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Re: Capacitor Container Failure



Tesla List wrote:
> 
> Original Poster: "Ross W. Overstreet" <ross-o-at-mindspring-dot-com>
> 
> I found an interesting puddle of liquid under my Tesla Coil this
> evening.  I really hoped that it was water, but I wasn't that lucky.
> Turned out to be good ole Shell Diala-AX.  I removed my rolled LDPE
> caps, started spreading the kitty litter, and took the caps to the work
> bench.  My capacitor containers are the typical 6" SDR sections with a
> regular flat end cap on the bottom and an end cap modified with a
> window, terminals, and vac attach fittings on top.
> 
> The failure turned out to be a hairline crack in the side of the lower
> end cap.  The crack extends 1.5" vertically from the base.  It doesn't
> appear to go along the bottom of the endcap, but the paint may be
> obscuring it.  I am puzzled by this failure.  This doesn't seem to be a
> highly stressed area.  Being on the side, it is twice and thick as
> anywhere else and definitely didn't see the stresses during vacuum
> degassing that the bottom did.  My only guess is that the PVC cement
> softened it and somehow weakened it.  The coil (6" sec, 12KV, 120ma, 50"
> arcs)  has been run  5 times and the caps are only a couple of months
> old.
> 
> Has anyone ever seen a rolled LDPE cap with a PVC container fail in this
> manner before?  Is painting PVC considered a bad practice?  Are there
> some crazy PVC glue dynamics that I need to be aware of?
> 
> --
> Ross Overstreet
> Huntington Beach, CA
> ICQ# 20762411
> http://www.geocities-dot-com/CapeCanaveral/Cockpit/3377/

Ross,

Bummer! I had a failure in one of my HDPE containers (Rubbermaid
trashcan), due to absorbtion and softenning of the HDPE from the mineral
oil over a 2.5 year span... fortunately it was in another container
which caught the oil! 

The end cap simply may have had a latent manufacturing defect, and was
probably under a degree of tensile stress, especially if the capacitor
roll was initially a "tight" fit, and the roll then swelled after the
poly absorbed some of the mineral oil (can 10%!). Was it possible that
the cap or end-cap was accidently dropped or subjected to extreme cold
and shocked? It also turns out that certain solvents that are used in
paints or plasticized plastics may cause stress corrosion cracking in
PVC that's under tensile stress. If you used a standard PVC solvent
glue, this is normally not a problem. 

However, if you painted over the PVC, there's some chance that a solvent
or plasticizer used in the paint itself may have made the PVC more
susceptible to stress cracking, particularly if there was already a
hidden "starter" crack. Some solvents that may induce this include
toluene, DOP (dioctylpthalate - a common plasticizer), benzene, diethyl
ketone, certain ethyl or methyl compounds, napthelene,
trichloroethylene, and Xylene... well you get the idea! Many of these
are found in paints. However, you'd think that there would only be a
small duration of exposure. A poser indeed!

Safe coilin' to you!

-- Bert --