Mechanisms of Fire-Retardant Action

Mechanisms of Fire-Retardant Action 315 Table 17.1 Limiting oxygen index values for selected polymers [29],... 

The combustion behavior of melamine pyrophosphate and dimelamine phosphate are different from those of melamine and the other melamine salts (Table 3.4.1). The former are ineffective at low concentrations (> 15%) and become effective at a loading of 20-30% because the intumescent char is formed on the surface of burning specimens. The mechanism of fire retardant action of both melamine pyrophosphate and dimelamine phosphate is similar to that of APP since, by analogy with ammonia melamine volatilizes, whereas the remaining phosphoric acids produce esters with nylon-6, which are precursors of the char [146]. Some part of the freed melamine condenses probably forming melem and melon [147]. 

Mechanism. No single mechanism explains the action of all fire retardants, so they probably work through a combination of several mechanisms. The mechanisms of fire retardants in wood involve a complex series of simultaneous reactions whose products may affect subsequent reactions. Pyrolysis of cellulose involves dehydration, depolymerization, decarbonylation, decomposition of smaller compounds, condensation, and other reactions. These pyrolysis reactions occur both in the solid phase and vapor phase. Addition of fire retardants will alter the reactions however, this alteration will depend on the additives, the material, and the thermal-physical environment. The presence of oxygen adds subsequent and competitive oxidation reactions to the above series. These oxidative reactions can take place in both the solid and vapor phases. Evidence indicates that most fire retardants reduce combustible volatiles production and limit combustion to the solid phase. The best retardants also inhibit solid-phase oxidation to effectively remove the fuel from the fire. 

Costa, L. Camino, G. Luda, M. P. Mechanism of condensed phase action in fire retardant bismuth compound-chloroparaffin-polypropylene mixtures Part I—The role of bismuth trichloride and oxychloride, Polymer Degradation and Stability, 1986, 14(2), 159-164. 

Camino, G. Duquesne, S. Delobel, R. et al. 2001. Mechanism of expandable graphite fire retardant action in polyurethanes. In Fire and Polymers. Materials and Solutions for Hazard Prevention, Nelson, G.L., Wilkie, C.A., Eds. ACS Symposium Series 797 American Chemical Society Washington, 2001 pp. 90-109. 

An early fire-retardant treatment for paper and cotton was to heat them with phosphoric acid and urea at 145-180°C to form insoluble ammonium polyphosphate, (NH4P03) . Ammonium polyphosphate is also used in intumescent paint formulations, where, like the orthophosphate, it releases ammonia and phosphoric acid on heating. The latter facilitates charring which, together with the release of ammonia, retards local combustion. A similar mechanism is believed to account, at least in part, for the fire-retardant action of many other phosphorus compounds. There is now evidence that impregnation of wood with phosphoric acid suppresses the formation of carcinogenic materials during pyrolysis [39]. 

Different behavior was found for SBS-MEL microcomposite, which had a TTI similar to that of pure SBS (85 s), but after 100-110 s a decrease in the HRR was observed, resulting in a delay in the PHRR (TTP = 239 than 204 s). Moreover, SBSMEL shows a PHRR similar to that for pure SBS. The mechanism of the fire retardant action of melamine derivatives is under investigation by many academic and industrial research groups however, it is well known that when exposed to heat and flames, melamine and derivatives decompose, absorbing heat and causing a cooling effect [46,47]. 

Camino G, Duquesne S, Delobel R, Eling B, Lindsay C, Roels T (2001) Mechanism of expandable graphite fire retardant action in polyurethanes. In Nelson GL, Wilkie CA (eds) Fire and polymers. ACS symposium series, vol 797. American Chemical Society, Washington, pp 90-109... 

The mechanism of action of an effective fire retardant acting in the vapor phase should inhibit one or both reactions (Equation 4.2 and Equation 4.3) because they have a paramount effect on the increase of the overall rate of thermal oxidation process occurring in the flame. Indeed, the reaction represented by Equation 4.2 increases radical concentration while reaction represented by Equation 4.3 increases the temperature. From a mass spectrometry study of species sampled in low-pressure flame,4 it is evident that the introduction of halogen species into a premixed CH4/02 flame leads to the production of the hydrogen halide, HX, early in the flame. It was also observed that the production of H2 is enhanced. This provides evidence for removal of H atoms from the flame and the predominant reaction is considered to be... 

Camino, G., Costa L., and Trossarelli, L. 1984. Study of the mechanism of intumescence in fire retardant polymers Part II—Mechanism of action in polypropylene-ammonium polyphosphate-pentaerythritol mixtures. Polym. Deg. Stab. 7(1) 25-31. 

Levchik, S.V., Camino, G., Costa, L., and Levchik, G.F. 1995. Mechanism of action of phosphorus-based flame retardants in nylon 6.1. Ammonium polyphosphate. Fire Mater. 19 1-10. 

In this chapter, an overview is presented of the principal fire-retardant filler types, including details of their origin, characteristics, and application. Consideration will then be given to their mechanism of action both as flame retardants and as smoke suppressants, and to means for potentially increasing their efficiency using synergists and nanoscale variants. 

Their fire-retardant mechanism is predominantly due to condensed phase action involving a combination of endothermic decomposition, water release, and oxide residue formation. 

Lewin M, Weil ED. Mechanisms and modes of action in flame retardancy of polymers. In Fire Retardant Materials. Horrocks AR, Price D, Eds. Woodhead Publishing Limited Cambridge, U.K., 2001 chap. 2, pp. 31-68. 

Vl/7 e have had a continuing interest in flame retardants, methods of test and mechanisms of action of such materials. Recently, we reported studies involving fire retardant additives, syntheses of monomers, and the preparation of copolymers to achieve flame resistance (41). In addition, considerable synthetic work in phosphorus chemistry has been pursued at our Stamford Research Laboratories. Some of this has been reported by Grayson, Rauhut, Buckler, Wystrach, and co-workers. This chapter and the one following result from this background. 

The mechanism of action of flame retardants in thermoplastic materials (polyethylene, polypropylene, polystyrene, cellulosics, PMMA, etc.) is unknown and is certainly quite complex. Broido (7) presented a good example in the difficulties of explaining how fire retardants work. He found that materials which were most effective in preventing flaming combustion of cellulose were also effective in causing sugar cubes to support flame ... 

Basic Mechanisms. Finally, further work is necessary on fundamental mechanisms of individual fire retardants. These mechanisms are a function of the particular chemicals involved and the environmental conditions of the fire exposure. There is a need to establish common methods and conditions for determining these mechanisms in order to compare different treatments. This would give us a better understanding of how these compounds work in action and would provide a more efficient approach for formulating fire-retardant systems than a trial and error approach. Correlations also need to be established between rapid precise thermal analysis methods and standard combustion tests. Retardant formulations could be evaluated initially on smaller (research and development size) samples. The more promising treatments could be tested for flame-spread index, heat release rate, and toxic smoke production. 

The borons are also eflFective and efficient fire retardants for wood. They are leachable but they do not reduce the strength or increase the hygroscopicity of the wood as some other compounds do. Little work has been done on the mechanism of action of the borons. 

Although much work has been carried out on the mode of action of flame retardants generally, the mechanisms associated with tin additives are only partially understood. It is clear that tin-based fire retardants can exert their action in both the condensed and vapor phases, and that the precise action in any particular system depends on a number of factors, including incorporation level, the amount and chemical nature of other additives present, and the nature of the polymer itself. 

Fire retardant additives are added to polymers during conversion or sometimes during manufacture by chemical reaction with the polymer substrate. Although they retard oxidation under burning conditions, they are not normally antioxidants at ambient temperatures, the mechanisms of action of hydrogen hahdes in flames are similar to those of antioxidants in the solid phase (see below). Inhibition of gas-phase oxidation is only one of the functions of flame retardants but, since more people are killed in fires by toxic fumes than by flames, it is of paramount importance to inhibit the initiation step in combustion. 

Another solution to improve the fire-retardant (FR) properties of polymers is the use of intumescent additives [2, 3]. Intumescent technology [4, 5] has found a place in polymer science as a method of imparting flame retardancy to polymeric materials. On heating, FR intumescent materials form a foamed cellular charred layer on their surfaces [6, 7], which protects the underlying materials from the action of heat flux and flame. The proposed mechanism [8] is based on the charred layer acting as a physical barrier, which retards heat and mass transfer between the gas and the condensed phase. 

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