Decomposition fire retardancy mechanisms

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

The most widely accepted theory of the mechanism of fire-retardant chemicals in reducing flaming combustion of wood is that the chemicals alter the pyrolysis reactions with formation of less flammable gases and tars and more char and water (4,5,8,21,24-29). Some fire retardants start and end the chemical decomposition at lower temperatures. Heat of combustion of the volatiles is reduced. Shafizadeh (21) suggests that a primary function of fire retardants is to promote dehydration and charring of cellulose. 

Furthermore, the effect of hydrated fillers on polymer fire retardancy will depend not only on the nature of the filler, including its particle characteristics (size, shape, and purity) and decomposition behavior, but also on the degradation mechanism of the polymer, together with any filler/ polymer interactions that might occur, influencing thermal stability of the polymer and possible char formation. 

However, this reaction is always in competition with the other reactions that are taking place (i.e., decarbonylation, condensation, decomposition). The mechanism of a particular fire retardant is the summed effect of all simultaneous reactions. This summed effect is especially evident in the synergism of some compounds the effect of two compounds together is greater than the summed effect of each individual one alone (9, 51, 71-73). 

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. 

Studies of polyamides " also illustrate mechanisms involving MglOlD. Mg(OH)2 decomposes before the decomposition of PA-6 and after the decomposition of PA-66. The water formed by Mg(OH)2 before the decomposition of PA-6 gives a fire retardant effect. However, it is possible that the water thus formed may hasten the degradation of the polymer by hydrolyzing it. 

As MFI appears approximately inverse to molecular weight, a low value is desired for augmenting the mechanical properties. The common range in MFI is 0.1-20. Stabilization against thermal and oxidative decomposition should always be undertaken, while light stabilizers (UVA) and fire retardants are incorporated, when required. 

In general, when compared with the conventional polymer composites, polymer nanocomposites exhibit significant improvements in different properties at relatively much lower concentration of filler. The efficiency of various additives in polymer composites can be increased manyfold when dispersed in the nanoscale. This becomes more noteworthy when the additive is used to address any specific property of the final composite such as mechanical properties, conductivity, fire retardancy, thermal stability, etc. In case of polyolefin/LDH nanocomposites, similar improvements are also observed in many occasions. For example, the thermal properties of PE/LDH showed that even a small amount of LDH improves the thermal stability and onset decomposition temperature in comparison with the unfilled PE [22] its mechanical properties revealed increasing LDH concentration brought about steady increase in modulus and also a sharp decrease in the elongation at break [25]. While in this section, fire-retardant properties and electric properties of polyolefin/LDH nanocomposite were described in detail. 

Borates with zinc, calcium, magnesium, melamine or barium as the cation, combine char formation with endothermic decomposition as a dual mechanism that provides an efficient flame retardant system. Although the exotherm reduction is not as great as that achieved with ATH, there is enough endotherm to delay the initiation of the polymer exotherm by as much as 100 °C in some systems. It is this endothermic reaction in the polymer condensed phase and the formation of a glassy char that makes borate products good s mergists with the fire retardant antimony trioxide. 80% of the boron from zinc borate remains in the char after PVC combustion. 

Fire retardant additives may therefore act in various ways and in any one polymer/additive composition there may be more than one effect. Attention is limited here to effects (chemical or physical) on the polymer decomposition process and to examples of investigations in which the nature of the interaction as it affects the degradation mechanism of the polymer has been established. Some of the most detailed investigations have been made in the absence of oxygen this is appropriate, however, because it reflects the situation in the condensed phase from which the fuel is generated, since the oxygen is removed from the polymer surface by the burning fuel. 

Antimony compounds are generally used as fire retardants in synergistic combinations with halogen compounds. The mechanism of action is not well understood, but may be associated with effects due to the release of the hydrogen halide, acting possibly both in the condensed phase to modify the decomposition of the polymer and in the gas phase as a flame quencher. 

---