From: "Barry L. Ornitz" <ornitz@dpnet.net>
Subject: Dielectric Coatings (part of discussion on toroids)
Date: 16 Dec 1998
Keywords: dielectrics. loss tangent, plastic, coatings
Newsgroups: rec.radio.amateur.homebrew

I would like to thank Roy, W7EL, for an excellent post on the measurements
he made on toroids with insulating coatings.  In a later post, Dave, KE9ED,
asked about beeswax as a coating.  Finally in private email to me, Nick,
KB1DEJ, asked about stearin candlewax.  Perhaps I can add a little to this
discussion by talking about the chemical compositions of the different
materials.

I cannot address the "black sealing tape" Roy mentioned but Duco Cement's
effects would be expected.  Duco Cement is a mixture of cellulose nitrate
and camphor in a mixed solvent consisting of acetone, butyl acetate, and
isopropanol.  Once the solvent evaporates, only the cellulose nitrate and
camphor are left behind.  This plastic solution is actually one of the
earliest manmade plastics; its older name is collodion.  The camphor acts
as a plasticizer to soften the very brittle cellulose nitrate. Cellulose
nitrate is a moderately polar compound by itself, and the camphor is quite
polar.  As a dielectric, this plastic would be expected to have much higher
dielectric losses than nonpolar compounds.  [By the way, cellulose nitrate
is slightly different from nitrocellulose in that not all of the hydroxyl
groups are nitrated.  It still is exceptionally flammable.]

Stearin candlewax is essentially the wax produced from animal fats or
tallow.  It is a long chain fatty acid mixture and is rather polar.
Likewise beeswax is a mixture of fatty acids and fatty acid esters.  It too
is rather polar.  Neither are particularly good dielectrics.  I would avoid
them in favor of paraffin wax which is a product of petrochemical refining.
Petroleum jelly is just a lower molecular weight variation of paraffin wax
that is semisolid at room temperature; it too is a good dielectric.

Roy mentioned true coil dope.  This used to be a cellulose ester dissolved
in a solvent (all solutions of cellulose esters are called "dopes" - I do
not know why).  Today this is generally polystyrene in a solvent.  It has
one of the lowest dielectric losses of any soluble plastics.  I make it by
dissolving a big bag of styrofoam "peanuts" in acetone and/or toluene.
[When you first mix it up, it makes a "fuzzy" mess; allow it to sit for a
few weeks for the polystyrene to fully go into solution and for various
pigments and filler materials to settle out.]

I have made a cement similar to Duco Cement by dissolving cellulose acetate
in acetone.  This is essentially the original coil dope.  The cellulose
acetate is slightly less polar and has slightly better dielectric
properties than the nitrate.  But like cellulose nitrate, it is brittle
unless a plasticizer is added.  Tricresyl phosphate may be used but it
hurts the dielectric properties again.  I have found dioctyl phthalate
(DOP) to be a better plasticizer for electrical applications but it still
leaves much to be desired.

As I mentioned earlier, the purpose of the plasticizer is to act as an
"internal lubricant" and "solvent which does not evaporate" to keep the
finished plastic flexible.  Soft, stretchy vinyl is another material that
relies on a plasticizer to keep it soft.  Polyvinyl chloride (a poor RF
dielectric) is a very brittle material; many commercial vinyl products can
be as much as 30% plasticizer.   DOP is commonly used with vinyl too.  As
vinyl ages, the plasticizer gradually oozes out leaving the surface sticky,
or it evaporates.  In your automobile in the summer heat, the plasticizer
evaporates from the vinyl seats and dashboard and condenses on the inside
of the windows leaving a sticky, messy coating that is difficult to remove
(*).  In the older styles of coaxial cable with vinyl jackets, the
plasticizer could leach out of the jacket and be absorbed by the
polyethylene insulation around the center conductor.  This is the so-called
"contaminating" coax.  It leads to high RF losses after a few years.  Newer
"noncontaminating" coax uses either improved plasticizers or a foil and/or
polyester film layer to prevent the plasticizer from reaching the
polyethylene.

The dielectric properties of silicones and epoxies are very dependent on
the actual compositions used.  Since these vary greatly with the products,
it is difficult to generalize.  The dielectric losses are usually less with
the cured materials, however.  This is especially important with one-part
silicones.  If you apply them in too thick a layer, only the surface will
cure.  This is why the silicone potting compounds are generally the
two-part (resin and catalyst) variety.

In commercial epoxy circuit board material, a large portion of the
dielectric loss is due to the flame retardant.  This is often
tetrabromo-bisphenol-A (TBBA).  The standard for glass-epoxy circuit boards
was generated when vacuum tubes were still popular and the flame retardant
was quite necessary.  With the low voltage, low current circuits of today,
there has been some industry interest in eliminating the flame retardant.

        73,  Barry L. Ornitz     WA4VZQ     ornitz@dpnet.net

(*)  Washing the windows first with denatured alcohol before using the
Windex seems to help.

From: "Barry L. Ornitz" <ornitz@dpnet.net>
Subject: Re: Wanted: teflon sheet
Date: 30 Nov 1998
Newsgroups: rec.radio.amateur.homebrew

Hello Jim,

It is too bad I am no longer with W. L. Gore; I could have gotten all the
scrap PTFE the Boatanchor group could have ever needed!

Do NOT use the suggestion of acetate sheet.  While it can take the
temperature of a fusing roll on a laser printer, it has lousy dielectric
properties.  A six meters, I would expect it to ignite in a very high
electric field.  Mylar, or PET, will handle the RF environment but its
temperature rating is slightly less than 100 C.  Very thin fiberglass
filled polyester sheet would work well if flexible enough.  If your
dimensions are finalized, you could make a custom tube from fiberglass
cloth and boat resin.  Use some PTFE plumber's tape on the ends to seal it.

          Barry L. Ornitz     WA4VZQ     ornitz@dpnet.net

Side note:  I have now worked for companies making cellulose acetate,
polyethylene terephthalate, and polytetrafluoroethylene, polyethylene,
polypropylene, and scads of specialty plastics.  I wonder what is next?

From: "Barry L. Ornitz" <ornitz@dpnet.net>
Subject: Re: 6cm waveguide filters - iron screws
Date: 20 Nov 1998
Newsgroups: rec.radio.amateur.homebrew,uk.radio.amateur,sci.electronics.design

Gary Peach wrote in message <733a4l$feh$1@plug.news.pipex.net>...
>What is the resistivity / Loss tangent and dielectric constant of Vaseline
>at uW frequencies?
>
>I am NOT a chemist so will bow willingly and kneel at the feet of anyone
>that is in matters chemical. I am always eager to learn.


Petroleum jelly (Vaseline) is essentially a nonpolar, long-chained alkane.
Its molecular weight is generally lower than paraffin wax and far less than
polyethylene, but the atomic structure is similar.  It has a very low loss
tangent through at least the normal microwave region.  I do not know its
properties at millimeter wavelengths, but it will certainly show infrared
spectra.  Offhand, I do not remember its dielectric constant, but I think
it is around 3 to 4.

When making dielectric measurements of polymer films in a parallel plate
(GenRad) sample holder, it was common to use very small amounts to fill in
the void spaces and bubbles between the polymer surface and the electrodes.
The error introduced was extremely small unless samples were being measured
with an exceptionally high dielectric constant.

           Barry L. Ornitz   WA4VZQ   ornitz@dpnet.net