TE>From all the discussion on this group about building ones own capacitors
TE>and what types of materials make good coil forms, I became interested in
TE>obtaining some hard (or semi-hard) data on different types of polymers. I

Good on you Ed, this is a great posting, and timely too.

TE>I thought I'd share the results of my library searches:

And I'll share a few facts as well.

I'll have to edit you reply a lot though, this e-mail costs me

TE>polymethyl methacrylate         PMMA            Plexiglas
TE>                                                Lucite

And in the English speaking world (!) it's called Perspex too.

TE>acrylonitrile-butadiene-styrene ABS             Cycopac ter-polymer of
TE>                                                        polystyrene

Well strictly speaking most ABS's are simple mixtures of
polystyrene and nitrile rubber. Nitrile rubber is a co-polymer of
acrylonitrile and butadiene. The nitrile is added to styrene to
give it added impact resistance. Unfortunately ABS is a very
generic term, ABS can have widely varying properties because of
this. It's best to think of it as "impact resistant polystyrene"

TE>there are plastics which are called co-polymers or ter-polymers. A co-
TE>polymer is just taking two different monomers and sticking them together in
TE>a unit cell before polymerizing. Example: PET.

PET is made by polymerising polyethylene glycol and terephthalic
acid together. Gees guy's I _am_ a chemist. So. Moving quickly

Likewsie, a ter-polymer is
TE>just sticking three monomers into a unit cell and then polymerizing. A very
TE>common example is ABS which is used as sewar and drainage pipe.

Wev'e already discussed ABS. Only very rarely three monomers

TE>some of the plastics I could find. However, nobody seems to have data on
TE>the frequency dependence of the dielectric strength. It is best just to
TE>assume that this was done at DC.

There's an ASTM method for this. I have it here... somedamnwhere.

TE>One property which is not well known for polymers is that the breakdown
TE>electric field or dielectric strength (VOLTS/INCH etc...) depends on the
TE>actual thickness of the film. Typically, as the film gets thinner, the
TE>dielectric strength goes up!!! For example, LDPE has a strength of 800volts
TE>per mil at 80 mils, but this goes up to 1400volts/mil at 20mils!!

Are you sure the figures are for the _same_ polymer exactly ?
Some subtle differences in polymer (or additives) can affect the
breakdown strength enormously. Some very good plastics have to be
tested in thin films to get a sensible figure. Others breakdown
too easily and thicker films must be used. Some grades of PE have
antioxidant added which causes poor electrical props, others have
none and are _much_ better.


Jim Oliver <jim.oliver-at-welcom.gen.nz> (3:771/370)

 * SLMR 2.1a *