Polybrominated diphenyl ethers and polybrominated biphenyls

These materials are examples of a wide range of brominated organic compounds that have been used for many years to impart flame retardant properties to a wide range of goods and products. The two polybrominated diphenyl ethers specifically mentioned by the RoHS directive are penta bromodiphenyl ether and octabromodiphenyl ether. There is a technical assessment underway on decabromodiphenyl ether and there appears to be a strong possibility that this may also be proscribed. The other group of compounds proscribed is the polybrominated diphenyls but these materials have long been out of favour because of the persistence and toxic properties and production ceased some years ago.

Unlike the situation for lead, where a single element is being proscribed via the RoHS Directive, the situation with brominated flame retardants is a little more complicated, especially since although the RoHS Directive only proscribes certain specific brominated flame retardants, the WEEE Directive requires plastics containing brominated flame retardant materials to be removed and disposed of separately from non-flame retardant polymers.

• Epoxy and phenolic resins
  Of the polybrominated diphenyl ethers, the pentabrominated material is not normally associated with direct electronics applications as it is used mainly in polyurethane foams and textiles. There are reports, however, that it has been used in some epoxy and phenolic resins used in electrical and electronic equipment (eg printed circuit boards). If an electronics assembly for some reason contained such a foam or one of these resins, it would be necessary for an alternative to be found.
  
  
• Flame retardants in plastics
  Octabromodiphenyl ether is more likely to find use in electrical and electronic applications since the majority supplied within Europe is used as a flame retardants in ABS (acrylonitrilebutadienestyrene) plastics.

  ABS is widely used in covers and casings for electrical or electronic equipment. It is also reportedly used in nylon, low density polyethylene, polycarbonate, phenol-formaldehyde resins and unsaturated polyesters, as well as in adhesives and coatings and so it may well appear in WEEE/RoHS affected applications.

  Decabromodiphenyl ether is a 'general purpose' flame retardant that has also been utilised with various plastics and textiles. It is typically used at loadings of 10 to 15% weight in polymers and is always used in conjunction with antimony trioxide to enhance the flame retardancy performance.

  A key application for it has been in the high impact polystyrene (HIPS) used for television set backplates, although it has also been used in polymers finding their way into electrical switch-gear, capacitors, connectors and a variety of other applications that will also be affected by the WEEE/RoHS Directive.

  For the range of materials known as polybrominated biphenyls, only decabromobiphenyl was used commercially in recent years, where it has traditionally been to give flame retardancy to styrene based polymers, engineering thermoplastics, and unsaturated polyester resins. Global production of this material ceased some years ago and so, from a WEEE and RoHS Directive perspective, it is only likely to be encountered in historic WEEE.

Summary - PBEs and PBDEs

There are large number of brominated flame retardants available commercially and an even greater number of non-brominated ones that are not affected by the new Directives and which might be suitable alternatives to the materials that will be proscribed.

This makes the selection of a suitable alternative more difficult because each type of replacement may have different properties and costs and they operate by a range of different flame retardancy mechanisms. The choice of an alternative material may, therefore, have significant cost or performance implications and a specific choice should only be made after consideration of the application and the key performance criteria which must be met.