Brominated epoxy resin structure

Kubens (9) found cyanate ester resins were compatible and reactive with epoxy resins. We have confirmed this in our laboratories. Such combinations can be formulated to yield cured materials containing both triazine and oxazoline ring structures. If brominated epoxy resins are employed in such blends, flame retardant materials can be obtained containing as low as 12% bromine versus 20% bromine required to flame retard epoxy systems. Such products show promise for use in printed wiring board laminates. 

Water absorption in epoxy resins, which aiways causes - even if slight - reduction in mechanicai properties, can also be counteracted by changing the structure of the resin/curing agent components. This requires an increase in hydrophobic properties in the network, which can be achieved by the addition of brominated epoxy resins that aiso increase flame resistance. 

Bremner [24] reported on various factors that affect the heat stability of brominated epoxy resins in the presence of air using TGA and isothermal methods. Figure 3.5 shows TGA curves for cured resins based on the structure (Equation 3.30). 

As in polyester resins, reactive halogens containing fire-retardant chemicals are most often used in epoxy materials. Tetrabromobisphenol A is perhaps the most widely used component for flame-retarding epoxy resins. Nara and Matsuyama (24) and Nara et al. (25) described the thermal degradation and flame retardance of tetrabrominated bisphenol A diglycidyl ether compared to the nonbrorainated structure. Their results indicate that bromine acts by vapor-phase as well as condensed-phase mechanisms of flame inhibition. 

Brominated flame retardants (BFRs) are a structurally diverse group of compounds including aromatics, cyclic aliphatics, phenolic derivatives, ahphatics, and phthahc anhydride derivatives (Figure 31.3). The most common BFRs are tetrabromobisphenol A (TBBPA), polybrominated diphenyl ethers (PBDE), hexabromocyclododecane (HBCD), and polybrominated biphenyls (PBB). The primary use of TBBPA is as reactive additive in epoxy resin circuit boards, while decabromodiphenyloxide (DBDO) is primarily used in high impact polystyrene for electronic enclosures. PBDEs are typically used as the additive type of flame retardant in high impact polystyrene, acrylonitrile butadiene styrene, flexible polyurethane foam, textile coatings, wire and cable insulation and electrical connectors. 

Intrinsically non-flammable polymers are few, but phenolic resins have a good reputation both in Are and smoke performance, which has resulted in their becoming increasingly favoured for reinforced plastics structures, for example, underground transport, where such concerns are greatest. Polyether ether ketone (PEEK) is also a low fire and smoke polymer. Unsaturated polyesters, vinyl esters and epoxy resins bum readily, but modified versions are available with improved behaviour. For example, both bromine and chlorine are used extensively in the form of chlorendic (HET) acid, tetrachlorophthalic anhydride (TCPA) and tetrabromo-phthalic anhydride (TBPA) which can be reacted into the polyester in small quantities and can act as permanent (non-migrating) flame retardants. 

Brominated epoxies have all the other characteristics of the resins in the epoxy family, and in addition they are relatively flame resistant because of the bromine constituents in the molecular structure. 

A wide variety of epoxy resins are commercially available monofunctional or polyfunctional, aliphatic, cyclic, or aromatic. Brominated epoxies may be useful where flammability is a concern. An oxirane functionality is all that is needed to make an epoxy resin, and structural adhesives are only one of over a dozen different uses for epoxy resins. Many epoxy resins on the market will not necessarily be suitable for adhesives, but their availability does expand the choices available for adhesive formulators. The specialty epoxy resins developed specifically for adhesive use sometimes will be more costly than the DGEBPA resins but may provide the basis for a specialty adhesive that can meet a unique need and therefore command a proportionally higher price. Examples of these are epoxy-fimctional dimer acids, urethanes, and various elastomers. 

A third category of vinyl ester resin is formed when tetrabromo bisphenol-A (TBBA) is used in the manufacture of the base epoxy starting resin. This affords a resin with up to about 20wt% bromine bound into its structure and is designed to achieve good fire retardancy. An added advantage of the bromine is that because of the large size of bromine atoms, it improves the chemical resistance, particularly towards caustic and hypochlorite solutions [1]. 

---