Cydrome Leader wrote in message
...
N_Cook wrote:
revised pic on that URL
the Philips 373 MKT was polyester as was the yellow TC probably, by that
flame test.
Yellow flame , sustained after flame removal, no drips, slow
progress,brittle remains, although HCl not smelled.
This film 15 microns, WVTR permeability of 25 micron films in g/m^2/day
polyester 4
Polythene 20
Polycarbonate 170, I didn;t realise this was so "porous"
From the boating industry epoxy has much better permeability figure than
polyester.

Now just to find the quantative deleterious effect of water/vapour on
dielectric strength.

polyster is cheap, so that's usually what you'll come across first for a
DC capacitor.

Tape and fill caps aren't considered hermetically sealed. If you want
that you need a metal can, but those cost more money, so moisture ingress
isn't really too surprising- if that's the problem.

in the self healing caps, once the dielecric punches though, the super
thin aluminum coating burns up like a fuse, isolating that are with the
pinhole though the plastic. Everytime this happens, you lose more and more
plate area and capacitance drops.

the low resistance reading is strange though. Even damp nylon probably
won't conduct at much, and it really lik to absorb water.






What happens to the "evaporated" aluminium from all those "35mm frames" ?
perhaps some of it migrates and settles at the edges as a slightly
conductive coating. It was interesting that as I unwound each layer of 2
"frames" the resistance incremented higher each time on a regular basis.
The real fault of these TC is the capacitor proper had to be squashed in the
outer shell and hence the moisture path in at the centre of those "frames"
as the potting was absent at those points , the silvering remains at the
tight curving sections of each turn . I'm amazed at how spectacularly
cleanly all the Al has erroded from those frames
, the remnant silvering is finely punctured though , looking via microscope.
Once an island of silvering emerges with a ring of eroded around it , what
mechanism ensures the erosion of the islands?