**7. Boron-based fire retardant coatings**

char forming additives are still prominent solution to address the flammability and environ-

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/

. 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/

for a range of wood products. Density, thickness and moisture content have a large influ-

The charring rate, speed at which charring depth advance in the material when exposed to high temperature, is a critical parameter for flammability of wooden samples because it allows the determination of the size of the residual section of wood. It depends on wood species, density, moisture content, permeability, composition and direction of burning [28]. For example, the charring rate of flame retardant-treated wood is linearly proportional to the applied heat flux in cone calorimeter and inversely proportional to the density of wood. Charred wood is bounded by the transition between the pyrolysis layer, the zone where thermal degradation of wood and char formation is actually occurring and the char layer, a zone of cracked charcoal that has no relevant strength or stiffness properties. Charring depth is the distance between the outer surface of the original member and the position of the char line. The base of char layer is widely occurs between 280 and 300°C. Beikircher et al. determined the charring rate, mass loss and temperature development of Norway wood coated with transparent and colored intumescent coatings using cone calorimeter and ISO 834 furnace test. They found that the intumescent coatings reduce the charring rate significantly at all irradiance and cellulosic fire exposure condition (ISO 834 test curve) in comparison with the uncoated (REF) wood sample (**Figure 2**). Intumescent coatings can delay the onset of charring

mental issues [24].

106 New Technologies in Protective Coatings

m2

m2

ence on the material dependent properties [27].

and reduce the charring rate of wood [29].

**6. Charring rate of wood**

Boric acid (H<sup>3</sup> BO<sup>3</sup> ) and borax (Na2 B4 O7 .10H2 O) are used as borates, which are water soluble and most commonly used flame retardants in wood products. Boron-containing formulations are also used to improve the service life of timbers, in terms of both increase in resistance to biological attacks and renders more resistant to burning. Atar and Keskin studied the flammability of varnish coated Uludag Fir wood and boron compound impregnation. The ASTM D1413-99 standard is used to impregnate the wood by vacuum technique using a mixture of boric acid and borax. The flammability of wood was characterized by ASTM E160-50. They found that boron impregnations showed decrease the flammability of wood as compared to varnish coated wood. It was suggested that the impregnation of wood with boron compounds before varnish coating can decrease the combustion temperature and provide security to wood structure. Further, they investigated the impact of boron compounds impregnation on combustion properties of the laminated veneer lumber obtained from European oak and Lombardy poplar woods. Because of the interaction of impregnation materials with wood structure, the lowest flame source combustion was observed in treated wood [30, 31].

Blasi et al. have investigated the pyrolysis products of fir wood impregnated with boric acid at heating temperatures of 377 and 527°C. It was found that the yields of char and water increase with boric acid concentration (below 2%), and the amounts of organic liquid products are reduced. The boric acid treatment lowers the activation energy and delayed the most important oxidation reaction of fir wood. The reaction temperature does not affect the pyrolysis product distribution but at lower temperature, the higher the char yields, water and non-flammable volatile products were observed [32]. Further, the effect of diammonium phosphate (DAP) and diammonium sulfate (DAS) treatment on the pyrolysis of wood was investigated. Two ammonium salts are widely used as flame retardants in wood substrate, and both are significantly alter the char reactivity. A lower activation energy and a higher reaction order are obtained for DAP-treated sample as compared to wood treated with DAS [33]. Both treatments produce an equal amount and composition of solid, liquid and gaseous products during decomposition. However, with increasing the concentration of salts and/or decreasing the heating temperature produce greater amount of char and water. It was concluded that DAP treatment showed better flame retardancy on the basis of the formation of higher yields of char, water and lower combustible flammable liquids. It also confirmed that the release of decomposed volatile products depends on the DAP concentration [34, 35]. The thermogravimetric analysis was carried out in air of wood and wood impregnated with DAP concentrations range up to 20% and heating rates between 5 and 20°C/min. Results showed a three step decompositions in sequence of wood decomposition, induction and char oxidation, and concluded that the estimated kinetic parameters are independent of the heating rate but vary with the DAP concentration. However, the activation energies of the various steps remain practically constant except for the decomposition of the cellulose component or the decomposition step, depending on the complexity of the mechanism [36].

The fire performance of Douglas fir wood was studied by both natural extractives and a mixture of boric acid and borax treatment. Dual treatments of wood with the natural extractives and borates were targeted to benefit from their potential cumulative protections, which are biological resistance and fire retardancy. It was observed that both treated wood specimens showed excellent fire retardant performance [37]. Tomark and Cavdar studied the effect of boron powder (BP), mixture of boric acid (BA) and borax (BX) and flame retardant agent (FA) based on liquid blend of limestone and silicone oil (SO) treatment on the oxygen index (OI) of Scots pine wood of bare and after leaching process. Leaching procedure was carried out to determine the permanent performance of the preservatives in wood. The oxygen index (OI) is the minimum percentage of oxygen required to continue flaming combustion of a sample under laboratory condition. Wood samples were initially vacuum treated with the preservative and then were subjected to leaching. **Figure 3** showed that the wood treated with flame retardants provided the best results, and moreover, leaching did not considerably change the

**Figure 3.** OI of un-leached and leached flame retardant-treated wood [38].

OI of wood. However, OI of treated sample was affected by leaching procedure, which may be due to the preservatives are not chemically adhered to the wood [38].

Disodium octaborate tetrahydrate (Na2 B8 O13.4H2 O) in waterborne paint treatment provides better fire retardant properties of Scot pine wood. The properties are further improved by tannin-based wood preservative solutions [39]. Further, the effect of wood preservative loading on the OI of fir wood was investigated. Wood preservatives have been widely used to extend the service life of wood. Study indicated that almost all treated samples showed higher OI values, and moreover, 3% copper-based preservative (Wolmanit-CB) was recommended [40]. The improvement in dimensional stability, durability and fire retardant properties of wood was investigated by using 1,3-dimethylol-4,5-dihydroxyethyleneurea treatment. The Oak wood showed considerable decrease in the pyrolysis temperature, heat release rate, and smoke production when methylolguanylurea phosphate and boric acid flame retardants were incorporated [41]. There is a current contentious toxicity problem with boron compounds, and as such, a need for wood products to move away from boron-based fire retardants is recognized.
