Halogenated fire retardants

Another factor potentially affecting the market for halogenated fire retardants is the waste disposal of plastics (see Wastes, industrial). As landfiU availabihty declines or becomes less popular, two alternatives are incineration and recycling (qv). The nature of the combustion products from halogenated products requires carefiil constmction and maintenance of incinerators (qv) to avoid damage to the incinerator itself and a pubHc health problem from the exhaust. The ease of recycling used products also has a potential effect on fire retardants. 

S.K. Brauman and A.S. Brolly, "Sb203 - Halogen Fire Retardance in Polymers I. General Mode of Action,"... 

Chamberlain, "Sb203-Halogen Fire Retardance in Polymers IV. Combustion Performance, "Journal of Fire Retardant Chemistry, 2, 225 (1976). 

Doyle M, Clemens M, Lees G, Briggs C, Day R (1994) Non-halogenated fire retardants for polypropylene. FR 94 Conference, BPF, London, pi93... 

The traditional halogen fire retardants used in styrenic copolymers are decabromodiphenyl ether and octabromodiphenyl ether, tetrabromobisphenol A, bis(tribromophenoxy) ethane, ethylene bis-tetrabromophthalimide, and chlorinated paraffins. Actually the octabromodiphenyl ether has been banned on precautionary principles, as will be explained below. The fire-retardant capabilities of the more effective halogen-containing compounds are in line with the quantity of halogen in the final polymer blend, with consideration for the use of synergists. Thus, the practical utility of these flame-retardant compounds (once the issue of degradation temperature is resolved) is often based on their ability to be blended into the polymer and to not substantially affect the physical properties of the polymers. 

As already seen in Section 4.3, the primary action of halogen fire-retardant action for polypropylene is in the gaseous phase, thus the fire-retardant additives for polypropylene are often based on aliphatic bromine compounds in order to develop bromine at its low ignition temperature. 

Toxicity and environmental concerns led to submission of a proposal to European Union to ban the use of PBDPE in 1989 (111-4301-89-EN Draft). The proposal was rejected on the basis of recommendations issued by a thorough debate between scientists, regulators, producers, and users of fire retardants, stating that banning would involve an unacceptable fire risk since alternatives were not available to replace halogen-based fire retardants with comparable effectiveness. Ever since, fire-retardant research has been mostly devoted to the development of nonhalogenated replacements for the halogen fire retardants. 

Besides the actions taken by the EU regulating bodies, an independent industrially supported thorough risk assessment has been planned and is still under execution on commercial halogenated fire retardants to assess their toxicity and environmental impact. 

Mechanistic studies described above show that halogenated fire-retardant systems can act by a condensed phase mechanism that in some cases could be induced by a halogen-free compound. 

In an attempt to look for alternatives to the use of halogenated fire retardants, which function in the gas phase, an approach has been pursued which controls the polymer flammability by modifying the condensed phase chemistry. Silica gel combined with potassium carbonate have been reported to be an effective fire retardant for a wide variety of common polymers, such as polypropylene, nylon, poly(methylmethacrylate), poly(vinyl alcohol), cellulose, and to a lesser extent, polystyrene and styrene-acrylonitrile.49 The cone calorimeter data shown in Table 8.5 indicate that the PHHR is reduced by up to 68% without significantly increasing the smoke or carbon monoxide levels during the combustion. 

As figure 3 shows the detected recovery rates for the halogenated fire retardent are increased for increasing extraction temperatures. This means that the extraction process depends not only on the density but is also diffusion controlled. The positive effect of the better diffusion of the fire retardent exceeds the lowered solubility of SC-C02. 

Table 17.2 shows the components of pwlymer combustion. In the presence of halogenated fire retardants, the active radical species such as OH, O, and H can be quenched in the gas phase to form species such as H2O, H2, and HX that are relatively less reactive in the combustion cyde. 

The mode of action of halogenated fire retardants depends on the reaction of halogen-based free radicals with excited state fire-propagating components in the gas phase resulting in lower system temperature and a decreased reaction rate for the fire-sustaining chemical processes. 

Thermoset composites are mainly considered due to their intrinsic fire resistance. They provide low flame spread and very low smoke emissions without any addition of halogenated fire retardants or higher loading of filler or reinforcing materials. Phenol-formaldehyde and urea-formaldehyde are the most common thermosetting resins used as adhesives for wood composite products, including plywood, particleboard, fibreboard and oriented strand board [66-69]. 

Sayers, D.R. et aL (1988) Development of low smoke, zero halogen, fire retardant composites based on methacrylate resins, Proc, British Plastics Federation Conference London. 

The advantage of this approaeh is the possibility to reaeh a desirable flame retardant effeet with a lower amount of halogenated compound. Moreover, the addition of metal oxides, sueh as AO, to halogenated fire retardants increases their effieieney through the formation of antimony trihalide, a volatile product that slows reactions in the flame, even tiiough the oxide itself has no effect. In... 

---