Halogenated flame retardants, environmental concerns

Several compounds may be used as flame-retardants for PS foams but generally halogen flame-retardants are the most used even if, due to environmental and human health concerns, a continuous search for halogen-free FR is going on. A very recent detailed review on FR for PS foams is reported by Levchik and Weil,79 other useful data may be found in patent literature. 

Despite some environmental concerns, halogenated flame retardants still play an important role. With the exception of a few chlorinated products (e.g., chlorinated paraffins), these flame retardants contain usually molecules with several bromo groups (Fig. 11.18). Bromo compounds are active during combustion in the gas phase, whereas HBr reacts with OH and H radicals under formation of less reactive halogen radicals. The flame retardancy of bromo compounds is often synergistically improved by the addition of antimony oxide Sb203. 

The environmental scrutiny that has impacted halogenated flame retardants has primarily focused on brominated diphenyl oxides such as DBDPO. There is concern that these compounds release dioxins when burned. Activity has primarily been in Europe. Currently there are no legislative bans or limits on halogenated flame retardants anywhere in the world, and there are not any on the near-term horizon. However, there are some voluntary bans on selected brominated compounds (particularly DBDPO and related types) in some of the green countries of Europe. In many cases, these brominated products are replaced by other brominated products that are not under immediate suspicion. 

In the modern polymer industry, the various existing types of polymer flame retardants based on halogens (Cl, Br), heavy and transition metals (Zn, V, Pb, Sb) or phosphorus-organic compounds reduce the risk from pol3nner combustion and p5n olysis, but may present ecological issues. The overall use of halogenated flame retardants is still showing an upward trend, but the above concerns have started a search for more environmentally friendly polymer additives. As a result it is quite possible that the future available flame retardants will be more limited than in the past. 

Halogen-based flame retardants have served a great need for effective flame retardancy for several years. Due to relatively recent environmental concerns, there is a continuing trend toward the development of nonhalogenated materials to replace these systems. While this has been underway for quite some time, it does not appear that nonhalogenated materials will be available in the near future. Hence it appears that there is still a need for these materials to prevent fires. 

This chapter has provided a concise account of an important type of flame retardants based on silicon. This class of flame retardants may provide an opportunity to develop systems for fire retardancy that are environmentally friendly. It seems that there is a growing interest in this type of flame retardant, and this trend most likely will continue, given the increasing concern over the release of the halogenated species into the environment. 

The flame retardant chemicals industry has historically been driven by regulations and standards. The normal fire-, smoke-, and toxicity-related standards have been joined by environmental standards provoked by the alleged environmental impact of halogens and the alleged toxicity of antimony. Although suitable replacements have not been found for these materials in all cases, the environmental concern has served to depress their growth levels from what it would otherwise have been and/or channel the growth into alternative chemical products. 

Because of environmental concerns regarding the use of halogenated FR derivatives [17], the demand for an alternative FR to replace the halogenated FR formulations has increased. One set of candidates is a group of aryl phosphates, whose chemical structures and applications for flame retardancy have been reviewed extensively in the literature [18, 19]. Some commonly used aryl phosphates and their properties are listed in Table 10.1. 

Under these conditions, regulations impose the use of materials possessing thermal stability as well as efficient fire retardant properties. In parallel, emissions of smoke must be low, not very opaque, not very toxic, and not very corrosive. This evolution toward greater safety seriously limits the use of many materials and involves the rejection of solutions largely used so far and, in particular, halogen-based flame retardants, on account of environmental concerns. Moreover, analysis of various statistics on plastic consumption (180 MT/yr, with a global annual growth rate of approximately 8%) shows the economic importance of this field and illustrates the world s industrial stake in it. 

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