PVC Water Absorption: fallacy?

From:  D.C. Cox [SMTP:DR.RESONANCE-at-next-wave-dot-net]
Sent:  Friday, May 29, 1998 1:37 PM
To:  Tesla List
Subject:  Re: PVC Water Absorption: fallacy?

to: Rich

They don't absorb it like a sponge but they absorb on surface through
adhesion and cohesion properties.  I did a lot of experiments with this
property and strongly recommend a good sealing process of PVC -- or else
pay the price especially with RF currents which are great "creepers".


> From: Tesla List <tesla-at-pupman-dot-com>
> To: 'Tesla List' <tesla-at-pupman-dot-com>
> Subject: PVC Water Absorption: fallacy?
> Date: Thursday, May 28, 1998 10:33 PM
> ----------
> 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
> in order to seal them against water ingress.
> Having spent an hour or so in our library with various materials
> 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
> (rigid polythene, alkathene). The volume resistivity is not affected by
> immersion in water, and the surface resistivity is only marginally
> (same is true for most amterials: even touching the test sample will
> an OM change in ohms per square). Nylon and PTFE do suffer, but pretty
> all of the thermoplastics are not prone to absorption. They are
> So, why is it recommended that PVC in particular is treated inside and
> 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
> 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
> 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