PVC Water Absorption: fallacy?

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