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 teflon coated wire I spec'd.  Apparently the wire in question is obsolete,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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