Re: General Questions

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>

Hi Antonio, All!

>This "quite well", as I could verify, is that Araldite is a bad
>insulator,
>almost useless for electrostatics. Just a bit better than dry wood.
>For Tesla coils, the presence of some glued parts close to the coils
>is not a great problem. A coating of the secondary with this material
>may be a bad idea, however.
>By the way, Nylon is a similarly bad insulator, maybe worse. After
>some bad experiences (have you seen what should be an insulator
drawing
>sparks?) I don't use it in anything involving high voltages.


This observation with epoxy is intriguing, especially as I'm sure some
on the list have used epoxy coatings on their secondaries!

I can understand nylon being very poor, as it will typically take up
2-3% water, and especially in the all-important surface layer.  This I
can get my head round easily, as nylon is chock full of polar
polyamide groups, but epoxy is pretty well devoid of anything polar.

I wonder if the conductivity problem with epoxy is due to residual
byproducts from manufacture?  Typically an epoxy is made from a
mixture of a sodium salt of a bisphenol plus epichlorohydrin.  This
means that the resulting resin as made contains common salt, which is
supposedly washed out in the purification process.  I'm wondering if
there is sufficient sodium chloride left behind in what you get sold
to explain this conductivity problem.  It could be . . . I know that
the epoxies used in the electronics industry have to be specially
purified to eliminate lead corrosion in the packaging of ICs etc and
that these resins are expensive for that reason.

The only other thing which occurs as a possible cause is the hardener
used, but as these are generally amines (they smell "fishy") and as I
have used amines as additives in styrene/acrylate copolymers for use
in photocopier toners, specifically because they hold a good
electrostatic charge (!) I don't feel that's the answer.

Interesting to me as a polymer chemist, but probably dead boring and
irritating to you!

This leaves the question of which plastics are good.  Polyethylene,
polypropylene, polystyrene, polytetrafluoroethylene ($$$) are all
first rate for rf high voltage as mentioned on this list many times by
many people; polyacrylates, polyacetals, polycarbonates, polyesters
nearly as good for many low frequency high voltage purposes (but not
rf) and I think I'd give the rest a miss if I had a choice in the
matter.

My personal favourite for lf ht would be polyacetals, as not only do
they have good ht performance, but excellent machining properties, and
if you have a lathe and some sharp tools you can turn insulators and
what-have-you very easily.  "Delrin" is one brand I've had good fun
with, as always YMMV.

Dunckx