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March 2015 • The PCB Magazine 13 FIRE RETARDANCy: WHAT, WHy, AND HOW continues Feature figure 2. as "free of halogenated flame retardant." Indeed the IEC defines halogen free thus [6] : • 900 ppm maximum chlorine • 900 ppm maximum bromine • 1500 ppm maximum total halogens It is necessary to have such a definition as halogens are always present to some extent. Ep- oxy resins, for example, are produced by react- ing bisphenol-A with epichlorohydrin, which contains chlorine. Therefore all epoxy resins contain measurable traces of chlorine; iodine is also widely used as a necessary constituent of the catalyst used in epoxy resin production. It is an interesting diversion to consider PTFE materials which are used in some PCB applications. PTFE contains fluorine which is highly toxic and the most reactive halogen but seems to have largely escaped attention in the context of halogen free materials. However, in PTFE the fluorine is tightly bound to the carbon atoms and is therefore not available as free fluo- rine. It is useful here to underline the distinc- tion between compounds and native elements and their respective physical and chemical properties. The best common example would be that of table salt, NaCl which comprises the ele- ments sodium and chlorine. Sodium is a highly reactive metal and is classified as being very hazardous in case of skin contact and chlorine is a highly corrosive toxic gas. However, the two elements combine to form sodium chloride which is benign and essential to human life. The same distinction must be drawn when considering flame retardants incorporating phosphorous and bromine. These are toxic in elemental form; however when used reactively as flame retardants have limited bioavailability and as such do not pose any significant threats to human life or the environment. V. Toxicology Studies There have been many studies into the toxi- cology of flame retardants and there is no sign of activity in this area abating. It is of great im- portance that people are protected from risk of death or serious injury by fire, but also that the systems used to provide that protection do so without adding risks to human health or the environment. The most widely used flame retar- dant in PCB materials is Tetrabromobisphenol- A or TBBPA, and is amongst the most studied flame retardants. A study carried out in 1999 by the Univer- sity of Surrey [7] assessed the risks posed by com-