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August 2002

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Tue, 13 Aug 2002 13:47:57 -0400
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Dave,
agree with you on the toxins... the engineering method is to determine what
is probability of burning at what temp. produce what kind of result.  Most
of the Flame retardant product is UL 94-0 rating... as long as you replace
something with the same performance: corrosion, UL, material compatibility
etc.etc. and low cost...I am sure everybody would be happy.  However, few of
the traps some of us went into are the "flashy" stuff, promise a lot,
implemented at the product level without sufficient test in real life data.
To replace a deficient product with an "unknown" is very scary.  Costly
lesson to learn.
...my 2.cents.
                                                      jk

>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]]On Behalf Of David Fish
>Sent: Tuesday, August 13, 2002 1:05 PM
>To: [log in to unmask]
>Subject: Re: [TN] Bromine
>
>
>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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