**8. Phosphorus-based fire retardant coatings**

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

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].

of the mechanism [36].

108 New Technologies in Protective Coatings

Fire retardant treatment on wood with flame retardants of both nitrogen compounds and phosphoric acid was examined. The fire retardance and endurance of wood were influenced by method of treatments, such as heat pressed treatment and heat dried treatment. The heat pressed treatment method was improved the properties of wood as compared to heat dried treatment. It was found that the flame retardant properties were further improved by the amount and functional reactivity of flame retardants with formaldehyde as in the dicyandiamide-formaldehyde-phosphoric acid or melamine-dicyandiamide-formaldehydephosphoric acid system. The endothermic release of water during the condensation of phosphoric acid can also cool the wood and dilute the volatile pyrolysis gases [42]. Similarly, Subyakto et al. also demonstrated the improvement in fire retardant properties of wood by phosphoric acid treatment, preheating and densifying the surface of wood. Trimethylol melamine formaldehyde resin mixed with phosphoric acid was coated on the wood surface, which was preheated and followed by hot pressing. The pressurized impregnation of coating was improved the fire retardancy of wood without reduction in the bending strength [43]. The fire retardancy of white pinewood was improved by treatment with orthophosphoric acid at different concentration. The aqueous solution of thiourea-formaldehyde resin and orthophosphoric acid was impregnated at different concentration for 1 h. It was found that the weight, compressive strength and fire retardant properties were improved after impregnation. However, the water uptake of treated wood was increased in a water soaking test for 168 h [44].

Organophosphorus flame retardant compounds are not often used in commercial wood applications, and some are known to have high volatility. The synergism between phosphorus and nitrogen is also observed in organophosphorus compounds. Rupper et al. investigated the surface chemistry of cellulose treated with fire retardants containing ethyl ester phosphoramidates. Evidence for a condensed-phase action of the fire retardants was found [45]. It was also showed that phosphoramides, when bonded to cellulose, increase the char yield and lower the weight loss rates in comparison to phosphorus pentoxide and amine-treated wood [46]. The modified pine sawdust with alkyl and phenyl chlorophosphorus compounds using pyridine was prepared by Stevens et al. They reported a reduction in the temperature of maximum pyrolysis rate of up to 90°C and an increase in char formation of up to 29%. The efficiency of the phenyl phosphates was favored compared to the alkyl analogues, and the order of effectiveness was, expectedly, attributed to the acidity and thermal stability, that is, phosphate > phosphonate > phosphinate [47]. Further, it was showed that coupled with a copper-based preservative, the impregnation of wood with an organophosphorus fire retardant reduced the FIGRA by a further 15% [48].

In another study, wood was treated with guanidine compounds, such as guanidine dihydrogen phosphate, diguanidine hydrogen phosphate, guanidine carbonate, and guanidine nitrate and analyzed thermal degradation properties by using thermogravimetric analysis. Char yields were increased compared to untreated wood by approximately 60, 55, 20 and 25%, respectively, which effectively demonstrates the synergism by phosphorus and nitrogen based fire retardants [49]. The effect of urea-formaldehyde oligomer reacted nitrogen-phosphorus flame retardant treatment on dimensional stability and the flame retardant properties of wood was investigated. The results showed that both dimensional stability and OI values were improved significantly in flame retardant impregnated wood. The better flame retardant properties of treated wood are due to formation of protective char layer through dehydration of polysaccharides, which terminate both heat and oxygen [50].

Stejskal et al. studied the flame retardant properties of wood coated with polyaniline, and the coating was made in hydrochloric or phosphoric acid solutions in the absence and presence of stabilizers, poly(n-vinylpyrolidone) or colloidal silica. The coated wood showed less mass loss and formation of charcoal layer on the surface when exposed to direct flame or in a furnace temperature at 400–600°C, as compared to uncoated wood. The similar observation was made with polypyrole and poly(1,4-phenylenediamine) as deposition polymers in wood. The soaking of wood in polyaniline colloids was badly affected the flame retardant properties, whereas the reaction between the cellulose fibers and polyaniline was required to enhance the stability of wood at high temperature. This is attributed to the formation of carbonaceous microtubes, which offered the higher stability of wood against flame and heat exposure [51]. Cyclophosphazene is a polymeric material containing both nitrogen and phosphorus, which has wide range of thermal and chemical stability in addition with fire retardant properties. El-Wahab et al. synthesized the three kinds of cyclodiphosh(V)azane compounds (I-III) and physically mixed in polyurethane varnish formulation at different concentration. It was found that OI of wood panels was increased with loadings (**Figure 4**). They claimed that improvement was mainly due to several factors, the high molecular weight and aromatic cyclophosphazenes containing chlorine, nitrogen and phosphorus that provides superior flame retardant properties. The presence of N-P bonds renders exceptionally thermally stable, and release less toxic and corrosive gases during burning [52].

**Figure 4.** OI of wood coated with varnish containing cyclodiphosh(V)azane compounds [52].

### **9. Coatings with synergistic flame retardants**

the surface chemistry of cellulose treated with fire retardants containing ethyl ester phosphoramidates. Evidence for a condensed-phase action of the fire retardants was found [45]. It was also showed that phosphoramides, when bonded to cellulose, increase the char yield and lower the weight loss rates in comparison to phosphorus pentoxide and amine-treated wood [46]. The modified pine sawdust with alkyl and phenyl chlorophosphorus compounds using pyridine was prepared by Stevens et al. They reported a reduction in the temperature of maximum pyrolysis rate of up to 90°C and an increase in char formation of up to 29%. The efficiency of the phenyl phosphates was favored compared to the alkyl analogues, and the order of effectiveness was, expectedly, attributed to the acidity and thermal stability, that is, phosphate > phosphonate > phosphinate [47]. Further, it was showed that coupled with a copper-based preservative, the impregnation of wood with an organophosphorus fire retar-

In another study, wood was treated with guanidine compounds, such as guanidine dihydrogen phosphate, diguanidine hydrogen phosphate, guanidine carbonate, and guanidine nitrate and analyzed thermal degradation properties by using thermogravimetric analysis. Char yields were increased compared to untreated wood by approximately 60, 55, 20 and 25%, respectively, which effectively demonstrates the synergism by phosphorus and nitrogen based fire retardants [49]. The effect of urea-formaldehyde oligomer reacted nitrogen-phosphorus flame retardant treatment on dimensional stability and the flame retardant properties of wood was investigated. The results showed that both dimensional stability and OI values were improved significantly in flame retardant impregnated wood. The better flame retardant properties of treated wood are due to formation of protective char layer through dehydration

Stejskal et al. studied the flame retardant properties of wood coated with polyaniline, and the coating was made in hydrochloric or phosphoric acid solutions in the absence and presence of stabilizers, poly(n-vinylpyrolidone) or colloidal silica. The coated wood showed less mass loss and formation of charcoal layer on the surface when exposed to direct flame or in a furnace temperature at 400–600°C, as compared to uncoated wood. The similar observation was made with polypyrole and poly(1,4-phenylenediamine) as deposition polymers in wood. The soaking of wood in polyaniline colloids was badly affected the flame retardant properties, whereas the reaction between the cellulose fibers and polyaniline was required to enhance the stability of wood at high temperature. This is attributed to the formation of carbonaceous microtubes, which offered the higher stability of wood against flame and heat exposure [51]. Cyclophosphazene is a polymeric material containing both nitrogen and phosphorus, which has wide range of thermal and chemical stability in addition with fire retardant properties. El-Wahab et al. synthesized the three kinds of cyclodiphosh(V)azane compounds (I-III) and physically mixed in polyurethane varnish formulation at different concentration. It was found that OI of wood panels was increased with loadings (**Figure 4**). They claimed that improvement was mainly due to several factors, the high molecular weight and aromatic cyclophosphazenes containing chlorine, nitrogen and phosphorus that provides superior flame retardant properties. The presence of N-P bonds renders exceptionally thermally stable, and release less toxic and corrosive

dant reduced the FIGRA by a further 15% [48].

110 New Technologies in Protective Coatings

gases during burning [52].

of polysaccharides, which terminate both heat and oxygen [50].

The combinations of silicon and phosphorus have proved popular in the fire retardancy community, in addition to silicon, phosphorus and nitrogen mixtures. The synergism is generally explained as a combination of the individual effects of each of the three additives: phosphorus provides the effective char formation, nitrogen produces non-combustible gases acting as diluents, and silicon offers thermal stability to the substrate by forming a protective layer over the forming char throughout decomposition. The white deposit of silicon dioxide covering the surface of the char will act as a radiant heat shield and help to reduce the rate of oxidation of the char [53]. Grexa and Lubke studied the effect of magnesium hydroxide (MDH), monoammonium phosphate (MAP), aluminum hydroxide (ATH), and boric acid on the flammability of particle board using cone calorimeter at external irradiance of 50 kW/m2 . The combination of MDH, MAP and boric acid showed better fire retardant properties, in terms of both heat release rate and smoke production as compared to the same composition contains ATH instead of boric acid. It is expected that while the phosphorus and nitrogen will have behaved synergistically to direct the pathway of pyrolysis toward more char and water and fewer flammable volatiles, the boron present will have become molten to form a glasslike barrier on the surface of the wood, stabilizing the char and enforcing the mass transport barrier. Further, the expandable graphite-based intumescent flame retardants showed lower heat release rate and mass loss rate of particle board when compared to the same loading of ammonium polyphosphate. It was found that the char layer forming flame retardants have a strong effect of flame retardation of wood. The forming char has a distinctive effect in the performance of the material when exposed to external heat in comparison to the same flame retardant without a char layer. They suggested that the intumescent flame retardant system has potential to be used to improve the reaction-to-fire performance of wood [54, 55].

Canosa et al. studied the role of reinforcing fibers on the flammability of intumescent flame retardant coated wood panel. Different film forming materials were chosen to be blended with active ingredients, pigment and several fibers, alumina, carbon, aramid, and glass fibers. 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 promotes the uniform char formation [57].

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 CO2 . 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 salts, Na2 WO<sup>4</sup> , Na2 SnO<sup>3</sup> , Na2 MoO<sup>4</sup> on the thermal decomposition and fire retardant properties 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 dehydration reaction, resulting in the formation of more H2 O, CO2 , and char, but less flammable vapors like levoglucosan and levoglucose [60].

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


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