as an insulating barrier and slows heat and mass transfer between the condensed and vapor phases. This intumesced char can increase to up to 50 times the original thickness of the applied coating. Two types of fire retardant coating either pigmented/colored or clear, transparent varnish are available on the market that are designed for use on different materials and that respond very differently when exposed to fire. They are mainly used in construction, transport, wall and ceiling linings, and other areas require products to satisfy the requirement of classes.

The burning of surface coating is believed to proceed by a free radical mechanism where both ˙H and ˙OH radicals are chain carriers and take part in a number of reactions in the flame 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 diluting or removing the flammable fuels and oxygen.
