General Flame Retardant Mechanisms

Condensed-phase mechanisms of action are more numerous than the gas-phase mechanisms. Charring, discussed briefly above, is the most conunon condensed-phase mode of action. Again, charring could be promoted either by chemical interaction of the flame retardant and the polymer or by physical retention of the polymer in the condensed phase. Charring could also be promoted by catalysis or oxidative dehydrogenation. 

Some flame retardants show almost exclusively a physical mode of action. 

Examples are aluminum hydroxide and magnesium hydroxide. On the other hand, there is no single flame retardant that will operate exclusively through a chemical mode of action. Chemical mechanisms are always accompanied by one or several physical mechanisms, most commonly endothermic dissociation or dilution of fuel. Combinations of several mechanisms can often be synergistic. 

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Combustion is prevented or stopped by affecting one or more of the three components necessary to support combustion (heat, fuel, and oxygen). Flame-retardant mechanisms cluster into four general classes. 

Physical or chemical vapor-phase mechanisms may be reasonably hypothesized in cases where a phosphoms flame retardant is found to be effective in a noncharring polymer, and especially where the flame retardant or phosphoms-containing breakdown products are capable of being vaporized at the temperature of the pyrolyzing surface. In the engineering of thermoplastic Noryl (General Electric), which consists of a blend of a charrable poly(phenylene oxide) and a poorly charrable polystyrene, experimental evidence indicates that effective flame retardants such as triphenyl phosphate act in the vapor phase to suppress the flammabiUty of the polystyrene pyrolysis products (36). 

Additives are needed not only to make resins processable and to improve the properties of the moulded product during use. As the scope of plastics has increased, so has the range of additives for better mechanical properties, resistance to heat, light and weathering, flame retardancy, electrical conductivity, etc. The demands of packaging have produced additive systems to aid the efficient production of film, and have developed the general need for additives which are safe for use in packaging and other applications where there is direct contact with food or drink. 

The use of phosphorus compounds as flame retardants has been reviewed by Lyons and others (1, 2, 3, 4 5). The mechanism of the action of this element is generally accepted to involve decomposition to produce acids which function as char promoters. Phosphorus compounds are particularly effective flame retardants for polyesters where they function to increase the char yields. 

The mechanism of the action of the phosphonate as a flame retardant is generally believed to be decomposition into acid fragments which contribute to char formation. These acidic species catalyze decomposition of the polyester, and give rise to species which on reaction with the phosphorus moiety cause char formation. TGA curves of the copolymers confirm that the incorporation of phosphorus into the polymer increases the char residue (Figure 4). These curves, however, show little evidence that the presence of phosphorus has any effect upon the temperature or rate of decomposition of the polyester. The curves are all fairly similar up to about 450°C. After that point, the amount of residue is proportional to the amount of phosphorus in the terpolymer. 

For more than a decade, numerous research studies have been carried out on the flame-retardant properties conferred by nanoparticles and mainly by organo-modified layered silicates (OMLS). Earlier work at Cornell University and National Institute of Standards and Technology in the United States showed that nanocomposites containing OMLS reduced polymer flammability and enhanced the formation of carbonaceous residue (char).14 Owing to a strong increase in polymer viscosity, impairing processability, and also due to the breakdown of ultimate mechanical properties, the acceptable rate of incorporation for nanoparticles to improve flame retardancy is generally restricted to less than 10 wt %. 

In some cases, there are also options for installation that can provide the needed flame retardancy instead of relying on the wire and cable materials. For example, a non-FR cable can be installed within a metal duct to provide the required level of flame resistance. Reliance on a FR or incombustible protective duct or other component that is functionally or mechanically distinct from the wire or cable is considered to be outside the scope of the present discussion, and is generally not a cost-effective approach to cable system design. 

The activity mechanism of most flame retardants currently used has not been established unambiguously so far. Therefore, the division of flame retardants according to mechanistic principles (2,3 abov can be only tentative, and is often based on very general concepts and conjectures about the role of the flame retardant. Actually, the effect of a flame retardant on the combustion process is frequently related both to the condensed-phase and to the gas-phase processes. 

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