Styrenic polymers halogen-based flame retardants

As mentioned previously, halogen-based flame retardants are the most widely used for styrenic polymers. Since halogen-based flame retardants act primarily in the vapor phase, the halogen-containing compounds need to decompose and evolve HX in the same temperature range in which polystyrene pyrolyzes (> 300 °C). Another consideration is that the flame retardant needs to be sufficiently thermally stable to be melt compounded with polystyrene. 

The most common halogen-based flame retardants used in styrenic polymers are listed in Table 29.1 [23]. The majority of these are brominated aromatic compounds used to flame retard HIPS and ABS. As mentioned in Section 4, roughly 10 wt% of bromine is required to pass UL 94 V-0 requirements. Antimony trioxide is also used in combination with these brominated compounds. 

Table 29.1 Halogen-based flame retardants used in styrenic polymers. 

In polymers such as polystyrene that do not readily undergo charring, phosphoms-based flame retardants tend to be less effective, and such polymers are often flame retarded by antimony—halogen combinations (see Styrene). However, even in such noncharring polymers, phosphoms additives exhibit some activity that suggests at least one other mode of action. Phosphoms compounds may produce a barrier layer of polyphosphoric acid on the burning polymer (4,5). Phosphoms-based flame retardants are more effective in styrenic polymers blended with a char-forming polymer such as polyphenylene oxide or polycarbonate. 

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. 

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