Brian,
Some of polyimide got Corona discharge problem... But the CR type solve
the problem (Corona resistance).  Nothing is perfect (otherwise, we
would out of job ;-).
                                                               jk

>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
>Sent: Thursday, April 24, 2003 4:44 AM
>To: [log in to unmask]
>Subject: Re: [TN] wire coatings
>
>
>May I come back to the chemical safety problem?
>
>PVC will support combustion and will give off toxic fumes
>(mainly hydrogen chloride and carbonyl chloride) if heated to
>beyond decomposition temperature. Carcinogenicity is not a
>valid reason as it is only by repeated exposure over a long
>period (chronic toxicity) will it kick in. A single exposure,
>such as in a fire, is a question of acute toxicity, where the
>metabolic action of the toxins is different (in this case,
>mostly from the HCL potentially causing damage to lung tissue).
>
>PTFE (Teflon (R)) also gives off potentially toxic fumes, in
>the event of heating to decomposition temperature, such as in
>a fire. However, because the molecule is perfluorinated, it
>will not burn. The decomposition products, in conjunction with
>other organic molecules or in the presence of water vapour are
>actually more toxic than with PVC (hydrogen fluoride and
>carbonyl fluoride) and deaths have been recorded due to a
>small chip of PTFE landing on a lit cigarette end.
>
>So the major difference, from the chemical point of view, is
>the higher combustibility of PVC.
>
> From the physical point of view, PTFE is an infinitely
>superior insulator, provided it is well sintered (pure PTFE
>does not melt before decomposition). It has a lower dielectric
>constant, so that connections will have a different
>characteristic impedance in high speed/frequency applications.
>However, because it is less "plastic", as others have
>observed, it's a bugger to strip off wires without damaging
>the metal with nicks and breaking off strands. Obviously, the
>thicker the strand, the less likely it will break, but there
>is always a higher risk of mechanical weakening of the metal
>where the insulation ends, than with PVC. Badly sintered PTFE
>insulation is often a cause for reliability problems, as well.
>Coloured PTFE is sometimes electrically inferior as some
>pigments have poorer characteristics than pure PTFE.
>
>Polyimides (e.g., Kevlar) are sometimes a good compromise but
>may cause insulation problems if mechanically stressed (sharp
>bends) under wet conditions.
>
>Brian
>
>Genny Gibbard wrote:
>> Do you remember my question regarding the midplane assembly?
>Turns out
>> that there was a reel of mislabelled wire in house, and a
>whole bunch
>> of assembly has been done using a PVC coated wire, instead of the
>> teflon coated wire I spec'd.  Apparently the wire in question is
>> obsolete, but they like using it better than the teflon, and want to
>> continue to use that. I don't know much about wire coatings
>- what is
>> the difference between these two coating types, and when would you
>> want to or not want to use them.  I need to decide whether
>we need to
>> force them to change out the wiring they've already done.
>> Sigh...
>> Thanks for your help in advance.
>>
>>
>> Genny Gibbard (mailto:[log in to unmask])
>>
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