PVC Water Absorption: fallacy?

From:  Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
Sent:  Saturday, May 30, 1998 7:12 AM
To:  Tesla List
Subject:  Re: PVC Water Absorption: fallacy?

Tesla List wrote:
> ----------
> From:  R M Craven [SMTP:craven-at-globalnet.co.uk]
> Sent:  Thursday, May 28, 1998 2:49 PM
> To:  Tesla List
> Subject:  PVC Water Absorption: fallacy?
> Many people recommend that PVC coilforms should be varnished inside and out,
> in order to seal them against water ingress.
> Having spent an hour or so in our library with various materials handbooks,
> I see no mention  of any significant water absorption of any of the rigid
> PVCs which are encountered. It is a very good plastic, comparable with HDPE
> (rigid polythene, alkathene). The volume resistivity is not affected by
> immersion in water, and the surface resistivity is only marginally worsened
> (same is true for most amterials: even touching the test sample will cause
> an OM change in ohms per square). Nylon and PTFE do suffer, but pretty well
> all of the thermoplastics are not prone to absorption. They are impermeable.
> So, why is it recommended that PVC in particular is treated inside and out
> with varnish? Is it actually the case that, in doing so, we create a more
> tacky surface on which to wind our secondary wire? Is it so that the wire
> will slightly embed itself in the varnish, get a grip, and thus exclude
> air-pockets which might harbour water vapour?
> I think the reason for varnishing might well be moisture related, but it is
> to exclude air pockets, nothing to do with water absorption. If so, then
> surface prep. should be carried out on all coilforms (which I acknowledge is
> recommended by the experienced builders on the list)
> If someone can quote me a BS or ASTM or other document that states
> hygroscopicity of PVC, then I guess i'm wrong!
> Any comments from people who've built an untreated PVC secondary and been
> able to do a comparison with a varnished one?
> Richard Craven, Malvern, England

Richard and all, 

I agree that technical information on this effect is sparse, and bulk
PVC certainly is an excellent insulator. However, there does seem to be
some empirical evidence for poorer performance at high humidity, at
least by some Tesla coilers. For example, I've got a tandem RQ/TCBOR
Static gap with a total of 12 gaps housed in single 6" PVC pipe. The PVC
was not specially pre-treated in any way. On humid days, there appears
to be a mechanisn whereby the surface of the PVC becomes significantly
more conductive, leading to sporadic flashovers exceeding 2" long along
the surface of the PVC. This doesn't happen on days with lower humidity. 

I also notice that the coil's output does not appear to be as great, and
there's more difficulty with breakout when the coil is run on humid
days. Best performance is in the winter when the relative humidity is
quite low. The PVC coilform was sanded and pre-sealed on the outside,
but not on the inside. 

I seem to remember reading somewhere (perhaps in the old archived notes
from Richard Quick) that bulk PVC itself was not the problem - it was a
surface layer (decomposition from ozone or UV photo-oxidation?) that
leads to degraded electrical performance and H20 absorbtion. The Modern
Plastics Encyclopedia, 1990 (McGraw-Hill) hints at this. A summary:
Dehydrochlorination is the most prevalent degradation mechanism for PVC,
breaking down the polymer to liberate HCl, which then acts as a catalyst
to aid in further decomposition of the resin. 

As decomposition progresses, the material gains a yellowish color (very
evident inside my UV-exposed spark gap), then amber, reddish brown, and
ultimately black. While not state directly, it should be expected that
these degradation products at the surface will detract from PVC's
otherwise excellent dielectric properties. Perhaps humdity/H2O
absorbtion is not within the bulk PVC, but confined to a [degraded]
surface layer??

-- Bert --