PVC Water Absorption: fallacy?

To: "'Tesla List'" <tesla-at-pupman-dot-com>
Subject: PVC Water Absorption: fallacy?
From: Tesla List <tesla-at-stic-dot-net>
Date: Mon, 1 Jun 1998 13:15:43 -0500
Approved: tesla-at-stic-dot-net

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From:  Robert W. Stephens [SMTP:rwstephens-at-headwaters-dot-com]
Sent:  Sunday, May 31, 1998 1:33 AM
To:  Tesla List
Subject:  Re: 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

Richard, All,

I cannot shed any light on the hygroscopic absobicity of PVC, however 
I know that PVC carbon tracks very efficiently with the slightest 
application of sprayed on corona, quickly resulting in catastrophic, 
deep carbon tracking of the material, and the rapid destruction of a 
PVC coilform, or PVC whatever.  A good quality varathane or 
better yet a high quality electrical insulating varnish like R-90 or 
Glyptol will provide a tough surface coating that in addition to 
repelling most harsh chemicals unscathed, will also survive somewhat better 
under the typical corona situations seen in TC's than bare PVC material.

Robert W. Stephens
Director
Lindsay Scientific Co.
RR1 Shelburne, ON Canada L0N-1S5
Tel: 1-519-925-1771   Fax: 
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