Phosphorus compounds, flame-retardant mechanism

The flame retardant mechanism for phosphorus compounds varies with the phosphorus compound, the polymer and the combustion conditions (5). For example, some phosphorus compounds decompose to phosphoric acids and polyphosphates. A viscous surface glass forms and shields the polymer from the flame. If the phosphoric acid reacts with the polymer, e.g., to form a phosphate ester with subsequent decomposition, a dense surface char may form. These coatings serve as a physical barrier to heat transfer from the flame to the polymer and to diffusion of gases in other words, fuel (the polymer) is isolated from heat and oxygen. 

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. 

Vapor-Phase Mechanisms. Phosphorus flame retardants can also exert vapor-phase llamc-rctardam acliun. Both physical and chemical vapor-phase mechanisms have been proposed lor the flamc-reuirdatit action of certain phosphorus compounds, such as triphenvl phosphate. 

There are, of course, other methods for treating fibers and rayon fabrics with organo-phosphorus compounds (9-13) in order to improve their flame-retardant properties. However, the modified fibers present the changed physical and mechanical properties due to large additives utilised and often these additives are eliminated after repeated washings. 

Mono- and diphosphonium halides have been found to be flame retardants for plastic materials. Their effectiveness can be related to the formation of various active phosphorus compounds, as well as to many of the postulated mechanisms for flame retardant action. The compounds are postulated to be effective because they decompose on ignition to thermally stable phosphine oxides or phosphonic acids which, in turn, are decomposed to continuous films of phosphate glass. In addition, the phosphonium halides form alkyl halides which cool the flame and/or form halogen acids which are fame retardants. 

Related problems must be considered in individual products. Bromine, chlorine, and antimony add to the smoke of a fire, while phosphorus and water do not, and some metal oxides can actually reduce it. Toxicity of combustion gases is a major concern but the main problem is that oxidation of carbon compounds in an enclosed space—indoors— produces carbon monoxide, no matter whether the carbon compounds are wood or plastics. Other problems include the cost of flame-retardants, difficulties in processing, and loss of mechanical or thermal properties. 

In contrast, phosphorus compounds act as char-forming agents which result in reduced generation of flammable gases. Both reaction mechanisms in flame retardance are different, and the combined use of the two is recommended. 

The structural chemical formulae of flame-retardant organic phosphorus compounds is given in Table 5.7. They can act either by an additive or a reactive mechanism. 

These studies clearly showed that the chemical structure of the phosphorus atom plays a major role in the flame retardancy, with phosphonate groups proving to be more active than the corresponding phosphate groups. Furthermore, phosphonated groups linked to the polymer backbone and obtained by copolymerization had a low effect on mechanical and physical properties in comparison with phosphonated compounds added to polystyrene (PS), where a decrease of these properties was obtained. The additives acted in the vapor phase, whereas phosphonated copolymers were active both in the vapor and condensed phases. 

Flame retardants reacted with the polymer backbone usually provide protection at lower levels and thus have a smaller, usually malign, influence on the compound s mechanical and physical properties. In the case of phosphorus-containing chemicals utilised for FR purposes, there exists a... 

Flame retardancy of m-PPO is improved by incorporating thermally stable phosphorus based additives, such as red phosphorus and organic phosphoms compounds like triphenyl phosphate, triphenyl phosphine and triphenyl phosphine oxide. The mechanism would appear to be gas phase activity rather than reactions in the condensed phase for all except red phosphoms where both are seen. 

A large number of organic phosphorus compounds are available to provide flame retardancy. However, it is known that such additives do not generally provide sufficient protection in epoxy resins. They are not resistant to migration and affect the mechanical properties. As a consequence epoxies are often proteeted by bromine containing types. 

In certain apphcations, flame resistance can be important. In this case, flame retarders may he added. They act hy one of four possible mechanisms. They may act to chemically interfere with the propagation of flame, react or decompose to absorb heaL form a fire resistant coating on the polymer, or produce gases that reduce the supply of air. Phosphates are an important class of flame retarders. Tritolyl phosphate and trixylyl phosphate are often used in PVC. Halogenated compounds such as chlorinated paraffins may also be used. Antimony oxide is often used in conjunction to obtain better results. Other flame retarders include titanium dioxide, zinc oxide, zinc borate, and red phosphorus. As with other additives, the proper selection of a flame retarder will depend on the particular thermoplastic. 

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