Halogen-based Flame Retardants for Styrenics

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 order of effectiveness of halogens is I Br Cl F [20]. Iodine-containing organic compounds are too thermally unstable to be melt compounded with polystyrene, and organoflorine comounds are too thermally stable to be effective as flame retardants. This leaves bromine- and chlorine-containing compounds as the most effective flame retardants. 

Aliphatic chlorine compounds find some utility as flame retardants for styrenic polymers, but aromatic chlorine compounds are probably too stable to be effective [21]. Aliphatic bromine compounds are too thermally unstable for com- 

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. 

Because expanded polystyrene foam is processed at a lower temperature, aliphatic bromine compounds such as hexabromocyclododerane (HBCD) can be used for this application. The flame retardant levels in these systems are family low, typically less than 3wt%. These levels are sufficient to pass the Steiner Tunnel test, and synergists such as antimony trioxide are not necessary. 

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