Brian
Bromine always seemed like such a 'nice' chemical, didn't it?
Let me take you in a different direction. Brominated flame retardant
incineration produces toxins:
* polybrominated biphenyl ether (PBBE)
* polybrominated biphenyl oxide (PBBO)
Dave Fish
----- Original Message -----
From: "b_ellis" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, August 13, 2002 5:14 AM
Subject: [TN] Bromine
> Yet once again, bromine has raised its head in a couple of recent
> threads on this list. I'm beginning to question whether brominated fire
> retardants are as bad as they are made out to be or whether they are
> just painted bad. Unfortunately, my knowledge of polymer chemistry is
> insufficient to form a judgement. Here is what I'm thinking:
>
> The basic manufacture of epoxy resins is in two stages, the
> prepolymerisation and the crosslinking. I won't go into more than the
> basics, as I know that the subject is very complex. The epoxy group is
> provided by epichlorohydrin which is reacted with bisphenol A (BPA) in
> the presence of an alkali (NaOH), the latter serving to react with the
> liberated chlorine atom to form NaCl, during the prepolymerisation. Most
> of the latter is removed for electrical grade resins, but some remains
> (hence the high dielectric constant and poor power factors).
>
> The epoxy groups are subsequently reacted with cross linking agents for
> the final polymerisation (curing) of the resin. Theoretically, if all
> the components are stoichiometric, the resultant cured resin is
> perfectly homogeneous with uniformly-sized molecules. In practice, this
> is never perfect.
>
> The bromine comes in with FR-4 by using tetrabromobisphenol A (TBBPA) in
> place of some of the BPA. This has the characteristic of releasing HBr
> and Br2 when heated to combustion temperature, this, being heavy,
> blankets the burning resin and excludes oxygen, thereby extinguishing
> the flame. TBBPA is identical to BPA other than that 4 of the H atoms
> are replaced by Br atoms, which are therefore covalently bonded to two
> of the non-cyclic carbon atoms. There is therefore no question of any
> ionic bromide being present in FR-4. Why then do bromide ions leach out
> of FR-4, as has been proved time and time again? (Let's discount bromide
> activated fluxes from this equation.)
>
> I have several thoughts on the matter:
> a) could it be that the TBBPA is supplied with excess free Br atoms or
> with brominated impurities that are less stable?
> b) could it be that pressing temperatures already start a breakdown of
> the TBBPA, allowing free HBr to float around the resin matrix? (This may
> explain why FR-4 is marginally poorer electrically than G-10)
> c) could it be that the NaOH is capturing some Br from the TBBPA during
> prepolymerisation?
> d) could it be that the TBBPA is astoichiometrically in excess,
> providing an easily attacked molecule?
> e) could it be that there are no free bromide ions at all, until our
> extraction fluid hydrolyses some organobromine compound? (If this is so,
> then our ion extraction/ion chromatography tests become less meaningful,
> as providing a means of a reaction!)
>
> I think we need the thoughts of a specialist polymer chemist to obtain a
> much better understanding of what is happening and its influence on the
> reliability of our FR-4 substrated assemblies.
>
> Anyone any ideas????
>
> Brian
>
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