**10. Intumescent fire retardant coatings**

Results suggested that chlorinated rubber, phenolic, and epoxy resin showed best performance in thermal conductivity test, 2-foot tunnel, OI and UL94 horizontal-vertical test and also fibers achieved the synergistic effect with intumescent coatings [56]. A 5A-zeolite-treated ammonium polyphosphate (APP) showed better fire performance properties, heat release rate (HRR), total heat released (THR), smoke production rate (SPR), total smoke released (TSR) and fire growth index (FGI) of sawdust board (SB) as compared to APP being used alone (**Table 1**). Further improvement of fire performance was observed by acid (4-picolinic acid) impregnated 5A-zeolite-treated APP. This can be because acid significantly altered the thermal decomposition and catalytically decomposed to retard the combustion, which pro-

Carbonates and hydrogen carbonates are known to have very high efficiency as gas-phase flame retardants. Potassium carbonate is reported as a compound with a high fire retardant efficiency for wood products. It has a relatively high decomposition temperature (800°C) and serves as a catalyst to the dehydration of wood to increase the production of char, water and

. However, the compound is unable to prevent the depolymerization of wood effectively, particularly at high concentrations, and also causes the evolution of CO. As such, the potassium carbonate is only used in low concentrations [58, 59]. The effects of various inorganic

ties of wood sample were demonstrated. Upon the treatment of these salts, the OI of wood sample was increased, which is caused by an increase in the amount of char on the surface. The activation energies of the samples were also decreased after treatment during both the charring stage and the calcining stage. The flame retardants were shown to be able to catalyze

The effects of pressure and microwave heating duration on the flammability of ammonium polyphosphate (APP) impregnated wood samples were studied. The flame retardant properties, such as peak heat release rate, total heat released and total smoke released, were measured for samples of pretreated and untreated with microwave and characterized by cone calorimeter. It was found that the treated wood showed better flame retardant properties, and moreover, the microwave pretreatment of wood can also increase the fire retardant properties of APP impregnated wood [61]. Wang et al. found that the expanded vermiculite treatments improve the flame retardant properties of plywood. Results showed that expanded vermiculite treatments increase the OI values of wood and at the same time decrease the thermal activation energy at the maximum degradation process. The increase in OI is due to the formation

on the thermal decomposition and fire retardant proper-

O, CO2

, and char, but less

motes the uniform char formation [57].

112 New Technologies in Protective Coatings

CO2

salts, Na2

WO<sup>4</sup>

**Sample Peak HRR** 

**(kW/m<sup>2</sup> )**

**Table 1.** Cone calorimetric date for sawdust board [57].

, Na2

SnO<sup>3</sup>

, Na2

MoO<sup>4</sup>

the dehydration reaction, resulting in the formation of more H2

flammable vapors like levoglucosan and levoglucose [60].

of protective layer on the surface of wood sample [62].

**Time to peak HRR (s)**

**Average HRR (kW/m<sup>2</sup> )**

SB-APP 122.0 40 29.2 4.4 310 4.1 5.3 SB-APP/5A 124.8 60 24.5 3.3 229 2.0 3.1 SB-APP/5A 50.1 50 8.5 1.1 215 1.9 1.4

**THR (MJ/m<sup>2</sup>**

**) SPR (m<sup>2</sup>**

**/s) TSR (m<sup>2</sup>**

**/m<sup>2</sup>**

**) FGI (kW/sm<sup>2</sup>**

**)**

The intumescent coating of either pigmented or clear was introduced to the fire retardation of wood-based products. Intumescent coatings swell and char when exposed to heat, giving carbonaceous foam that insulates the surface from the fire. The char layer is also responsible for the limitation of oxygen diffusion and the reduction of the volatilization of the fuel in order to prevent the continuation of the combustion cycle. Intumescence resulted from the application of flame retardant coating systems to timber can reduce the char formation and heat buildup and also delay the onset of combustion of a wood [63]. Gardner and Thomson have studied the flammability of forest products, including sawn boards, plywood, hardboards and particleboard as per ASTM E906–Standard test method for heat and visible smoke release rates for materials and products. The intumescent flame retardant was used to treat the plywood by pressure impregnation. The ignition time of treated sawn board was not increased with density when exposed to heat fluxes of 20 and 40 kW/m2 . However, the ignition times of plywood were increased by pressure impregnated flame retardants. The heat release properties of both samples were reduced by flame retardant coatings, which is dependent on the exposed heat flux [64].

An intumescent fire-retardant coating based on unsaturated polyester and epoxy resin was prepared by using ammonium polyphosphate, pentaerythritol, melamine, and expandable graphite, and studied the fire endurance performance of wooden board. Results showed that 2 mm film thickness of intumescent coating provides excellent fire endurance time [65]. Chou et al. investigated the usefulness of an artificial graphite powder and a sericrite (Al<sup>4</sup> (OH)<sup>4</sup> (KAl-Si<sup>3</sup> O10) 2 ), and a mixture of the two on plywood. The intumescent fire retardant coating formulation contains 19.8% of the flame retardant, 15% of the dehydration agent, 18% of the foaming agent, 7.2% of the resin binder and 40% of the solvent which was prepared and applied to the surface of plywood. They showed that when sericrite was in excess of 75% in the fire retardant composition, the mixture obtained the lowest flammability grade possible in Taiwan Standard CNS 7614. Furthermore, for sericrite to be effective in inhibiting combustion and also carbonizing agents, such as graphite powder, are not required [66]. The burning behavior of intumescent coating-treated flax board was studied by cone calorimeter and single burning item (SBI) test. Intumescent coating provides significant increase in ignition time and decrease in heat release rate of flax board. It was also confirmed that experimental results are comparable to numerical predictions, factional factor, the ratio of the heat flux at the interface of the intumescent surface and the char layer of flax board to the surface heat flux when the absence of intumescent coating layer, based on analytical solutions for charring materials and burning rates in SBI tests [67]. Intumescent coatings are commonly used for protecting steel structures. Their application on wood has also been studied, and some commercial products intended for wood are available. At present, however, fire protection of wooden and wood-based products with intumescent coatings is not widely employed.

#### **11. Transparent fire retardant coatings**

The transparent ultraviolet (UV) curable intumescent fire retardant coating on wood was prepared by using cyclotriphosphazene as a flame retardant. A series of (2-hydroxyethylmethacrylate)

(n-propoxy)cyclotriphosphazenes were prepared by the reaction of N<sup>3</sup> P3 Cl6 with n-propanol and 2-hydroxyethylmethacrylate sequentially in the appropriate solvents. It was observed that phosphazene-based compounds showed better fire retardant properties of wood and without affecting the aesthetic appearance of wood structure [68]. Chen et al. prepared the dual cured (UV-radiation and moisture) flame retardant coatings based on silicone and phosphate modified acrylates. The oligomer was made by reacting 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide and 3-glycidoxypropyltriemethoxysilane, and reacted with 2,4-toluene diisocyanate and 2-hydroxyethyl acrylate. The dual cured film showed high-flame retardance, which is attributed to the synergistic effect of phosphorus-silicon and phosphorusnitrogen [69]. Shi and Wang synthesized the transparent intumescent flame retardant coatings from epoxy/phosphate flame retardant and amino resins. Five types of flame retardants were prepared by a two-step reaction using 1-oxo-4-hydroxymethyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane, polyphosphoric acid, bisphenol-A epoxy resin and 1,3-butanediol diglycidyl ether with different proportions. The structure of flame retardant was confirmed by nuclear magnetic resonance (<sup>1</sup> H-NMR) and Fourier transforms infrared (FTIR) spectroscopy. It was found that the fire retardant properties of plywood were significantly improved by intumescence and also enable to maintain the visibility of the surface features of wood [70].

#### **12. Nanocomposite coatings**

Nanoparticles have recently been used to prepare the nanocomposites for the improvement in fire retardant properties. The major concern of these materials is dispersion. The surface modification is essential for nanoparticles to achieve better compatible and homogeneous dispersion. The necessary loading of nanoparticles is usually lower than for their micron-sized filler counterparts which are an enormous advantage industrially and economically. Clay nanopowder composed of montmorillonite and cellulose nanofibers which is used as a transparent fire retardant coating for wood was demonstrated. Fire performance was assessed by cone calorimeter at irradiance of 35 kW/m2 . It was found that nanopowder coated wood showed a strong increase in time to ignition, and a 33% total heat release was reduced along with the 46% reduction in maximum average heat emission rate. Both thermal shielding and gas barrier functions contribute to delayed thermal degradation of wood and delayed emission of volatile combustible gases [71]. Hassan et al. prepared the flame retardant intumescent polyurethane coatings for wood products. The limitation of intumescent additives, such as incompatibility and loading issue, was addressed by using butyl acrylate and montmorillonite clay. They found that the flame retardant properties of wood were improved by addition of acrylate and nanoclay using cone calorimeter [72]. Giudice and Pereyra showed through the use of oxygen index and a two-foot tunnel tests, (ASTM E84 2013—the regulatory test for interior flammability of building materials in the United States), the silica nanoparticle-treated wood provides several significant advantages: high fire retardancy, low thermal expansion, reduced smoke production, and low cost [73]. The reaction-to-fire properties of titanium dioxide (TiO2 ) and/or clay nanoparticles coated spruce wood were investigated by Fufa et al. using a small-scale cone calorimeter. They found that the negative results of reaction-tofire performance along with water vapor permeability were observed on specimens treated with TiO2 and/or clay nanoparticles treatments [74]. Chuang et al. have investigated the fire performance of intumescent coated plywood with the addition of commercial organoclays (Cloisite 30B, Cloisite 10A, and Cloisite 15A) at different loadings, 1, 3, 5, and 10%. According to the cone calorimeter test, compared to uncoated plywood, the intumescent coating exhibited lower peak heat release rate (peak HRR) and extend the time to reach peak HRR. Further improvement was observed with the incorporation of organoclays; however, the type and amount of organoclay are very important. For instance, 3% Cloisite 30B and 5% Cloisite 10A displayed better fire retardancy as compared to other loadings of the clay. It was demonstrated that adding organoclay extends the survival duration of the phosphocarbonaceous char structures, which is confirmed by spectroscopic studies [75].

(n-propoxy)cyclotriphosphazenes were prepared by the reaction of N<sup>3</sup>

magnetic resonance (<sup>1</sup>

114 New Technologies in Protective Coatings

dioxide (TiO2

**12. Nanocomposite coatings**

by cone calorimeter at irradiance of 35 kW/m2

and 2-hydroxyethylmethacrylate sequentially in the appropriate solvents. It was observed that phosphazene-based compounds showed better fire retardant properties of wood and without affecting the aesthetic appearance of wood structure [68]. Chen et al. prepared the dual cured (UV-radiation and moisture) flame retardant coatings based on silicone and phosphate modified acrylates. The oligomer was made by reacting 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide and 3-glycidoxypropyltriemethoxysilane, and reacted with 2,4-toluene diisocyanate and 2-hydroxyethyl acrylate. The dual cured film showed high-flame retardance, which is attributed to the synergistic effect of phosphorus-silicon and phosphorusnitrogen [69]. Shi and Wang synthesized the transparent intumescent flame retardant coatings from epoxy/phosphate flame retardant and amino resins. Five types of flame retardants were prepared by a two-step reaction using 1-oxo-4-hydroxymethyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane, polyphosphoric acid, bisphenol-A epoxy resin and 1,3-butanediol diglycidyl ether with different proportions. The structure of flame retardant was confirmed by nuclear

found that the fire retardant properties of plywood were significantly improved by intumes-

Nanoparticles have recently been used to prepare the nanocomposites for the improvement in fire retardant properties. The major concern of these materials is dispersion. The surface modification is essential for nanoparticles to achieve better compatible and homogeneous dispersion. The necessary loading of nanoparticles is usually lower than for their micron-sized filler counterparts which are an enormous advantage industrially and economically. Clay nanopowder composed of montmorillonite and cellulose nanofibers which is used as a transparent fire retardant coating for wood was demonstrated. Fire performance was assessed

showed a strong increase in time to ignition, and a 33% total heat release was reduced along with the 46% reduction in maximum average heat emission rate. Both thermal shielding and gas barrier functions contribute to delayed thermal degradation of wood and delayed emission of volatile combustible gases [71]. Hassan et al. prepared the flame retardant intumescent polyurethane coatings for wood products. The limitation of intumescent additives, such as incompatibility and loading issue, was addressed by using butyl acrylate and montmorillonite clay. They found that the flame retardant properties of wood were improved by addition of acrylate and nanoclay using cone calorimeter [72]. Giudice and Pereyra showed through the use of oxygen index and a two-foot tunnel tests, (ASTM E84 2013—the regulatory test for interior flammability of building materials in the United States), the silica nanoparticle-treated wood provides several significant advantages: high fire retardancy, low thermal expansion, reduced smoke production, and low cost [73]. The reaction-to-fire properties of titanium

) and/or clay nanoparticles coated spruce wood were investigated by Fufa et

al. using a small-scale cone calorimeter. They found that the negative results of reaction-tofire performance along with water vapor permeability were observed on specimens treated

cence and also enable to maintain the visibility of the surface features of wood [70].

H-NMR) and Fourier transforms infrared (FTIR) spectroscopy. It was

P3 Cl6

. It was found that nanopowder coated wood

with n-propanol

Zhang et al. studied the ignition and burning behavior of intumescent coating and sepiolite nanoparticles applied on flaxboard using cone calorimeter, single burning item (SBI) test, and reduced scale (one-third) ISO room test. Both cone calorimeter and SBI test represent an open and well-ventilated condition, whereas reduced ISO room test is a fire in a confined space and represents a more realistic burning condition in most compartment fires. Intumescent coatings effectively delaying the ignition and mass loss rate in both cone calorimeter and SBI tests. Further improvement was observed in addition of nanoparticles. This because of intumescent coating forms carbonaceous char on the surface, which acts as a thermal and physical barrier preventing heat, mass and gas transfer, and further improvement is due to an increase in the thermal stability of the char as confirmed by the thermogravimteric analysis. In addition, it was also confirmed that intumescent coatings do not cause an increase of the toxic gases, and ventilation plays a vital role in the development of the fire [76].

The high thermal conductivity of nanosilver coatings was tested to improve heat transfer in wood and enhance fire retardant performance. Nanosilver treatment clearly showed potential in improving some of the fire retarding properties in solid wood products. It can be observed that coating may delay thermal degradation and carbonization by reducing the accumulation of heat that is rapidly transferred [77]. Nano-wollastonite is demonstrated as multifunctional additive in wood. Wollastonite nanofibers were also reported the same group to improve durability and fire retardant properties of poplar wood and solid wood composites. It was found that both fire retardant and dimensional stability were improved at 10% nano-wollastonite impregnated wood. As a mineral material of wollastonite, acts as an impermeable physical barrier toward the penetration of flames into the wood structure [78]. The synthesis of hexagonal boron nitride nanosheets was demonstrated through a facile shear force liquid phase exfoliation method and used as a binder free oxidation and fire retardant wood coatings. Because of intrinsic low thermal diffusivity and thermal effusivity, nanosheet coatings showed an excellent fire retardation and oxidation resistance up to 900°C [79]. The effect of organically modified alpha-zirconium phosphate (OZrP) on the thermal and fire retardant properties UV curable system was studied. The flame retardant coating system comprises of phenyl di(acryloyloxyethyl)phosphate (PDHA), triglycidylisocyanurate acrylate (TGICA) and 2-phenoxyethyl acrylate (PHEA). Results showed that addition of 0.5 wt% OZrP, the peak heat release rate and total heat of combustion were reduced significantly. This is because of the effective char formation and improvement of anti-oxidation performance of the coating [80].

## **13. Sol-gel method**

In recent years, sol-gel processes have also become recognized for the purposes of incorporating fire retardants into products. The process comprises hydrolysis and condensation reactions that lead to the formation of inorganic or organic-inorganic hybrid coatings. This technique is well documented for different polymers. Giudice et al. synthesized the polysiloxanes in wood pores by sol-gel process using aminopropylmethyldiethoxysilane, aminopropyltriethoxysilane and a mixture of both (50/50 ratio), and then, impregnated panels were subjected to 2-foot tunnel test (flame spread index, panel consumption, and smoke density). Impregnation process was carried out at 40–50°C in an autoclave and controlling the operating conditions for achieving different weight gains. It was shown by the authors that aminopropyltriethoxysilane-treated wood sample showed best fire retardant efficiency. This is because of more reactivity of alkoxide, which forms hybrid structure [81]. Another study showed that the transparent fire retardant coating for wood (pine and larch) was prepared by a sol-gel method using vinyl functionalized zirconium oxy-clusters copolymerized with vinyl trimethoxysilane. Results showed that coating has improved the fire retardant properties and also without affecting the macroscopic appearance of wood surface [82].

#### **14. Fire resistance of timber**

The use of timber components in the loading structure in a building relies on fire engineering design to ensure that the building can retain its structural integrity for sufficient time either for building occupants to be evacuated, or for the fire to be extinguished. In construction using large cross-section timber members, like cross-laminated timber, this may be done by assuming a rate at which the timber chars and therefore the cross-section of timber remaining after a given time [83]. The strength and stiffness of timber both reduce at lower temperatures than steel and concrete. For example, timber's strength is reduced by more than 50% at 100°C, compared with that at 20°C [84]. Timber structural members may still perform well at high temperatures in comparison with steel, however, since the char layer can act to insulate the material within, whereas the high thermal conductivity of steel means that the complete section quickly heats up. Where steel is used to connect timber elements, heat can be quickly conducted through the connectors, degrading the strength and stiffness of the wood around them. The behavior of timber in fire is fundamentally different to steel and reinforced concrete; however, since it is combustible, research groups have identified that the key research needs to be addressed for the next generation of large timber buildings [85, 86]. They address the performance of systems with various levels of encapsulation, the effect of flame spread due to a combustible structural material, and the fire performance of connections. Another potential use for coatings is to increase the fire resistance of structural timbers. It was showed that significant increases in fire resistance can be achieved by using fire resistant coatings. This concept should be pursued by the timber industry, as it would be possible to improve the fire resistance of structural timbers in old buildings that are being remodeled. The application of a fire resistive coating would be simpler and cheaper than cladding the members in fire resistive board materials, or replacing the timber member with a concrete or steel member.
